WorldWideScience

Sample records for accretion powered spin-up

  1. The accretion powered spin-up of GRO J1750-27

    DEFF Research Database (Denmark)

    Shaw, S.E.; Hill, A.B.; Kuulkers, E.

    2009-01-01

    ), respectively. The model is then used to compare the time-resolved variation of the X-ray flux and intrinsic spin-up against the accretion torque model of Ghosh & Lamb; this finds that GRO J1750-27 is likely located 12-22 kpc distant and that the surface magnetic field of the neutron star is similar to 2 x 10...

  2. Evolution of Accreting Binary Systems on the Spin-up Line

    CERN Document Server

    Taani, Ali; Khasawneh, Awni

    2014-01-01

    The measured characteristics of binary pulsars provide valuable insights into the evolution of these systems. We study the aspects of binary evolution particularly relevant to binary Millisecond Pulsars (MSPs), and the formation of close binaries involving degenerate stars through a spin-evolution diagram (spin-up line). For this task, we use a wide variety of binaries, including those with compact components that observed in different energy bands, which we analyze them according to the spin-up line. Their formation and evolution over timescales of binary evolution models are investigated in order to grab any constraint on their evolution, and to estimate the masses of neutron stars with different mass-transfer histories.

  3. Limits On The Spin Up Of Stellar-Mass Black Holes Through A Spiral Stationary Accretion Shock Instability

    CERN Document Server

    Méndez, Enrique Moreno

    2015-01-01

    The spin of a number of black holes (BHs) in binary systems has been measured. In the case of BHs found in low-mass X-ray binaries (LMXBs) the observed values are in agreement with some theoretical predictions based on binary stellar evolution. However, using the same evolutionary models, the calculated spins of BHs in high-mass X-ray binaries (HMXBs) fall short compared to the observations. A possible solution to this conundrum is the accretion of high-specific-angular-momentum material after the formation of the BH, although this requires accretion above the Eddington limit. Another suggestion is that the observed high values of the BHs spin could be the result of an asymmetry during Core Collapse (CC). The only available energy to spin up the compact object during CC is its binding energy. A way to convert it to rotational kinetic energy is by using a Standing Accretion Shock Instability (SASI), which can develop during CC and push angular momentum into the central compact object through a spiral mode ($m ...

  4. Formation of millisecond pulsars with CO white dwarf companions - II. Accretion, spin-up, true ages and comparison to MSPs with He white dwarf companions

    CERN Document Server

    Tauris, Thomas M; Kramer, Michael

    2012-01-01

    Millisecond pulsars (MSPs) are mainly characterised by their spin periods, B-fields and masses - quantities which are largely affected by previous interactions with a companion star in a binary system. In this paper, we investigate the formation mechanism of MSPs by considering the pulsar recycling process in both intermediate-mass X-ray binaries (IMXBs) and low-mass X-ray binaries (LMXBs). The IMXBs mainly lead to the formation of binary MSPs with a massive carbon-oxygen (CO) or an oxygen-neon-magnesium white dwarf (ONeMg WD) companion, whereas the LMXBs form recycled pulsars with a helium white dwarf (He WD) companion. We discuss the accretion physics leading to the spin-up line in the PPdot-diagram and demonstrate that such a line cannot be uniquely defined. We derive a simple expression for the amount of accreted mass needed for any given pulsar to achieve its equilibrium spin and apply this to explain the observed differences of the spin distributions of recycled pulsars with different types of companion...

  5. The Accretion Powered Spin-up of GRO 1750–27

    DEFF Research Database (Denmark)

    Kretschmar, P.; Shaw, S.; Hill, A. B.

    2009-01-01

    The transient Be X-ray pulsar GRO J1750-27 was originally detected in 1995 by CGRO/BATSE during a giant outburst. After a long period of quiescence the source was detected in another outburst early 2008. Following this outburst with hard X-ray data from INTEGRAL and Swift, the orbital parameters ...

  6. Observational Limits on the Spin-down Torque of Accretion Powered Stellar Winds

    Science.gov (United States)

    Zanni, Claudio; Ferreira, Jonathan

    2011-01-01

    The rotation period of classical T Tauri stars (CTTS) represents a longstanding puzzle. While young low-mass stars show a wide range of rotation periods, many CTTS are slow rotators, spinning at a small fraction of breakup, and their rotation period does not seem to shorten, despite the fact that they are actively accreting and contracting. Matt & Pudritz proposed that the spin-down torque of a stellar wind powered by a fraction of the accretion energy would be strong enough to balance the spin-up torque due to accretion. Since this model establishes a direct relation between accretion and ejection, the observable stellar parameters (mass, radius, rotation period, magnetic field) and the accretion diagnostics (accretion shock luminosity) can be used to constrain the wind characteristics. In particular, since the accretion energy powers both the stellar wind and the shock emission, we show in this Letter how the accretion shock luminosity L UV can provide upper limits to the spin-down efficiency of the stellar wind. It is found that luminous sources with L UV >= 0.1 L sun and typical dipolar field components UV Lt 0.1 L sun) are compatible with a zero-torque condition, but the corresponding stellar winds are still very demanding in terms of mass and energy flux. We therefore conclude that accretion powered stellar winds are unlikely to be the sole mechanism to provide an efficient spin-down torque for accreting CTTS.

  7. Observational limits on the spin-down torque of Accretion Powered Stellar Winds

    CERN Document Server

    Zanni, C

    2010-01-01

    The rotation period of classical T Tauri stars (CTTS) represents a longstanding puzzle. While young low-mass stars show a wide range of rotation periods, many CTTS are slow rotators, spinning at a small fraction of break-up, and their rotation period does not seem to shorten, despite the fact that they are actively accreting and contracting. Matt & Pudritz (2005) proposed that the spin-down torque of a stellar wind powered by a fraction of the accretion energy would be strong enough to balance the spin-up torque due to accretion. Since this model establishes a direct relation between accretion and ejection, the observable stellar parameters (mass, radius, rotation period, magnetic field) and the accretion diagnostics (accretion shock luminosity), can be used to constraint the wind characteristics. In particular, since the accretion energy powers both the stellar wind and the shock emission, we show in this letter how the accretion shock luminosity L_UV can provide upper limits to the spin-down efficiency ...

  8. Accretion-powered Compact Binaries

    Science.gov (United States)

    Mauche, Christopher W.

    2003-12-01

    Preface; The workshop logo; A short history of the CV workshop F. A. Córdova; Part I. Observations: 1. Low mass x-ray binaries A. P. Cowley, P. C. Schmidtke, D. Crampton, J. B. Hutchings, C. A. Haswell, E. L. Robinson, K. D. Horne, H. M. Johnston, S. R. Kulkarni, S. Kitamoto, X. Han, R. M. Hjellming, R. M. Wagner, S. L. Morris, P. Hertz, A. N. Parmar, L. Stella, P. Giommi, P. J. Callanan, T. Naylor, P. A. Charles, C. D. Bailyn, J. N. Imamura, T. Steiman-Cameron, J. Kristian, J. Middleditch, L. Angelini and J. P. Noris; 2. Nonmagnetic cataclysmic variables R. S. Polidan, C. W. Mauche, R. A. Wade, R. H. Kaitchuck, E. M. Schlegel, P. A. Hantzios, R. C. Smith, J. H. Wood, F. Hessman, A. Fiedler, D. H. P. Jones, J. Casares, P. A. Charles, J. van Paradijs, E. Harlaftis, T. Naylor, G. Sonneborn, B. J. M. Hassall, K. Horne, C. A. la Dous, A. W. Shafter, N. A. Hawkins, D. A. H. Buckley, D. J. Sullivan, F. V. Hessman, V. S. Dhillon, T. R. Marsh, J. Singh, S. Seetha, F. Giovannelli, A. Bianchini, E. M. Sion, D. J. Mullan, H. L. Shipman, G. Machin, P. J. Callanan, S. B. Howell, P. Szkody, E. M. Schlegel and R. F. Webbink; 3. Magnetic cataclysmic variables C. Hellier, K. O. Mason, C. W. Mauche, G. S. Miller, J. C. Raymond, F. K. Lamb, J. Patterson, A. J. Norton, M. G. Watson, A. R. King, I. M. McHardy, H. Lehto, J. P. Osborne, E. L. Robinson, A. W. Shafter, S. Balachandran, S. R. Rosen, J. Krautter, W. Buchholz, D. A. H. Buckley, I. R. Tuoly, D. Crampton, B. Warner, R. M. Prestage, B. N. Ashoka, M. Mouchet, J. M. Bonnet-Bidaud, J. M. Hameury, P. Szkody, P. Garnavich, S. Howell, T. Kii, M. Cropper, K. Mason, J. Bailey, D. T. Wickramasinghe, L. Ferrario, K. Beuermann, A. D. Schwope, H.-C. Thomas, S. Jordan, J. Schachter, A. V. Filippenko, S. M. Kahn, F. B. S. Paerels, K. Mukai, M. L. Edgar, S. Larsson, R. F. Jameson, A. R. King, A. Silber, R. Remillard, H. Bradt, M. Ishida, T. Ohashi and G. D. Schmidt; Part II. Accretion Theory: 4. Nonmagnetic W. Kley, F. Geyer, H. Herold, H

  9. Self consistent modeling of accretion columns in accretion powered pulsars

    Science.gov (United States)

    Falkner, Sebastian; Schwarm, Fritz-Walter; Wolff, Michael Thomas; Becker, Peter A.; Wilms, Joern

    2016-04-01

    We combine three physical models to self-consistently derive the observed flux and pulse profiles of neutron stars' accretion columns. From the thermal and bulk Comptonization model by Becker & Wolff (2006) we obtain seed photon continua produced in the dense inner regions of the accretion column. In a thin outer layer these seed continua are imprinted with cyclotron resonant scattering features calculated using Monte Carlo simulations. The observed phase and energy dependent flux corresponding to these emission profiles is then calculated, taking relativistic light bending into account. We present simulated pulse profiles and the predicted dependency of the observable X-ray spectrum as a function of pulse phase.

  10. Power Spectrum Density of Stochastic Oscillating Accretion Disk

    Indian Academy of Sciences (India)

    G. B. Long; J. W. Ou; Y. G. Zheng

    2016-06-01

    In this paper, we employ a stochastic oscillating accretion disk model for the power spectral index and variability of BL Lac object S5 0716+714. In the model, we assume that there is a relativistic oscillation of thin accretion disks and it interacts with an external thermal bath through a friction force and a random force. We simulate the light curve and the power spectrum density (PSD) at (i) over-damped, (ii) critically damped and (iii) under-damped cases, respectively. Our results show that the simulated PSD curves depend on the intrinsic property of the accretion disk, and it could be produced in a wide interval ranging from 0.94 to 2.05 by changing the friction coefficient in a stochastic oscillating accretion disk model. We argue that accretion disk stochastic oscillating could be a possible interpretation for observed PSD variability.

  11. Are Radio AGN Powered by Accretion or Black Hole Spin?

    CERN Document Server

    McNamara, B R; Nulsen, P E J

    2010-01-01

    We compare accretion and black hole spin as potential energy sources for outbursts from AGN in brightest cluster galaxies (BCGs). We find that the distribution of AGN power estimated from X-ray cavities is consistent with a broad range of both spin parameter and accretion rate. Sufficient quantities of molecular gas are available in most BCGs to power their AGN by accretion alone. However, we find no correlation between AGN power and molecular gas mass. For a given AGN power, the BCG's gas mass and accretion efficiency vary by more than two orders of magnitude. Most of the molecular gas in BCGs is apparently consumed by star formation or is driven out of the nucleus by the AGN before it reaches the nuclear black hole. Bondi accretion from hot atmospheres is generally unable to fuel powerful AGN, unless their black holes are more massive than their bulge luminosities imply. We identify several powerful AGN that reside in relatively gas-poor galaxies, indicating an unusually efficient mode of accretion, or that...

  12. SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS

    Energy Technology Data Exchange (ETDEWEB)

    Matt, Sean P. [Laboratoire AIM Paris-Saclay, CEA/Irfu Universite Paris-Diderot CNRS/INSU, 91191 Gif-sur-Yvette (France); Pinzon, Giovanni [Observatorio Astronomico Nacional, Facultad de Ciencias, Universidad Nacional de Colombia, Bogota (Colombia); Greene, Thomas P. [NASA Ames Research Center, M.S. 245-6, Moffett Field, CA 94035-1000 (United States); Pudritz, Ralph E., E-mail: sean.matt@cea.fr, E-mail: thomas.p.greene@nasa.gov, E-mail: gapinzone@unal.edu.co, E-mail: pudritz@physics.mcmaster.ca [Physics and Astronomy Department, McMaster University, Hamilton, ON L8S 4M1 (Canada)

    2012-01-20

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh and Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

  13. BeppoSAX observations of the accretion-powered X-ray pulsar SMC X-1

    CERN Document Server

    Naik, S

    2004-01-01

    We present here results obtained from three BeppoSAX observations of the accretion-powered X-ray pulsar SMC X-1 carried out during the declining phases of its 40--60 days long super-orbital period. Timing analysis of the data clearly shows a continuing spin-up of the neutron star. Energy-resolved timing analysis shows that the pulse-profile of SMC X-1 is single peaked at energies less than 1.0 keV whereas an additional peak, the amplitude of which increases with energy within the MECS range, is present at higher energies. Broad-band pulse-phase-averaged spectroscopy of the BeppoSAX data, which is done for the first time since its discovery, shows that the energy spectrum in the 0.1--80 keV energy band has three components, a soft excess that can be modeled as a thermal black-body, a hard power-law component with a high-energy exponential cutoff and a narrow and weak iron emission line at 6.4 keV. Pulse-phase resolved spectroscopy indicates a pulsating nature of the soft spectral component, as seen in a few ot...

  14. Spin Evolution of Accreting Young Stars. II. Effect of Accretion-Powered Stellar Winds

    CERN Document Server

    Matt, Sean P; Greene, Thomas P; Pudritz, Ralph E

    2011-01-01

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind. For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1--10 days in the age range of 1--3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to s...

  15. Ice Accretion Prediction on Wind Turbines and Consequent Power Losses

    Science.gov (United States)

    Yirtici, Ozcan; Tuncer, Ismail H.; Ozgen, Serkan

    2016-09-01

    Ice accretion on wind turbine blades modifies the sectional profiles and causes alteration in the aerodynamic characteristic of the blades. The objective of this study is to determine performance losses on wind turbines due to the formation of ice in cold climate regions and mountainous areas where wind energy resources are found. In this study, the Blade Element Momentum method is employed together with an ice accretion prediction tool in order to estimate the ice build-up on wind turbine blades and the energy production for iced and clean blades. The predicted ice shapes of the various airfoil profiles are validated with the experimental data and it is shown that the tool developed is promising to be used in the prediction of power production losses of wind turbines.

  16. Black hole mass, jet power and accretion in AGN

    CERN Document Server

    Chen, Yong-Yun; Xiong, Dingrong; Yu, Xiaoling

    2015-01-01

    We study the relation between accretion, black hole mass and jet power in AGN, by using a large group of blazars detected by the Fermi Large Area Telescope and radio galaxies. Our main results are as follows. (i) The jet power of FSRQs and FRII-HEG depends on the black hole mass, which suggests that the FSRQs and FRII-HEG are in Radiation-Pressure Dominated regime. The jet power of BL Lacs and FRI-LEG depends on the accretion, which suggests that the BL Lacs and FRI-LEG are in the Gas-Pressure Dominated regime. (ii) We find that most of FSRQs and BL Lacs have $\\rm{P_{jet}>L_{BZ}^{max}}$, which suggests that the Blandford-Znajek mechanism is insufficient to explain the jet power of these objects. (iii) The FSRQs are roughly separated from BL Lacs by the Ledlow-Owen's dividing line in the $\\rm{\\log P_{jet}-\\log M}$ plane, which supports the unified scheme of AGN. (iv) The FSRQs and BL Lacs have a clear division at $\\rm{L_{bol}/L_{Edd}\\sim0.01}$, and the distribution of Eddington ratios of BL Lacs and FSRQs exhi...

  17. CHANDRA Observations of V407 Vul: Confirmation of the Spin-up

    Science.gov (United States)

    Strohmayer, T.

    2004-01-01

    V407 Vu1 (RX J1914.4+2456) is a candidate double-degenerate binary with a putative 1.756 mHz (9.5 min) orbital frequency. In a previous timing study using archival ROSAT and ASCA data we reported evidence for an increase of this frequency at a rate consistent with expectations for gravitational radiation from a detached ultracompact binary system. Here we report the results of new Chandra timing observations which confirm the previous indications of spin-up of the X-ray frequency, and provide much tighter constraints on the frequency derivative, nu (raised dot). We obtained with Chandra a total of 90 ksec of exposure in two epochs separated in time by 11.5 months. The total time span of the archival ROSAT, ASCA and new Chandra data is now approximately equal to 10.5 years. This more than doubles the interval spanned by the ROSAT and ASCA data alone, providing much greater sensitivity to a frequency derivative. With the addition of the Chandra data an increasing frequency is unavoidable, and the mean nu (raised dot) is 7.0 plus or minus 0.8 x l0(exp -18) Hz per second. Although a long-term spin-up trend is confirmed, there is excess variance in the phase timing residuals, perhaps indicative of shorter timescale torque fluctuations or phase instability associated with the source of the X-ray flux. Power spectral searches for periods longward of the 9.5 minute period do not find any significant modulations, however, the sensitivity of searches in this frequency range are somewhat compromised by the dithering of the Chandra attitude. The observed spin-up is of a magnitude consistent with that expected from gravitational radiation decay, however, the factor of approximately equal to 3 variations in flux combined with the timing noise could conceivably result from accretion-induced spin-up of a white dwarf. Continued monitoring to explore correlations of torque with X-ray flux could provide a further test of this hypothesis.

  18. Spinning-Up: the Case of the Symbiotic X-Ray Binary 3A 1954+319

    Science.gov (United States)

    Fuerst, F.; Marcu, D. M.; Pottschmidt, K.; Grinberg, V.; Wilms, J.; CadolleBel, M.

    2011-01-01

    We present a timing and spectral analysis of the variable X-ray source 3A 1954+319. Our analysis is mainly based on an outburst serendipitously observed during INTEGRAL Key Program observations of the Cygnus region in 2008 fall and on the Swift/BAT longterm light curve. Previous observations, though sparse, have identified the source to be one of only nine known symbiotic X-ray binaries, i.e., systems composed of an accreting neutron star orbiting in a highly inhomogeneous medium around an M-giant companion. The spectrum of3A 1954+319 above > 20 keV can be best described by a broken power law model. The extremely long pulse period of approx.5.3 hours is clearly visible in the INTEGRAL/ISGRI light curve and confirmed through an epoch folding period search. Furthermore, the light curve allows us to determine a very strong spin up of -2 x 10(exp -4) h/h during the outburst. This spin up is confirmed by the pulse period evolution calculated from Swift/BAT data. The Swift/BAT data also show a long spin-down trend prior to the 2008 outburst, which is confirmed in archival INTEGRAL/ISGRI data. We discuss possible accretion models and geometries allowing for the transfer of such large amounts of angular momentum and investigate the harder spectrum of this outburst compared to previously published results.

  19. "Circularization" vs. Accretion -- What Powers Tidal Disruption Events?

    CERN Document Server

    Piran, Tsvi; Krolik, Julian; Cheng, Roseanne M; Shiokawa, Hotaka

    2015-01-01

    A tidal disruption event (TDE) takes place when a star passes near enough to a massive black hole to be disrupted. About half the star's matter is given elliptical trajectories with large apocenter distances, the other half is unbound. To "circularize", i.e., to form an accretion flow, the bound matter must lose a significant amount of energy, with the actual amount depending on the characteristic scale of the flow measured in units of the black hole's gravitational radius (~ 10^{51} (R/1000R_g)^{-1} erg). Recent numerical simulations (Shiokawa et al., 2015) have revealed that the circularization scale is close to the scale of the most-bound initial orbits, ~ 10^3 M_{BH,6.5}^{-2/3} R_g ~ 10^{15} M_{BH,6.5}^{1/3} cm from the black hole, and the corresponding circularization energy dissipation rate is $\\sim 10^{44} M_{BH,6.5}^{-1/6}$~erg/s. We suggest that the energy liberated during circularization, rather then energy liberated by accretion onto the black hole, powers the observed optical TDE candidates (e.g.A...

  20. Swinging between rotation and accretion power in a binary millisecond pulsar

    NARCIS (Netherlands)

    Papitto, A.; Ferrigno, C.; Bozzo, E.; Rea, N.

    2014-01-01

    We present the discovery of IGR J18245-2452, the first millisecond pulsar observed to swing between a rotation-powered, radio pulsar state, and an accretion-powered X-ray pulsar state [31]. This transitional source represents the most convincing proof of the evolutionary link shared by accreting

  1. Observations of Accreting Pulsars

    Science.gov (United States)

    Bildsten, Lars; Chakrabarty, Deepto; Chiu, John; Finger, Mark H.; Koh, Danny T.; Nelson, Robert W.; Prince, Thomas A.; Rubin, Bradley C.; Scott, D. Matthew; Stollberg, Mark; Vaughan, Brian A.; Wilson, Colleen A.; Wilson, Robert B.

    1997-01-01

    We summarize 5 years of continuous monitoring of accretion-powered pulsars with the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. Our 20-70 keV observations have determined or refined the orbital parameters of 13 binaries, discovered five new transient accreting pulsars, measured the pulsed flux history during outbursts of 12 transients (GRO J1744-28, 4U 0115+634, GRO J1750-27, GS 0834-430, 2S 1417-624, GRO J1948+32, EXO 2030+375, GRO J1008-57, A0535+26, GRO J2058+42, 4U 1145-619, and A1118-616), and also measured the accretion torque history during outbursts of six of those transients whose orbital param- eters were also known. We have also continuously measured the pulsed flux and spin frequency for eiaht persistently accreting pulsars (Her X-1, Cen X-3, Vela X-1, OAO 1657-415, GX 301-2, 4U 1626-67, 4U 1538-52, and GX 1+4). Because of their continuity and uniformity over a long baseline, BATSE observations have provided new insights into the long-term behavior of accreting magnetic neutron stars. We have found that all accreting pulsars show stochastic variations in their spin frequencies and luminosities, including those displaying secular spin-up or spin-down on long timescales, which blurs the con- ventional distinction between disk-fed and wind-fed binaries. Pulsed flux and accretion torque are strongly correlated in outbursts of transient accreting pulsars but are uncorrelated, or even anti- correlated, in persistent sources. We describe daily folded pulse profiles, frequency, and flux measurements that are available through the Compton Observatory Science Support Center at NASA/Goddard Space Flight Center.

  2. On the release of binding energy and accretion power in core collapse-like environments

    CERN Document Server

    Socrates, Aristotle

    2008-01-01

    All accretion models of gamma-ray bursts share a common assumption: accretion power and gravitational binding energy is released and then dissipated locally, with the mass of its origin. This is equivalent to the Shakura-Sunyaev 1973 (SS73) prescription for the dissipation of accretion power and subsequent conversion into radiate output. Since their seminal paper, broadband observations of quasars and black hole X-ray binaries insist that the SS73 prescription cannot wholly describe their behavior. In particular, optically thick black hole accretion flows are almost universally accompanied by coronae whose relative power by far exceeds anything seen in studies of stellar chromospheric and coronal activity. In this note, we briefly discuss the possible repercussions of freeing accretion models of GRBs from the SS73 prescription. Our main conclusion is that the efficiency of converting gravitational binding energy into a GRB power can be increased by an order of magnitude or more.

  3. The power of relativistic jets is larger than the luminosity of their accretion disks

    CERN Document Server

    Ghisellini, G; Maraschi, L; Celotti, A; Sbarrato, T

    2014-01-01

    Theoretical models for the production of relativistic jets from active galactic nuclei predict that jet power arises from the spin and mass of the central black hole, as well as the magnetic field near the event horizon. The physical mechanism mechanism underlying the contribution from the magnetic field is the torque exerted on the rotating black hole by the field amplified by the accreting material. If the squared magnetic field is proportional to the accretion rate, then there will be a correlation between jet power and accretion luminosity. There is evidence for such a correlation, but inadequate knowledge of the accretion luminosity of the limited and inhomogeneous used samples prevented a firm conclusion. Here we report an analysis of archival observations of a sample of blazars (quasars whose jets point towards Earth) that overcomes previous limitations. We find a clear correlation between jet power as measured through the gamma-ray luminosity, and accretion luminosity as measured by the broad emission...

  4. Spinning-up: the case of the symbiotic X-ray binary 3A 1954+319

    CERN Document Server

    Fürst, F; Pottschmidt, K; Grinberg, V; Wilms, J; Bel, M Cadolle

    2011-01-01

    We present a timing and spectral analysis of the variable X-ray source 3A 1954+319. Our analysis is mainly based on an outburst serendipitously observed during INTEGRAL Key Program observations of the Cygnus region in 2008 fall and on the Swift/BAT longterm light curve. Previous observations, though sparse, have identified the source to be one of only nine known symbiotic X-ray binaries, i.e., systems composed of an accreting neutron star orbiting in a highly inhomogeneous medium around an M-giant companion. The spectrum of 3A 1954+319 above 20 keV can be best described by a broken power law model. The extremely long pulse period of ~5.3 hours is clearly visible in the INTEGRAL/ISGRI light curve and confirmed through an epoch folding period search. Furthermore, the light curve allows us to determine a very strong spin up of -2x10^-4 h/h during the outburst. This spin up is confirmed by the pulse period evolution calculated from Swift/BAT data. The Swift/BAT data also show a long spin-down trend prior to the 2...

  5. Spin-Up/Spin-Down models for Type Ia Supernovae

    CERN Document Server

    Di Stefano, R; Claeys, J S W

    2011-01-01

    In the single degenerate scenario for Type Ia supernova (SNeIa), a white dwarf (WD) must gain a significant amount of matter from a companion star. Because the accreted mass carries angular momentum, the WD is likely to achieve fast spin periods, which can increase the critical mass, $M_{crit}$, needed for explosion. When $M_{crit}$ is higher than the maximum mass achieved by the WD, the WD must spin down before it can explode. This introduces a delay between the time at which the WD has completed its epoch of mass gain and the time of the explosion. Matter ejected from the binary during mass transfer therefore has a chance to become diffuse, and the explosion occurs in a medium with a density similar to that of typical regions of the interstellar medium. Also, either by the end of the WD's mass increase or else by the time of explosion, the donor may exhaust its stellar envelope and become a WD. This alters, generally diminishing, explosion signatures related to the donor star. Nevertheless, the spin-up/spin...

  6. CHANDRA Observations of RX J1914.4+ 2456: Spin-up of a White Dwarf?

    Science.gov (United States)

    Strohmayer, Tod E.

    2004-01-01

    RX 51914.4+2456 is a candidate double-degenerate binary with a putative 1.756 mHz orbital frequency. In a previous timing study using archival ROSAT and ASCA data we reported evidence for an increase of the putative orbital frequency at a rate consistent with expectations for gravitational radiation from the system. Here we report the results of new Chandra timing observations which confirm the previous indications of spin-up of the X-ray frequency, and provide much tighter constraints on the frequency derivative, u. We obtained with Chandra a total of 75 ksec of exposure in two epochs separated in time by 10.3 months. The total time span of the archival ROSAT, ASCA and new Chandra data is now 10.2 years. This more than doubles the interval spanned by the ROSAT and ASCA data alone, providing much greater sensitivity to a frequency derivative. With the addition of the Chandra data an increasing frequency is unavoidable, and the mean i/ is 5.9f0.9 x 10-l' Hz s-'. Interestingly, power spectra of the longest Chandra pointing show evidence for a sideband structure to the 1.756 mHz frequency. The fundamental and first harmonic show evidence for upper sidebands with a frequency separation of E 0.5 mHz from their parent peaks. Additionally, the first and second harmonics show evidence for lower sidebands with approximately half the frequency separation of the upper sidebands. Similar sideband structure is a common feature of Intermediate Polars (Ips)-although it is usually observed in the optical-and suggests the presence of a longer period in the system, perhaps the previously unseen orbital period. If this is correct the sideband structure indicates an orbital period close to 1 hr, and the observed u likely represents the accretion-induced spin-up of a white dwarf. We discuss the implications of these findings for the nature of RX J1914.4+2456.

  7. An Exoplanet Spinning Up Its Star

    Science.gov (United States)

    Kohler, Susanna

    2016-11-01

    , for transiting planets with orbital periods shorter than 2 days and masses greater than 0.1 Jupiter masses. HATS-18b is denoted by the red star. [Penev et al. 2016]Tidal InteractionsWhat happens when a massive planet orbits this close to its star? Tidal interactions between the star and the planet cause tidal dissipation in the star, resulting in decay of the planets orbit. But there may be an additional effect of this interaction in the case of HATS-18b, the authors claim: the planet may be transferring some of its angular momentum to the star.As stars age, they should gradually spin slower as they lose angular momentum viastellar winds. But Penev and collaborators note that this exoplanets host star, HATS-18, spins roughly three times as fast asits inferred age suggests it should. The authors conclude that the angular momentum lost by the planet as its orbit shrinks is deposited in the star, causing the star to spin up.HATS-18 is an excellent laboratory for studying how very short-period planets interact with their stars in fact, Penev and collaborators have already used their observations of the system to constrain models of tidal dissipation from Sun-like stars. Additional observations of HATS-18 and other short-period systems should allow us to further test models of how planetary systems form and evolve.CitationK. Penev et al 2016 AJ 152 127. doi:10.3847/0004-6256/152/5/127

  8. Asteroid spin-up fission systems

    Science.gov (United States)

    Pravec, P.

    2014-07-01

    Among asteroids smaller than about 15 km in diameter, there is a population of binary and multiple asteroid systems that show characteristics strongly suggesting their formation by spin-up fission. I will review the current observational data we have on the systems and compare them with predictions from theories of formation of asteroid systems. I will show that the best explanation of their observed properties is provided by the theory of fission of cohesionless (rubble-pile) asteroids spun up to the critical spin frequency by the YORP effect. Observed asteroid systems are of two kinds: bound and unbound. Bound asteroid systems typically consist of a larger primary and one or two smaller satellites. Unbound systems consist of two asteroids orbiting the Sun on highly similar orbits, again with one being typically larger (primary) and the other being smaller (secondary). These two groups are not exclusive; there exist systems with one or two bound and an unbound secondary. Our current sample consists of 133 bound asteroid systems (binaries or triples) with primary sizes between 0.12 and 13 km and of 178 asteroid pairs with similar primary sizes. Bound systems have been observed in heliocentric orbits from near the Earth to the outer main belt, while asteroid pairs are recognizable only in the main belt where their orbits are only slowly dispersed so the pairs can be identified for up to 2 Myr after formation. The leading observational techniques for discovery and characterization of asteroid systems are radar imagery (for near-Earth asteroid systems) and lightcurve photometry (for main-belt ones). The observed characteristics of asteroid systems suggesting their formation by rotational fission of parent rubble-pile asteroids after being spun up by the YORP effect are as follows. The angular momentum content of binary asteroids is close to critical. The orientations of satellite orbits are non-random; the orbital poles concentrate near the obliquities of 0 and 180

  9. NuSTAR discovery of an unusually steady long-term spin-up of the Be binary 2RXP J130159.6-635806

    CERN Document Server

    Krivonos, Roman A; Lutovinov, Alexander A; Tomsick, John A; Chakrabarty, Deepto; Bachetti, Matteo; Boggs, Steven E; Chernyakova, Masha; Christensen, Finn E; Craig, William W; Fuerst, Felix; Hailey, Charles J; Harrison, Fiona A; Lansbury, George B; Rahoui, Farid; Stern, Daniel; Zhang, William W

    2015-01-01

    We present spectral and timing analysis of NuSTAR observations of the accreting X-ray pulsar 2RXP J130159.6-635806. The source was serendipitously observed during a campaign focused on the gamma-ray binary PSR B1259-63 and was later targeted for a dedicated observation. The spectrum has a typical shape for accreting X-ray pulsars, consisting of a simple power law with an exponential cutoff starting at ~7 keV with a folding energy of E_fold=~18 keV. There is also an indication of the presence of a 6.4 keV iron line in the spectrum at the ~3 sigma significance level. NuSTAR measurements of the pulsation period reveal that the pulsar has undergone a strong and steady spin-up for the last 20 years. The pulsed fraction is estimated to be ~80%, and is constant with energy up to 40 keV. The power density spectrum shows a break towards higher frequencies relative to the current spin period. This, together with steady persistent luminosity, points to a long-term mass accretion rate high enough to bring the pulsar out ...

  10. Powerful jets from accreting black holes: evidence from the optical and infrared

    NARCIS (Netherlands)

    Russell, D.M.; Fender, R.P.; Wachter, A.D.; Propst, R.J.

    2010-01-01

    A common consequence of accretion onto black holes is the formation of powerful, relativistic jets that escape the system. In the case of supermassive black holes at the centres of galaxies this has been known for decades, but for stellar-mass black holes residing within galaxies like our own, it ha

  11. Super-Eddington mechanical power of an accreting black hole in M83.

    Science.gov (United States)

    Soria, R; Long, K S; Blair, W P; Godfrey, L; Kuntz, K D; Lenc, E; Stockdale, C; Winkler, P F

    2014-03-21

    Mass accretion onto black holes releases energy in the form of radiation and outflows. Although the radiative flux cannot substantially exceed the Eddington limit, at which the outgoing radiation pressure impedes the inflow of matter, it remains unclear whether the kinetic energy flux is bounded by this same limit. Here, we present the detection of a radio-optical structure, powered by outflows from a non-nuclear black hole. Its accretion disk properties indicate that this black hole is less than 100 solar masses. The optical-infrared line emission implies an average kinetic power of 3 × 10(40) erg second(-1), higher than the Eddington luminosity of the black hole. These results demonstrate kinetic power exceeding the Eddington limit over a sustained period, which implies greater ability to influence the evolution of the black hole's environment.

  12. Extremes of the jet-accretion power relation of blazars, as explored by NuSTAR

    CERN Document Server

    Sbarrato, T; Tagliaferri, G; Perri, M; Madejski, G M; Stern, D; Boggs, S E; Christensen, F E; Craig, W W; Hailey, C J; Harrison, F A; Zhang, W W

    2015-01-01

    Hard X-ray observations are crucial to study the non-thermal jet emission from high-redshift, powerful blazars. We observed two bright z>2 flat spectrum radio quasars (FSRQs) in hard X-rays to explore the details of their relativistic jets and their possible variability. S5 0014+81 (at z=3.366) and B0222+185 (at z=2.690) have been observed twice by the Nuclear Spectroscopic Telescope Array (NuSTAR) simultaneously with Swift/XRT, showing different variability behaviours. We found that NuSTAR is instrumental to explore the variability of powerful high-redshift blazars, even when no gamma-ray emission is detected. The two sources have proven to have respectively the most luminous accretion disk and the most powerful jet among known blazars. They are located at the extreme end of the jet-accretion disk relation previously found for gamma-ray detected blazars.

  13. Super-Eddington Mechanical Power of an Accreting Black Hole in M83

    Science.gov (United States)

    Soria, R.; Long, K. S.; Blair, W. P.; Godfrey, L.; Kuntz, K. D.; Lenc, E.; Stockdale, C.; Winkler, P. F.

    2014-01-01

    Mass accretion onto black holes releases energy in the form of radiation and outflows. Although the radiative flux cannot substantially exceed the Eddington limit, at which the outgoing radiation pressure impedes the inflow of matter, it remains unclear whether the kinetic energy flux is bounded by this same limit. Here, we present the detection of a radio-optical structure, powered by outflows from a non-nuclear black hole. Its accretion disk properties indicate that this black hole is less than 100 solar masses. The optical-infrared line emission implies an average kinetic power of 3 × 10(exp 40) erg second(exp -1), higher than the Eddington luminosity of the black hole. These results demonstrate kinetic power exceeding the Eddington limit over a sustained period, which implies greater ability to influence the evolution of the black hole's environment.

  14. Swinging between rotation and accretion power in a millisecond binary pulsar

    CERN Document Server

    Papitto, A; Bozzo, E; Rea, N; Pavan, L; Campana, S; Romano, P; Burderi, L; Di Salvo, T; Riggio, A; Torres, D F; Falanga, M; Hessels, J W T; Burgay, M; Sarkissian, J M; Wieringa, M H; Filipović, M D; Wong, G F

    2013-01-01

    Radio pulsars are neutron stars that emit radiation modulated and powered by the rotation of their magnetic field, and which consequently decelerate (Pacini, 1967). The very fast millisecond spin periods measured in old radio pulsars (Backer et al. 1982) are thought to be the outcome of an earlier X-ray bright phase, during which the neutron star accretes matter and angular momentum from a low mass companion star in a binary system (Alpar et al. 1982; Radhakrishnan & Srinivasan 1982). This evolutionary scenario has been supported by the detection of X-ray millisecond pulsations from several accreting neutron stars in the past fifteen years (Wijnands & van der Klis 1998), as well as by the indirect evidence for the presence of a disk in the past around a millisecond radio pulsar now powered by rotation (Archibald et al. 2009). However, a transition between a rotation-powered and an accretion-powered state was never observed. Here we present the detection of millisecond X-ray pulsations from an accretin...

  15. Accretion-caused deceleration of a gravitationally powerful compact stellar object moving within a dense Fermi gas

    CERN Document Server

    Tito, Elizabeth P

    2016-01-01

    We consider accretion-caused deceleration of a gravitationally-powerful compact stellar object traveling within a cold Fermi-gas medium. We provide analytical and numerical estimates of the effect manifestation.

  16. AGN jet power and feedback controlled by Bondi accretion in brightest cluster galaxies

    CERN Document Server

    Fujita, Yutaka; Shlosman, Isaac

    2014-01-01

    (Abridged) We propose a new method to estimate the Bondi (hot gas) accretion rates onto the supermassive black holes (SMBHs) at the centres of elliptical galaxies. It can be applied even if the Bondi radius is not well-resolved in X-ray observations. This method is based on two simple assumptions: (1) hot gas outside the Bondi radius is in nearly a hydrostatic equilibrium in a gravitational potential, and (2) the gas temperature near the galaxy centre is close to the virial temperature of the galaxy. We apply this method to 28 bright elliptical galaxies in nearby galaxy clusters (27 of them are the brightest cluster galaxies; BCGs). We find a strong correlation between the Bondi accretion rates and the power of jets associated with the SMBHs. For most galaxies, the accretion rates are large enough to account for the jet powers. Our results indicate that hot gas in the elliptical galaxies directly controls the feedback from the active galactic nuclei (AGN), which leads to a stable heating of the cluster cool c...

  17. The spin-up of contracting red supergiants

    CERN Document Server

    Heger, A; Heger, Alexander; Langer, Norbert

    1998-01-01

    We report on a mechanism which may lead to a spin-up of the surface of a rotating single star leaving the Hayashi line, which is much stronger than the spin-up expected from the mere contraction of the star. By analyzing rigidly rotating, convective stellar envelopes, we qualitatively work out the mechanism through which these envelopes may be spun up or down by mass loss through their lower or upper boundary, respectively. We find that the first case describes the situation in retreating convective envelopes, which tend to retain most of the angular momentum while becoming less massive, thereby increasing the specific angular momentum in the convection zone and thus in the layers close to the stellar surface. We explore the spin-up mechanism quantitatively in a stellar evolution calculation of a rotating 12 M_sun star, which is found to be spun up to critical rotation after leaving the red supergiant branch. We discuss implications of this spin-up for the circumstellar matter around several types of stars, i...

  18. Stratified spin-up in a sliced, square cylinder

    Energy Technology Data Exchange (ETDEWEB)

    Munro, R. J. [Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD (United Kingdom); Foster, M. R. [Department of Mathematical Sciences, Rensselaer Polytechnic Institute, Troy, New York 12180 (United States)

    2014-02-15

    We previously reported experimental and theoretical results on the linear spin-up of a linearly stratified, rotating fluid in a uniform-depth square cylinder [M. R. Foster and R. J. Munro, “The linear spin-up of a stratified, rotating fluid in a square cylinder,” J. Fluid Mech. 712, 7–40 (2012)]. Here we extend that analysis to a “sliced” square cylinder, which has a base-plane inclined at a shallow angle α. Asymptotic results are derived that show the spin-up phase is achieved by a combination of the Ekman-layer eruptions (from the perimeter region of the cylinder's lid and base) and cross-slope-propagating stratified Rossby waves. The final, steady state limit for this spin-up phase is identical to that found previously for the uniform depth cylinder, but is reached somewhat more rapidly on a time scale of order E{sup −1/2}Ω{sup −1}/log (α/E{sup 1/2}) (compared to E{sup −1/2}Ω{sup −1} for the uniform-depth cylinder), where Ω is the rotation rate and E the Ekman number. Experiments were performed for Burger numbers, S, between 0.4 and 16, and showed that for S≳O(1), the Rossby modes are severely damped, and it is only at small S, and during the early stages, that the presence of these wave modes was evident. These observations are supported by the theory, which shows the damping factors increase with S and are numerically large for S≳O(1)

  19. The Early History of Stellar Spin: the Theory of Accretion onto Young Stellar Objects

    Directory of Open Access Journals (Sweden)

    Pudritz Ralph E.

    2014-01-01

    Full Text Available The interaction of the magnetospheres of forming stars with their surrounding protostellar disks results in magnetospheric accretion flow onto the star. How is the associated angular momentum of accreting material channelled? The resolution of this issue is crucial for understanding the origin of the spins of pre main sequence stars. A significant fraction of these rotate very slowly, which indicates that an efficient angular momentum transport mechanism is at work to counteract the strong accretion spin up torques. We review the observational, theoretical, and computational advances in the field and argue that an accretion powered stellar winds together with highly time variable mass ejections from the disk/magnetosphere interface is a likely solution.

  20. An accretion disk swept up by a powerful thermonuclear X-ray burst

    Science.gov (United States)

    Degenaar, Nathalie

    Type-I X-ray bursts are thermonuclear explosions occurring in the surface layers of accreting neutron stars. These events are powerful probes of the physics of neutron stars and their surrounding accretion flow. Swift recently caught a very energetic type-I X-ray burst from the neutron star IGR J17062-6143 that displayed exceptional features. Firstly, the light curve of the 18 minute long X-ray burst tail shows an episode of 10 minutes with wild X-ray intensity fluctuations. Secondly, X-ray spectral analysis revealed a highly significant emission line around 1 keV, which can be interpreted as an Fe-L shell line caused by the irradiation of cold gas. Finally, the detection of significant absorption lines and edges in the Fe-K band are strongly suggestive of the presence of hot, highly ionized gas along the line of sight. None of these features are present in the persistent emission of the source. The X-ray burst of IGR J17062-6143 shows the first unambiguous detection of atomic features at CCD resolution. The timescale of the strong intensity variations, the velocity width of the Fe-L emission line, and photo-ionization modeling of the Fe-K absorption features each independently point to swept-up gas at a radius of ~1000 km from the neutron star. The unusual X-ray light curve and spectral properties could have plausibly been caused by a disruption of the accretion disk due to the super-Eddington fluxes reached during the X-ray burst.

  1. Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques

    CERN Document Server

    Matt, Sean

    2008-01-01

    [Abridged] In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar winds in torquing down the stars. For this mechanism to be effective, the stellar winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed 2-dimensional (axisymmetric) stellar wind solutions, in order to determine the spin down torque on the star. We explore a range of parameters relevant for CTTSs, including variations in the stellar mass, radius, spin rate, surface magnetic field strength, the mass loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the stellar wind torque is of sufficient magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss rate of $\\sim 10^{-9} M_\\odot$ yr$^{-1}$. The winds are wide-angle, ...

  2. Revitalizing Physics Departments: The Spin-UP Reports

    Science.gov (United States)

    Hehn, J. G.; Czujko, R.; Hilborn, R.

    2007-12-01

    The American Institute of Physics (AIP) has carefully measured education trends in the physics and related sciences community for nearly five decades. During the 1990s, the community realized that the number of undergraduate physics majors was declining significantly. A number of efforts were launched in the physics community intending to reverse that decline and the number of bachelor's degrees has been rebounding for the last 7 years. The National Task Force on Undergraduate Physics (NTFUP) was one such effort that identified thriving physics departments and analyzed strategies shared among those departments. In 2003 NTFUP issued a report entitled: Strategic Programs for Innovations in Undergraduate Physics, referred to as Spin-UP. A subsequent study of physics programs in two-year colleges, Spin-UP TYC, produced many similar findings published in 2005. Lessons learned as stated in the Spin-UP reports and several conferences will be reviewed. Some lessons learned include: A thriving department demonstrated (1) a widespread attitude among the faculty that the department has the primary responsibility for maintaining or improving the undergraduate program; (2) a challenging, but supportive and encouraging undergraduate program that includes a well-developed curriculum, advising and mentoring, an undergraduate research participation program, and many opportunities for informal student-faculty interactions, enhanced by a strong sense of community among the students and faculty; (3) strong and sustained leadership within the department and a clear sense of the mission of its undergraduate program; and (4) a strong disposition toward continuous evaluation of and experimentation with the undergraduate program. In short, thriving departments paid attention to undergraduates and made majors feel like members of their physics department and members of a physics community.

  3. Simulations of the magnetospheres of accreting millisecond pulsars

    CERN Document Server

    Parfrey, Kyle; Beloborodov, Andrei M

    2016-01-01

    Accreting pulsars power relativistic jets, and display a complex spin phenomenology. These behaviours may be closely related to the large-scale configuration of the star's magnetic field. The total torque experienced by the pulsar comprises spin-up and spin-down contributions from different bundles of magnetic field lines; the spin-down `braking' torque is applied both by closed stellar field lines which enter the disc beyond the corotation radius, and those which are open and not loaded with disc material. The rates of energy and angular momentum extraction on these open field lines have lower bounds in the relativistic, magnetically dominated limit, due to the effective inertia of the electromagnetic field itself. Here we present the first relativistic simulations of the interaction of a pulsar magnetosphere with an accretion flow. Our axisymmetric simulations, with the pseudospectral PHAEDRA code, treat the magnetospheric, or coronal, regions using a resistive extension of force-free electrodynamics. The m...

  4. Swings between rotation and accretion power in a binary millisecond pulsar

    NARCIS (Netherlands)

    Papitto, A.; Ferrigno, C.; Bozzo, E.; Rea, N.; Pavan, L.; Burderi, L.; Burgay, M.; Campana, S.; Di Salvo, T.; Falanga, M.; Filipović, M.D.; Freire, P.C.C.; Hessels, J.W.T.; Possenti, A.; Ransom, S.M.; Riggio, A.; Romano, P.; Sarkissian, J.M.; Stairs, I.H.; Stella, L.; Torres, D.F.; Wieringa, M.H.; Wong, G.F.

    2013-01-01

    It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods1, 2, 3. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When

  5. An ultraluminous X-ray source powered by an accreting neutron star

    DEFF Research Database (Denmark)

    Bachetti, M.; Harrison, F. A.; Walton, D. J.;

    2014-01-01

    the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects....

  6. An Electromagnetic Model for Jet Power from an Advection Dominated Accretion Flow around a Rotating Black Hole

    Institute of Scientific and Technical Information of China (English)

    GONG Xiao-Long; WANG Ding-Xiong

    2005-01-01

    @@ We discuss jet production from an advection dominated accretion flow (ADAF) around a rotating black hole (BH) in an electromagnetic regime. An analytical expression for the jet power is derived by using an equivalent circuit in the BH magnetosphere. It turns out that a large fraction of jet powers is contributed from the inner region of the ADAF, and the jet power depends sensitively on the degree to which the flow is advection-dominated. In addition, we use our model to fit the strong jet powers of several BL Lac objects, which cannot be explained by virtue of the BZ process.

  7. A High-Frequency Doppler Feature in the Power Spectra of Simulated GRMHD Black Hole Accretion Disks

    CERN Document Server

    Wellons, Sarah; Psaltis, Dimitrios; Narayan, Ramesh; McClintock, Jeffrey E

    2013-01-01

    Black hole binaries exhibit a wide range of variability phenomena, from large-scale state changes to broadband noise and quasi-periodic oscillations, but the physical nature of much of this variability is poorly understood. We examine the variability properties of three GRMHD simulations of thin accretion disks around black holes of varying spin, producing light curves and power spectra as would be seen by observers. We find that the simulated power spectra show a broad feature at high frequency, which increases in amplitude with the inclination of the observer. We show that this high-frequency feature is a product of the Doppler effect and that its location is a function of the mass and spin of the black hole. This Doppler feature demonstrates that power spectral properties of the accretion disk can be tied to, and potentially used to determine, physical properties of the black hole.

  8. An Ultraluminous X-ray Source Powered by An Accreting Neutron Star

    CERN Document Server

    Bachetti, M; Walton, D J; Grefenstette, B W; Chakrabarty, D; Fürst, F; Barret, D; Beloborodov, A; Boggs, S E; Christensen, F E; Craig, W W; Fabian, A C; Hailey, C J; Hornschemeier, A; Kaspi, V; Kulkarni, S R; Maccarone, T; Miller, J M; Rana, V; Stern, D; Tendulkar, S P; Tomsick, J; Webb, N A; Zhang, W W

    2014-01-01

    Ultraluminous X-ray sources (ULX) are off-nuclear point sources in nearby galaxies whose X-ray luminosity exceeds the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes. Their luminosity ranges from $10^{40}$ erg s$^{-1} $10^{40}$ erg s$^{-1}$), which require black hole masses MBH >50 solar masses and/or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries. Here we report broadband X-ray observations of the nuclear region of the galaxy M82, which contains two bright ULXs. The observations reveal pulsations of average period 1.37 s with a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to $L_X$(3 - 30 keV) = $4.9 \\times 10^{39}$ erg s$^{-1}$. The pulsating source is spatially coincident with a variable ULX which can reach $L_X$ (0.3 - 10 keV) = $1.8 \\times 10^{40}$ erg s$^{-1}$. This ...

  9. Timing of the first eclipsing accretion-powered millisecond X-ray pulsar

    CERN Document Server

    Altamirano, D; Patruno, A; Watts, A; Linares, M; Degenaar, N; Kalamkar, M; van der Klis, M; Rea, N; Casella, P; Padilla, M Armas; Kaur, R; Yang, Y J; Soleri, P; Wijnands, R

    2010-01-01

    We report on the timing analysis of the first eclipsing accretion-powered millisecond X-ray pulsar (AMXP): SWIFT J1749.4-2807. The neutron star rotates at a frequency of ~517.9 Hz and is in a binary system with an orbital period of 8.8 hrs and a projected semi-major axis of ~1.90 lt-s. Based on the mass function and the eclipse half-angle, we constrain the inclination of the system to be between ~76 and ~80 deg. This is to date the tightest constraint on the orbital inclination of any AMXP. We also estimate the mass of the companion to be in the 0.6-0.8 Msun range. As in other AMXPs, the pulse profile shows harmonic content up to the 3rd overtone. However, this is the first AMXP to show a 1st overtone with rms amplitudes between 5 and 25%, which is the strongest ever seen, and which can be more than two times stronger than the fundamental. The fact that SWIFT J1749.4-2807 is an eclipsing system which shows uncommonly strong harmonic content suggests that it might be the best source to date to set constraints ...

  10. A model for electromagnetic extraction of rotational energy and formation of accretion-powered jets in radio galaxies

    CERN Document Server

    Hujeirat, A

    2004-01-01

    A self-similar solution for the 3D axi-symmetric radiative MHD equations, which revisits the formation and acceleration of accretion-powered jets in AGNs and microquasars, is presented. The model relies primarily on electromagnetic extraction of rotational energy from the disk plasma and forming a geometrically thin super-Keplerian layer between the disk and the overlying corona. The outflowing plasma in this layer is dissipative, two-temperature, virial-hot, advective and electron-proton dominated. The innermost part of the disk in this model is turbulent-free, sub-Keplerian rotating and advective-dominated. This part ceases to radiate as a standard disk, and most of the accretion energy is converted into magnetic and kinetic energies that go into powering the jet. The corresponding luminosities of these turbulent-truncated disks are discussed.

  11. Application of the Ghosh & Lamb Relation to the Spin-up/down Behavior in the X-ray Binary Pulsar 4U 1626-67

    CERN Document Server

    Takagi, Toshihiro; Sugizaki, Mutsumi; Makishima, Kazuo; Morii, Mikio

    2016-01-01

    We analyzed continuous MAXI/GSC data of the X-ray binary pulsar 4U 1626-67 from 2009 October to 2013 September, and determined the pulse period and the pulse-period derivative for every 60-d interval by the epoch folding method. The obtained periods are consistent with those provided by the Fermi/GBM pulsar project. In all the 60-d intervals, the pulsar was observed to spin up, with the spin-up rate positively correlated with the 2-20 keV flux. We applied the accretion torque model proposed by Ghosh & Lamb (1979, ApJ, 234, 296) to the MAXI/GSC data, as well as the past data including both spin-up and spin-down phases. The Ghosh & Lamb relation was confirmed to successfully explain the observed relation between the spin-up/down rate and the flux. By comparing the model-predicted luminosity with the observed flux, the source distance was constrained as 5-13 kpc, which is consistent with that by Chakrabarty (1998, ApJ, 492, 342). Conversely, if the source distance is assumed, the data can constrain the m...

  12. 14 CFR Appendix D to Part 23 - Wheel Spin-Up and Spring-Back Loads

    Science.gov (United States)

    2010-01-01

    ... 14 Aeronautics and Space 1 2010-01-01 2010-01-01 false Wheel Spin-Up and Spring-Back Loads D.... D Appendix D to Part 23—Wheel Spin-Up and Spring-Back Loads D23.1 Wheel spin-up loads. (a) The... spring-back of the landing gear and adjacent structure at the instant just after the wheels come up...

  13. Periodic Accretion-powered Flares from Colliding EMRIs as TDE Imposters

    Science.gov (United States)

    Metzger, Brian D.; Stone, Nicholas C.

    2017-07-01

    When a main-sequence star undergoes Roche lobe overflow onto a supermassive black hole (SMBH) in a circular extreme mass ratio inspiral (EMRI), a phase of steady mass transfer ensues. Over millions of years, the binary evolves to a period minimum before reversing course and migrating outward as a brown dwarf. Because the time interval between consecutive EMRIs is comparable to the mass-transfer timescale, the semimajor axes of two consecutive mass-transferring EMRIs will cross on a radial scale of less than a few au. We show that such EMRI crossing events are inevitably accompanied by a series of mildly relativistic, grazing physical collisions between the stars. Each collision strips a small quantity of mass, primarily from the more massive star, which generally increases their radial separation to set up the next collision after a delay of decades to centuries (or longer) set by further gravitational radiation. Depending on the mass of the SMBH, this interaction can result in {N}{{c}}˜ 1{--}{10}4 gas production events of mass ˜ {M}⊙ /{N}{{c}}, thus powering a quasi-periodic sequence of SMBH accretion-powered flares over a total duration of thousands of years or longer. Although the EMRI rate is 2-3 orders of magnitude lower than the rate of tidal disruption events (TDEs), the ability of a single interacting EMRI pair to produce a large number of luminous flares—and to make more judicious use of the available stellar fuel—could make their observed rate competitive with the TDE rate, enabling them to masquerade as “TDE imposters.” Gas produced by EMRI collisions is easier to circularize than the highly eccentric debris streams produced in TDEs. We predict flares with bolometric luminosities that decay both as power laws shallower than {t}-5/3 and as decaying exponentials in time. Viscous spreading of the gaseous disks produced by the accumulation of previous mass-stripping events will place a substantial mass of gas on radial scales ≳ 10{--}100 {au

  14. Episodic jet power extracted from a spinning black hole surrounded by a neutrino-dominated accretion flow in gamma-ray bursts

    Energy Technology Data Exchange (ETDEWEB)

    Cao, Xinwu [Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030 (China); Liang, En-Wei [Department of Physics and GXU-NAOC Center for Astrophysics and Space Sciences, Guangxi University, Nanning 530004 (China); Yuan, Ye-Fei, E-mail: cxw@shao.ac.cn, E-mail: lew@gxu.edu.cn, E-mail: yfyuan@ustc.edu.cn [Department of Astronomy, University of Sciences and Technology of China, Hefei, Anhui 230026 (China)

    2014-07-10

    It was suggested that the relativistic jets in gamma-ray bursts (GRBs) are powered via the Blandford-Znajek (BZ) mechanism or the annihilation of neutrinos and anti-neutrinos from a neutrino cooling-dominated accretion flow (NDAF). The advection and diffusion of the large-scale magnetic field of an NDAF is calculated, and the external magnetic field is found to be dragged inward efficiently by the accretion flow for a typical magnetic Prandtl number P{sub m}=η/ν∼1. The maximal BZ jet power can be ∼10{sup 53}-10{sup 54} erg s{sup –1} for an extreme Kerr black hole, if an external magnetic field with 10{sup 14} Gauss is advected by the NDAF. This is roughly consistent with the field strength of the disk formed after a tidal disrupted magnetar. The accretion flow near the black hole horizon is arrested by the magnetic field if the accretion rate is below than a critical value for a given external field. The arrested accretion flow fails to drag the field inward and the field strength decays, and then the accretion re-starts, which leads to oscillating accretion. The typical timescale of such episodic accretion is of an order of one second. This can qualitatively explain the observed oscillation in the soft extended emission of short-type GRBs.

  15. A large spin-up rate measured with INTEGRAL in the High Mass X-ray Binary Pulsar SAXJ2103.5+4545

    CERN Document Server

    Sidoli, L; Larsson, S; Chernyakova, M; Kreykenbohm, I; Kretschmar, P; Paizis, A; Santangelo, A; Ferrigno, C; Falanga, M

    2005-01-01

    The High Mass X-ray Binary Pulsar SAXJ2103.5+4545 has been observed with INTEGRAL several times during the last outburst in 2002-2004. We report a comprehensive study of all INTEGRAL observations, allowing a study of the pulse period evolution during the recent outburst. We measured a very rapid spin-up episode, lasting 130days, which decreased the pulse period by 1.8s. The spin-up rate, pdot=-1.5e-7 s/s, is the largest ever measured for SAXJ2103.5+4545, and it is among the fastest for an accreting pulsar. The pulse profile shows evidence for temporal variability, apparently not related to the source flux or to the orbital phase. The X-ray spectrum is hard and there is significant emission up to 150keV. A new derivation of the orbital period, based on RXTE data, is also reported.

  16. Can the anomalous X-ray pulsars be powered by accretion?

    CERN Document Server

    Li, X D

    1999-01-01

    The nature of the 5-12 s "anomalous" X-ray pulsars remains a mystery. Among the models that have been proposed to explain the properties of AXPs, the most likely ones are: (1) isolated accreting neutron stars evolved from the Thorne-\\.{Z}ytkow objects due to complete spiral-in during the common envelope evolution of high-mass X-ray binaries, and (2) magnetars, which are neutron stars with ultra-high ($\\sim 10^{14}-10^{15}$ G) surface magnetic fields. We have critically examined the predicted change of neutron star's spin in the accretion model, and found that it is unable to account for the steady spin-down observed in AXPs. A simple analysis also shows that any accretion disk around an isolated neutron star has extremely limited lifetime. A more promising explanation for such objects is the magnetar model.

  17. The 2015 outburst of the accretion-powered pulsar IGR J00291+5934: INTEGRAL and Swift observations

    Science.gov (United States)

    De Falco, V.; Kuiper, L.; Bozzo, E.; Galloway, D. K.; Poutanen, J.; Ferrigno, C.; Stella, L.; Falanga, M.

    2017-03-01

    The pulsar IGR J00291+5934 is the fastest-known accretion-powered X-ray pulsar, discovered during a transient outburst in 2004. In this paper, we report on INTEGRAL and Swift observations during the 2015 outburst, which lasts for 25 d. The source has not been observed in outburst since 2008, suggesting that the long-term accretion rate has decreased by a factor of two since discovery. The averaged broad-band (0.1-250 keV) persistent spectrum in 2015 is well described by a thermal Comptonization model with a column density of NH ≈ 4 × 1021 cm-2, a plasma temperature of kTe ≈ 50 keV, and a Thomson optical depth of τT ≈ 1. Pulsations at the known spin period of the source are detected in the INTEGRAL data up to the 150 keV energy band. We also report on the discovery of the first thermonuclear burst observed from IGR J00291+5934, which lasts around 7 min and occurs at a persistent emission level corresponding to roughly 1.6% of the Eddington accretion rate. The properties of the burst suggest it is powered primarily by helium ignited at a depth of yign ≈ 1.5 × 109 g cm-2 following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first 20 s of the burst provide indications of a photospheric radius expansion phase. Assuming this is the case, we infer a source distance of d = 4.2 ± 0.5 kpc.

  18. Swinging between rotation and accretion power in a binary millisecond pulsar

    Directory of Open Access Journals (Sweden)

    Papitto A.

    2014-01-01

    While accreting mass, the X-ray emission of IGR J18245–2452 varies dramatically on time-scales ranging from a second to a few hours. We interpret a state characterised by a lower flux and pulsed fraction, and by sudden increases of the hardness of the X-ray emission, in terms of the onset of a magnetospheric centrifugal inhibition of the accretion flow. Prospects of finding new members of the newly established class of transitional pulsars are also briefly discussed.

  19. The 2015 outburst of the accretion-powered pulsar IGR J00291+5934: INTEGRAL and Swift observations

    CERN Document Server

    De Falco, V; Bozzo, E; Galloway, D K; Poutanen, J; Ferrigno, C; Stella, L; Falanga, M

    2016-01-01

    IGR J00291+5934 is the fastest-known accretion-powered X-ray pulsar, discovered during a transient outburst in 2004. In this paper, we report on Integral and Swift observations during the 2015 outburst, which lasts for $\\sim25$ d. The source has not been observed in outburst since 2008, suggesting that the long-term accretion rate has decreased by a factor of two since discovery. The averaged broad-band (0.1 - 250 keV) persistent spectrum in 2015 is well described by a thermal Comptonization model with a column density of $N_{\\rm H} \\approx4\\times10^{21}$ cm$^{-2}$, a plasma temperature of $kT_{\\rm e} \\approx50$ keV, and a Thomson optical depth of $\\tau_{\\rm T}\\approx1$. Pulsations at the known spin period of the source are detected in the Integral data up to the $\\sim150$ keV energy band. We also report on the discovery of the first thermonuclear burst observed from IGR J00291+5934, which lasts around 7 min and occurs at a persistent emission level corresponding to roughly $1.6\\%$ of the Eddington accretion ...

  20. The 2009 outburst of accretion-powered millisecond pulsar IGR J17511-3057 as observed by Swift and RXTE

    Science.gov (United States)

    Ibragimov, Askar; Poutanen, Juri; Kajava, Jari

    Accretion-powered millisecond pulsars (AMPs) are very interesting astrophysical objects. Mat-ter from accretion disk is captured by star's magnetic field and falls along the field lines, creating "hotspots" near magnetic poles of the star. Typical spectrum of an AMP contains a disk emis-sion, blackbody emission of a hotspot and a powerlaw tail, produced by thermal Comptonizaion in accreting shock. Pulse profiles of these sources are modified by relativistic effects and can be used to put geometrical constraints and to understand physical processes near the compact object. IGR J17511-3057 was discovered on September 12, 2009 during the INTEGRAL Galactic Bulge monitoring program. The source has the pulse frequency of 245 Hz. In this work, we study spectral and temporal characheristics of IGR J17511-3057 during the outburst, based on Swift and RXTE data. We analyze its energy spectra in range 0.6-150 keV, phase-resolved spectra, pulse profiles, time lags and discuss physical conditions in the source.

  1. EXPERIMENTAL INVESTIGATION ON VORTEX BREAKDOWN IN SPIN-UP AND SPIN-DOWN PROCESSES VIA PIV

    Institute of Scientific and Technical Information of China (English)

    Liu Ying-zheng; Koyama Hide S.; Chen Han-ping

    2003-01-01

    The whole field measurements of swirling flow in spin-up and spin-down processes via PIV are presented in the paper. Investigation of the flow patterns at H/R=1.50 was experimentally carried out for the first time in both processes. By means of symmetry analysis, it is found that the overall flow structure in the spin-up process still keep axisymmetric to a great extent, but deteriorated very fast in the spin-down process. The time to settle to the state of rest in the spin-down process is found to be greatly shorter than the time to achieve the steady state in the spin-up process. Temporarily oscillatory vortex breakdown was discovered during the spin-up process, although no breakdown in the steady state at the same Reynolds number is found in precious researches.

  2. Episodic jet power extracted from a spinning black hole surrounded by a neutrino-dominated accretion flow in gamma-ray bursts

    CERN Document Server

    Cao, Xinwu; Yuan, Ye-Fei

    2014-01-01

    It was suggested that the relativistic jets in gamma-ray bursts (GRBs) are powered via the Blandford-Znajek (BZ) mechanism or the annihilation of neutrinos and anti-neutrinos from a neutrino cooling-dominated accretion flow (NDAF). The advection and diffusion of the large-scale magnetic field of a NDAF is calculated, and the external magnetic field is found to be dragged inward efficiently by the accretion flow for a typical magnetic Prandtl number P_m=1. The maximal BZ jet power can be ~10^53-10^54 erg/sec for an extreme Kerr black hole, if an external magnetic field with 10^14 Gauss is advected by the NDAF. This is roughly consistent with the field strength of the disk formed after a tidal disrupted magnetar. The accretion flow near the black hole horizon is arrested by the magnetic field if the accretion rate is below than a critical value for a given external field. The arrested accretion flow fails to drag the field inward and the field strength decays, and then the accretion re-starts, which leads to os...

  3. Application of the Ghosh & Lamb relation to the spin-up/down behavior in the X-ray binary pulsar 4U 1626-67

    Science.gov (United States)

    Takagi, Toshihiro; Mihara, Tatehiro; Sugizaki, Mutsumi; Makishima, Kazuo; Morii, Mikio

    2016-06-01

    We analyzed continuous Monitor of All-sky X-ray Image/Gas Slit Camera (MAXI/GSC) data of the X-ray binary pulsar 4U 1626-67 from 2009 October to 2013 September, and determined the pulse period and the pulse-period derivative for every 60-d interval by the epoch folding method. The obtained periods are consistent with those provided by the Fermi/Gamma-ray Burst Monitor pulsar project. In all the 60-d intervals, the pulsar was observed to spin up, with the spin-up rate positively correlated with the 2-20 keV flux. We applied the accretion torque model proposed by Ghosh and Lamb (1979, ApJ, 234, 296) to the MAXI/GSC data, as well as the past data including both spin-up and spin-down phases. The "Ghosh & Lamb" relation was confirmed to successfully explain the observed relation between the spin-up/down rate and the flux. By comparing the model-predicted luminosity with the observed flux, the source distance was constrained as 5-13 kpc, which is consistent with that found by Chakrabarty (1998, ApJ, 492, 342). Conversely, if the source distance is assumed, the data can constrain the mass and radius of the neutron star, because the Ghosh & Lamb model depends on these parameters. We attempted this idea, and found that an assumed distance of, e.g., 10 kpc gives a mass in the range of 1.81-1.90 solar mass, and a radius of 11.4-11.5 km, although these results are still subject to considerable systematic uncertainties, other than distance.

  4. Ultraluminous X-ray Sources Powered by Radiatively Efficient Two-Phased Super-Eddington Accretion onto Stellar Mass Black holes

    CERN Document Server

    Socrates, A; Socrates, Aristotle; Davis, Shane W.

    2005-01-01

    The radiation spectra of many of the brightest ultraluminous X-ray sources (ULXs) are dominated by a hard power law component, likely powered by a hot, optically thin corona that Comptonizes soft seed photons emitted from a cool, optically thick black hole accretion disk. Before its dissipation and subsequent conversion into coronal photon power, the randomized gravitational binding energy responsible for powering ULX phenomena must separate from the mass of its origin by a means other than, and quicker than, electron scattering-mediated radiative diffusion. Therefore, the release of accretion power in ULXs is not necessarily subject to Eddington-limited photon trapping, as long as it occurs in a corona. Motivated by these basic considerations, we present a model of ULXs powered by geometrically thin accretion onto stellar mass black holes. We argue that the radiative efficiency of the flow remains high if the corona is magnetized or optically thin and the majority of the accretion power escapes in the form o...

  5. The accretion history of dark matter halos II: The connections with the mass power spectrum and the density profile

    CERN Document Server

    Correa, Camila A; Schaye, Joop; Duffy, Alan R

    2015-01-01

    We explore the relation between the structure and mass accretion histories of dark matter halos using a suite of cosmological simulations. We confirm that the formation time, defined as the time when the virial mass of the main progenitor equals the mass enclosed within the scale radius, correlates strongly with concentration. We provide a semi-analytic model for halo mass history that combines analytic relations with fits to simulations. This model has the functional form, $M(z) = M_{0}(1+z)^{\\alpha}e^{\\beta z}$, where the parameters $\\alpha$ and $\\beta$ are directly correlated with concentration. We then combine this model for the halo mass history with the analytic relations between $\\alpha$, $\\beta$ and the linear power spectrum derived by Correa et al. (2014) to establish the physical link between halo concentration and the initial density perturbation field. Finally, we provide fitting formulas for the halo mass history as well as numerical routines, we derive the accretion rate as a function of halo ma...

  6. Study of luminosity and spin-up relation in X-ray binary pulsars with long-term monitoring by MAXI/GSC and Fermi/GBM

    CERN Document Server

    Sugizaki, Mutsumi; Nakajima, Motoki; Yamaoka, Kazutaka

    2015-01-01

    We study the relation between luminosity and spin-period change in X-ray binary pulsars using long-term light curve obtained by the MAXI/GSC all-sky survey and pulse period data from the Fermi/GBM pulsar project. X-ray binaries, consisting of a highly magnetized neutron star and a stellar companion, originate X-ray emission according to the energy of the accretion matter onto the neutron star. The accretion matter also transfers the angular momentum at the Alfven radius, and then spin up the neutron star. Therefore, the X-ray luminosity and the spin-up rate are supposed to be well correlated. We analyzed the luminosity and period-change relation using the data taken by continuous monitoring of MAXI/GSC and Fermi/GBM for Be/X-ray binaries, GX 304$-$1, A 0535$+$26, GRO J1008$-$57, KS 1947$+$300, and 2S 1417$-$624, which occurred large outbursts in the last four years. We discuss the results comparing the obtained observed relation with that of the theoretical model by Ghosh \\& Lamb (1979).

  7. Spin-up and hot spots can drive mass out of a binary

    Science.gov (United States)

    van Rensbergen, W.; De Greve, J. P.; De Loore, C.; Mennekens, N.

    2008-09-01

    Context: The observed distribution of orbital periods of Algols with a B-type primary at birth agrees fairly well with the prediction from conservative theory. Conservative evolution fails, however, to produce the rather large fraction of Algols observed with a high mass-ratio, especially: q in [0.4-0.6]. Aims: In order to keep Algols for a longer time with a higher mass-ratio without disturbing the distribution of orbital periods too much, interacting binaries have to lose a significant fraction of their total mass without losing much angular momentum before or during Algolism. We propose a mechanism that meets both requirements. Methods: In the case of direct impact the gainer spins up: sometimes up to critical velocity. Equatorial material on the gainer is therefore less bound. A similar statement applies to material located at the edge of an accretion disc. The incoming material moreover creates a hot spot in the area of impact. The sum of the rotational and radiative energy of hot spot material depends on the mass-transfer-rate. The sum of both energies overcomes the binding energy at a well defined critical value of the mass-transfer-rate. As long as the transfer-rate is smaller than this critical value RLOF happens conservatively. But as soon as the critical rate is exceeded the gainer will acquire no more than the critical value and RLOF runs into a liberal era. Results: Low-mass binaries never achieve mass-transfer-rates larger than the critical value. Intermediate-mass binaries evolve mainly conservatively but mass will be blown away from the system during the short era of rapid mass-transfer soon after the onset of RLOF. We have calculated the evolution of binaries with a 9 M⊙ primary and a 5.4 M⊙ companion over a range of initial orbital periods, covering case-A RLOF. Mass-loss from the system is achieved during direct impact only. Conclusions: We find systems that show Algolism for more than ten million years. RLOF occurs almost always

  8. Inconsistent strategies to spin up models in CMIP5: implications for ocean biogeochemical model performance assessment

    Science.gov (United States)

    Séférian, Roland; Gehlen, Marion; Bopp, Laurent; Resplandy, Laure; Orr, James C.; Marti, Olivier; Dunne, John P.; Christian, James R.; Doney, Scott C.; Ilyina, Tatiana; Lindsay, Keith; Halloran, Paul R.; Heinze, Christoph; Segschneider, Joachim; Tjiputra, Jerry; Aumont, Olivier; Romanou, Anastasia

    2016-05-01

    During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were made to systematically assess the skill of Earth system models. One goal was to check how realistically representative marine biogeochemical tracer distributions could be reproduced by models. In routine assessments model historical hindcasts were compared with available modern biogeochemical observations. However, these assessments considered neither how close modeled biogeochemical reservoirs were to equilibrium nor the sensitivity of model performance to initial conditions or to the spin-up protocols. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESMs) contributes to model-to-model differences in the simulated fields. We take advantage of a 500-year spin-up simulation of IPSL-CM5A-LR to quantify the influence of the spin-up protocol on model ability to reproduce relevant data fields. Amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) is assessed as a function of spin-up duration. We demonstrate that a relationship between spin-up duration and assessment metrics emerges from our model results and holds when confronted with a larger ensemble of CMIP5 models. This shows that drift has implications for performance assessment in addition to possibly aliasing estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to-model uncertainty. This requires more attention in future model intercomparison exercises in order to provide quantitatively more correct ESM results on marine biogeochemistry and carbon cycle feedbacks.

  9. Inconsistent Strategies to Spin up Models in CMIP5: Implications for Ocean Biogeochemical Model Performance Assessment

    Science.gov (United States)

    Seferian, Roland; Gehlen, Marion; Bopp, Laurent; Resplandy, Laure; Orr, James C.; Marti, Olivier; Dunne, John P.; Christian, James R.; Doney, Scott C.; Ilyina, Tatiana; Romanou, Anastasia

    2015-01-01

    During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were made to systematically assess the skill of Earth system models. One goal was to check how realistically representative marine biogeochemical tracer distributions could be reproduced by models. In routine assessments model historical hindcasts were compared with available modern biogeochemical observations. However, these assessments considered neither how close modeled biogeochemical reservoirs were to equilibrium nor the sensitivity of model performance to initial conditions or to the spin-up protocols. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESMs) contributes to model-to-model differences in the simulated fields. We take advantage of a 500-year spin-up simulation of IPSL-CM5A-LR to quantify the influence of the spin-up protocol on model ability to reproduce relevant data fields. Amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) is assessed as a function of spin-up duration. We demonstrate that a relationship between spin-up duration and assessment metrics emerges from our model results and holds when confronted with a larger ensemble of CMIP5 models. This shows that drift has implications for performance assessment in addition to possibly aliasing estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to- model uncertainty. This requires more attention in future model intercomparison exercises in order to provide quantitatively more correct ESM results on marine biogeochemistry and carbon cycle feedbacks.

  10. Inconsistent strategies to spin up models in CMIP5: implications for ocean biogeochemical model performance assessment

    Science.gov (United States)

    Séférian, R.; Gehlen, M.; Bopp, L.; Resplandy, L.; Orr, J. C.; Marti, O.; Dunne, J. P.; Christian, J. R.; Doney, S. C.; Ilyina, T.; Lindsay, K.; Halloran, P.; Heinze, C.; Segschneider, J.; Tjiputra, J.

    2015-10-01

    During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were carried out on the systematic assessment of the skill of Earth system models. One goal was to check how realistically representative marine biogeochemical tracer distributions could be reproduced by models. Mean-state assessments routinely compared model hindcasts to available modern biogeochemical observations. However, these assessments considered neither the extent of equilibrium in modeled biogeochemical reservoirs nor the sensitivity of model performance to initial conditions or to the spin-up protocols. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESM) contribute to model-to-model differences in the simulated fields. We take advantage of a 500 year spin-up simulation of IPSL-CM5A-LR to quantify the influence of the spin-up protocol on model ability to reproduce relevant data fields. Amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) is assessed as a function of spin-up duration. We demonstrate that a relationship between spin-up duration and assessment metrics emerges from our model results and is consistent when confronted against a larger ensemble of CMIP5 models. This shows that drift has implications on their performance assessment in addition to possibly aliasing estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to-model uncertainty. This requires more attention in future model intercomparison exercices in order to provide realistic ESM results on marine biogeochemistry and carbon cycle feedbacks.

  11. Dust particle spin-up caused by cross-field plasma flow and turbulence.

    Science.gov (United States)

    Shukla, P. K.; Shevchenko, V. I.; Krasheninnikov, S. I.

    2006-10-01

    Spinning of dust particles adds new interesting features to dust particle dynamics and to the dusty plasma physics. Several reasons for dust particle spin-up have been suggested (e.g. Ref. 1): i) sheared flow of plasmas around charge dust particles, ii) dust particle surface irregularities, and iii) sheath effects resulting from the interactions of a charge dipole of a dust particle (caused by plasma flows into the sheath) with the sheath electric field. Here we present a novel mechanism for charged dust particle spin-up. The physics of the present mechanism is simple and robust, and is associated with the interaction of a charge dipole of a dust particle, D, induced by the ExB cross-field flow of a magnetized plasma (D ExB), where E and B are the electric and ambient magnetic fields. Since the resulting torque is proportional to | E |^2, the presented mechanism of charged dust particle spin-up works for both stationary and non-stationary (turbulent in particular) electric fields. In many cases the turbulent electric field stremgth is much larger than the laminar one so that the impact of turbulence can be dominant. We present theoretical analyses for charged dust particle spin-up and estimate the maximum value for the angular velocity charged dust particle can acquire due to our new spin-up mechanism. [1] N. Sato ``Spinning Motion of Fine Particles in Plasmas'', AIP Conference Proceedings No. 799, p. 97; AIP, New York, 2005.

  12. A Newton-Krylov solver for fast spin-up of online ocean tracers

    Science.gov (United States)

    Lindsay, Keith

    2017-01-01

    We present a Newton-Krylov based solver to efficiently spin up tracers in an online ocean model. We demonstrate that the solver converges, that tracer simulations initialized with the solution from the solver have small drift, and that the solver takes orders of magnitude less computational time than the brute force spin-up approach. To demonstrate the application of the solver, we use it to efficiently spin up the tracer ideal age with respect to the circulation from different time intervals in a long physics run. We then evaluate how the spun-up ideal age tracer depends on the duration of the physics run, i.e., on how equilibrated the circulation is.

  13. Inconsistent strategies to spin up models in CMIP5: implications for ocean biogeochemical model performance assessment

    Science.gov (United States)

    Seferian, R.; Gehlen, M.; Bopp, L.; Resplandy, L.; Orr, J. C.; Marti, O.

    2016-12-01

    During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were made to systematically assess the skills of Earth system models against available modern observations. However, most of these skill-assessment approaches can be considered as "blind" given that they were applied without considering models' specific characteristics and treat models a priori as independent of observations. Indeed, since these models are typically initialized from observations, the spin-up procedure (e.g. the length of time for which the model has been run since initialization, and therefore the degree to which it has approached it's own equilibrium) has the potential to exert a significant control over the skill-assessment metrics calculated for each model. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESM) contributes to model-to-model differences in the simulated fields. We focus on the amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) as a function of spin-up duration in a dedicated 500-year-long spin-up simulation performed with IPSL-CM5A-LR as well as an ensemble of 24 CMIP5 ESMs. We demonstrate that a relationship between spin-up duration and skill-assessment metrics emerges from the results of a single model and holds when confronted with a larger ensemble of CMIP5 models. This shows that drift in biogeochemical fields has implications for performance assessment in addition to possibly influence estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to-model uncertainty. This requires more attention in future model intercomparison exercises in order to provide quantitatively more correct ESM results on marine biogeochemistry and carbon cycle feedbacks.

  14. Numerical Experiments on the Spin-up Time for Seasonal-Scale Regional Climate Modeling

    Institute of Scientific and Technical Information of China (English)

    ZHONG Zhong; HU Yijia; MIN Jinzhong; XU Honglei

    2007-01-01

    In this paper, the numerical experiments on the issue of spin-up time for seasonal-scale regional climate modeling were conducted with the newly Regional Climate Model (RegCM3), in the case of the abnormal climate event during the summer of 1998 in China. To test the effect of spin-up time on the regional climate simulation results for such abnormal climate event, a total of 11 experiments were performed with different spin-up time from 10 days to 6 months, respectively. The simulation results show that, for the meteorological variables in the atmosphere, the model would be running in "climate mode" after 4-8-day spin-up time, then,it is independent of the spin-up time basically, and the simulation errors are mainly caused by the model's failure in describing the atmospheric processes over the model domain. This verifies again that the regional climate modeling is indeed a lateral boundary condition problem as demonstrated by earlier research work.The simulated mean precipitation rate over each subregion is not sensitive to the spin-up time, but the precipitation scenario is somewhat different for the experiment with different spin-up time, which shows that there exists the uncertainty in the simulation to precipitation scenario, and such a uncertainty exhibits more over the areas where heavy rainfall happened. Generally, for monthly-scale precipitation simulation, aspin-up time of 1 month is enough, whereas a spin-up time of 2 months is better for seasonal-scale one.Furthermore, the relationship between the precipitation simulation error and the advancement/withdrawal of East Asian summer monsoon was analyzed. It is found that the variability of correlation coefficient for precipitation is more significant over the areas where the summer monsoon is predominant. Therefore, the model's capability in reproducing precipitation features is related to the heavy rainfall processes associated with the advancement/withdrawal of East Asian summer monsoon, which suggests

  15. The neutron star transient and millisecond pulsar in M28: from sub-luminous accretion to rotation-powered quiescence

    CERN Document Server

    Linares, Manuel; Heinke, Craig; Wijnands, Rudy; Patruno, Alessandro; Altamirano, Diego; Homan, Jeroen; Bogdanov, Slavko; Pooley, David

    2013-01-01

    The X-ray transient IGR J18245-2452 in the globular cluster M28 contains the first neutron star (NS) seen to switch between rotation-powered and accretion-powered pulsations. We analyse its 2013 March-April 25d-long outburst as observed by Swift, which had a peak bolometric luminosity of ~6% of the Eddington limit (L$_{E}$), and give detailed properties of the thermonuclear burst observed on 2013 April 7. We also present a detailed analysis of new and archival Chandra data, which we use to study quiescent emission from IGR J18245-2452 between 2002 and 2013. Together, these observations cover almost five orders of magnitude in X-ray luminosity (L$_X$, 0.5-10 keV). The Swift spectrum softens during the outburst decay (photon index $\\Gamma$ from 1.3 above L$_X$/L$_{E}$=10$^{-2}$ to ~2.5 at L$_X$/L$_{E}$=10$^{-4}$), similar to other NS and black hole (BH) transients. At even lower luminosities, deep Chandra observations reveal hard ($\\Gamma$=1-1.5), purely non-thermal and highly variable X-ray emission in quiesce...

  16. A relation of jet power to the central black hole and its accretion

    Directory of Open Access Journals (Sweden)

    Liu Xiang

    2013-12-01

    Full Text Available We have developed an integrated jet power formula in the context of the Blandford-Znajek and Blandford-Payne models, and applied this model to the Foschini sample. The result suggests that there is a positive correlation of the jet power versus the product of the disk luminosity and black hole mass within each type of source, and the di↵erent linear correlation slopes imply that the disk emissivity efficiency and/or the SMBH spin are quite di↵erent for FSRQs, BL Lacs and γ – NLS 1s.

  17. Discovery of a Transition to Global Spin-up in EXO 2030+375

    Science.gov (United States)

    Wilson, Colleen A.; Fabregat, Juan; Coburn, Wayne

    2005-01-01

    EXO 2030+375, a 42 s transient X-ray pulsar with a Be star companion, has been observed to undergo an outburst at nearly every periastron passage for the last 13.5 years. From 1994 through 2002, the global trend in the pulsar spin frequency was spin-down. Using Rossi X-Ray Timing Explorer (RXTE) data from 2003 September, we have observed a transition to global spin-up in EXO 2030+375. Although the spin-frequency observations are sparse, the relative spin-up between 2002 June and 2003 September observations, along with an overall brightening of the outbursts since mid-2002 observed with the RXTE All-Sky Monitor, accompanied by an increase in density of the Be disk, indicated by infrared magnitudes, suggest that the pattern observed with BATSE of a roughly constant spin frequency, followed by spin-up, followed by spin-down is repeating. If so, this pattern has approximately an 11 yr period, similar to the 15 +/- 3 yr period derived by Wilson et al. for the precession period of a one-armed oscillation in the Be disk. If this pattern is indeed repeating, we predict a transition from spin-up to spin-down in 2005.

  18. Interfacial mass transfer to a cylinder endwall during spin-up/spin-down

    Science.gov (United States)

    Larrousse, Mark F.; Wilcox, William R.

    1990-01-01

    The local rate of mass transfer to the bottom endwall of a large aspect ratio cylinder was measured during spin-up/spin-down. The local mass transfer rate was a strong function radial position along the endwall. At the center during spin-up from rest, the maximum enhancement in mass transfer occurred after the Ekman time scale and before the viscous time scale. At the center during spin-down to rest, a stagnation vortex formed, causing the mass transfer rate to decay and then increase back to the original value of the order of the viscous time scale. Away from the center a much more complicated pattern was observed, but spin-up and spin-down were similar. Two peaks in mass transfer rate occurred for an Ekman number over 0.0074. Alternating spin-up and spin-down with a short period caused the center of the endwall to experience a nearly sinusoidal variation in mass transfer with the frequency equal to the forcing frequency. Near the edge the frequency was twice the forcing frequency.

  19. Formation of columnar baroclinic vortices in thermally stratified nonlinear spin-up

    NARCIS (Netherlands)

    Pacheco, J.R.; Verzicco, R.

    2012-01-01

    We investigate the mechanisms that affect the formation of columnar vortices for spin-up in a cylinder where the temperatures at the horizontal walls are prescribed. Numerical results from the three-dimensional Navier–Stokes equations show that a short-lived instability, suppressed by the combined e

  20. Porting marine ecosystem model spin-up using transport matrices to GPUs

    Directory of Open Access Journals (Sweden)

    E. Siewertsen

    2013-01-01

    Full Text Available We have ported an implementation of the spin-up for marine ecosystem models based on transport matrices to graphics processing units (GPUs. The original implementation was designed for distributed-memory architectures and uses the Portable, Extensible Toolkit for Scientific Computation (PETSc library that is based on the Message Passing Interface (MPI standard. The spin-up computes a steady seasonal cycle of ecosystem tracers with climatological ocean circulation data as forcing. Since the transport is linear with respect to the tracers, the resulting operator is represented by matrices. Each iteration of the spin-up involves two matrix-vector multiplications and the evaluation of the used biogeochemical model. The original code was written in C and Fortran. On the GPU, we use the Compute Unified Device Architecture (CUDA standard, a customized version of PETSc and a commercial CUDA Fortran compiler. We describe the extensions to PETSc and the modifications of the original C and Fortran codes that had to be done. Here we make use of freely available libraries for the GPU. We analyze the computational effort of the main parts of the spin-up for two exemplar ecosystem models and compare the overall computational time to those necessary on different CPUs. The results show that a consumer GPU can compete with a significant number of cluster CPUs without further code optimization.

  1. Porting marine ecosystem model spin-up using transport matrices to GPUs

    Directory of Open Access Journals (Sweden)

    E. Siewertsen

    2012-07-01

    Full Text Available We have ported an implementation of the spin-up for marine ecosystem models based on the "Transport Matrix Method" to graphics processing units (GPUs. The original implementation was designed for distributed-memory architectures and uses the PETSc library that is based on the "Message Passing Interface (MPI" standard. The spin-up computes a steady seasonal cycle of the ecosystem tracers with climatological ocean circulation data as forcing. Since the transport is linear with respect to the tracers, the resulting operator is represented in so-called "transport matrices". Each iteration of the spin-up involves two matrix-vector multiplications and the evaluation of the used biogeochemical model. The original code was written in C and Fortran. On the GPU, we use the CUDA standard, a specialized version of the PETSc toolkit and a CUDA Fortran compiler. We describe the extensions to PETSc and the modifications of the original C and Fortran codes that had to be done. Here we make use of freely available libraries for the GPU. We analyze the computational effort of the main parts of the spin-up for two exemplary ecosystem models and compare the overall computational time to those necessary on different CPUs. The results show that a consumer GPU can beat a significant number of cluster CPUs without further code optimization.

  2. Reducing spin-up time for DNS and LES of turbulent channel flow

    Science.gov (United States)

    Nelson, Kurt; Fringer, Oliver

    2016-11-01

    In DNS or LES of turbulent channel flow, significant computational resources are wasted on simulation of flow evolution as it approaches statistical equilibrium. Although the driving pressure gradient that produces the desired time-averaged bottom stress is known a-priori, during flow spin-up this pressure gradient is typically not in balance with the time-averaged bottom stress, leading to flow acceleration beyond the target velocity which can significantly prolong the time to reach statistical equilibrium. Through DNS of turbulent channel flow with Reτ = 500, we present a method that ensures a time invariant volume-averaged streamwise velocity. While the method eliminates spin-up time related to approaching the target volume-averaged velocity, spin-up time is still needed for the turbulence to reach statistical equilibrium. To this end, we study the evolution of the turbulence in response to different initial velocity profiles and initial random perturbations and show that initialization with a laminar velocity profile significantly reduces spin-up time because the linear distribution of vertical shear triggers turbulence faster than it would with a log-law velocity profile. We gratefully acknowledge ONR Grant N00014-15-1-2287.

  3. Magnetized accretion

    Science.gov (United States)

    Heyvaerts, J.

    This lecture reviews in simple terms the general subject of large scale magnetic field coupling to plasma flows in the vicinity of accreting compact stars. The relevant astrophysical phenomenology is summarized. Disk interaction with the magnetosphere of accreting stars is first discussed, in particular the structure of the magnetopause, its stability and plasma ejection in so-called propeller systems. The physics of accretion/ejection is then considered. Acceleration and focusing mechanisms of jets from accretion disks around compact stars or black holes and the question of the self-consistency of accretion and ejection are described. By contrast, small scale MHD turbulence in disks is not discussed, neither are accretion columns near the polar caps of neutron stars or white dwarfs. The reader is only assumed to have some basic knowledge of astrophysics and of fluid mechanics and electromagnetism.

  4. Spin Period Evolution of Recycled Pulsar in Accreting Binary

    CERN Document Server

    Wang, J; Zhao, Y H; Kojima, Y; Yin, H X; SOng, L M

    2010-01-01

    We investigate the spin-period evolutions of recycled pulsars in binary accreting systems. Taking both the accretion induced field decay and spin-up into consideration, we calculate their spin-period evolutions influenced by the initial magnetic-field strengths, initial spin-periods and accretion rates, respectively. The results indicate that the minimum spin-period (or maximum spin frequency) of millisecond pulsar (MSP) is independent of the initial conditions and accretion rate when the neutron star (NS) accretes $\\sim> 0.2\\ms$. The accretion torque with the fastness parameter and gravitational wave (GW) radiation torque may be responsible for the formation of the minimum spin-period (maximum spin frequency). The fastest spin frequency (716 Hz) of MSP can be inferred to associate with a critical fastness parameter about $\\omega_{c}=0.55$. Furthermore, the comparisons with the observational data are presented in the field-period ($B-P$) diagram.

  5. Do we see accreting magnetars in X-ray pulsars?

    Directory of Open Access Journals (Sweden)

    Postnov K.A.

    2014-01-01

    Full Text Available Strong magnetic field of accreting neutron stars (1014 G is hard to probe by Xray spectroscopy but can be indirectly inferred from spin-up/spin-down measurement in X-ray pulsars. The existing observations of slowly rotating X-ray pulsars are discussed. It is shown that magnetic fields of neutron stars derived from these observations (or lower limits in some cases fall within the standard 1012-1013 G range. Claims about the evidence for accreting magnetars are critically discussed in the light of recent progress in understanding of accretion onto slowly rotating neutron stars in the subsonic regime.

  6. Hot-spot model for accretion disc variability as random process - II. Mathematics of the power-spectrum break frequency

    CERN Document Server

    Pechacek, Tomas; Karas, Vladimir; Czerny, Bozena; Dovciak, Michal

    2013-01-01

    We study some general properties of accretion disc variability in the context of stationary random processes. In particular, we are interested in mathematical constraints that can be imposed on the functional form of the Fourier power-spectrum density (PSD) that exhibits a multiply broken shape and several local maxima. We develop a methodology for determining the regions of the model parameter space that can in principle reproduce a PSD shape with a given number and position of local peaks and breaks of the PSD slope. Given the vast space of possible parameters, it is an important requirement that the method is fast in estimating the PSD shape for a given parameter set of the model. We generated and discuss the theoretical PSD profiles of a shot-noise-type random process with exponentially decaying flares. Then we determined conditions under which one, two, or more breaks or local maxima occur in the PSD. We calculated positions of these features and determined the changing slope of the model PSD. Furthermor...

  7. Streaks to Rings to Vortex Grids: Generic Patterns in Transient Convective Spin-Up

    CERN Document Server

    Zhong, Jin-Qiang; Wettlaufer, J S

    2010-01-01

    We observe the transient formation of a ringed pattern state during spin-up of an evaporating fluid on a time scale of order a few Ekman spin-up times. The ringed state is quantified using infrared thermometry and particle image velocimetry and it is demonstrated to be a consequence of the temporary balance between Coriolis and viscous forces which dominate inertia, each of which are extracted from the velocity field. The breakdown of the ringed state is quantified in terms of antiphasing of these force components driving a Kelvin-Helmholtz instability and we show that the resulting vortex grid spacing scales with the ring wavelength. This transient route to quasi-two dimensional turbulent vortex flow has implications across a broad range of fields, but particularly in astrophysics and geophysics.

  8. Technical Note: Reducing the spin-up time of integrated surface water–groundwater models

    KAUST Repository

    Ajami, H.

    2014-12-12

    One of the main challenges in the application of coupled or integrated hydrologic models is specifying a catchment\\'s initial conditions in terms of soil moisture and depth-to-water table (DTWT) distributions. One approach to reducing uncertainty in model initialization is to run the model recursively using either a single year or multiple years of forcing data until the system equilibrates with respect to state and diagnostic variables. However, such "spin-up" approaches often require many years of simulations, making them computationally intensive. In this study, a new hybrid approach was developed to reduce the computational burden of the spin-up procedure by using a combination of model simulations and an empirical DTWT function. The methodology is examined across two distinct catchments located in a temperate region of Denmark and a semi-arid region of Australia. Our results illustrate that the hybrid approach reduced the spin-up period required for an integrated groundwater–surface water–land surface model (ParFlow.CLM) by up to 50%. To generalize results to different climate and catchment conditions, we outline a methodology that is applicable to other coupled or integrated modeling frameworks when initialization from an equilibrium state is required.

  9. Technical Note: Reducing the spin-up time of integrated surface water–groundwater models

    KAUST Repository

    Ajami, H.

    2014-06-26

    One of the main challenges in catchment scale application of coupled/integrated hydrologic models is specifying a catchment\\'s initial conditions in terms of soil moisture and depth to water table (DTWT) distributions. One approach to reduce uncertainty in model initialization is to run the model recursively using a single or multiple years of forcing data until the system equilibrates with respect to state and diagnostic variables. However, such "spin-up" approaches often require many years of simulations, making them computationally intensive. In this study, a new hybrid approach was developed to reduce the computational burden of spin-up time for an integrated groundwater-surface water-land surface model (ParFlow.CLM) by using a combination of ParFlow.CLM simulations and an empirical DTWT function. The methodology is examined in two catchments located in the temperate and semi-arid regions of Denmark and Australia respectively. Our results illustrate that the hybrid approach reduced the spin-up time required by ParFlow.CLM by up to 50%, and we outline a methodology that is applicable to other coupled/integrated modelling frameworks when initialization from equilibrium state is required.

  10. Technical Note: Reducing the spin-up time of integrated surface water–groundwater models

    Directory of Open Access Journals (Sweden)

    H. Ajami

    2014-06-01

    Full Text Available One of the main challenges in catchment scale application of coupled/integrated hydrologic models is specifying a catchment's initial conditions in terms of soil moisture and depth to water table (DTWT distributions. One approach to reduce uncertainty in model initialization is to run the model recursively using a single or multiple years of forcing data until the system equilibrates with respect to state and diagnostic variables. However, such "spin-up" approaches often require many years of simulations, making them computationally intensive. In this study, a new hybrid approach was developed to reduce the computational burden of spin-up time for an integrated groundwater-surface water-land surface model (ParFlow.CLM by using a combination of ParFlow.CLM simulations and an empirical DTWT function. The methodology is examined in two catchments located in the temperate and semi-arid regions of Denmark and Australia respectively. Our results illustrate that the hybrid approach reduced the spin-up time required by ParFlow.CLM by up to 50%, and we outline a methodology that is applicable to other coupled/integrated modelling frameworks when initialization from equilibrium state is required.

  11. CHANDRA Observations of V407 Vul: Confirmation of the Spin-up

    CERN Document Server

    Strohmayer, T E

    2004-01-01

    V407 Vul is a candidate double-degenerate binary with a putative 1.756 mHz (9.5 min) orbital frequency. In a previous timing study using archival ROSAT and ASCA data we reported evidence for an increase of this frequency at a rate consistent with expectations for gravitational radiation from a detached ultracompact binary system. Here we report the results of new CHANDRA timing observations which confirm the previous indications of spin-up of the X-ray frequency, and provide much tighter constraints on the frequency derivative. We obtained with CHANDRA a total of 90 ksec of exposure in two epochs separated in time by 11.5 months. The total time span of the archival ROSAT, ASCA and new CHANDRA data is now 10.5 years. This more than doubles the interval spanned by the ROSAT and ASCA data alone, providing much greater sensitivity to a frequency derivative. With the addition of the CHANDRA data an increasing frequency is unavoidable, with a value df/dt = 7.0 x 10-18 Hz/s. Although a long-term spin-up trend is con...

  12. Spin-up, adjustment and equilibrium state of the Antarctic Circumpolar Current

    Science.gov (United States)

    Johnson, H. L.; Allison, L. C.; Marshall, D. P.; Munday, D. R.

    2009-04-01

    An analytical model will be presented for the spin-up, adjustment and equilibrium state of the Antarctic Circumpolar Current (ACC) in a reduced-gravity ocean, forced by wind stress, buoyancy forcing and diapycnal mixing. The initial spin-up from a shallow pycnocline (consistent with Sandstrom's theorem) is controlled by diapycnal mixing, but the eventual equilibration/adjustment time-scale is controlled by the Gent and McWilliams eddy transfer coefficient. This has important implications for climate model studies, which greatly underestimate the sensitivity of eddy fluxes to changes in the mean strength of the ACC. We also find that diapycnal mixing plays an important role in setting the mean strength of the ACC, in particular in the limit of strong diapycnal mixing (10-4 m2 s-1). This is for precisely the same reasons that diapycnal mixing may be important in setting the strength of the Atlantic Meridional Overturning Circulation (AMOC). The theoretical predictions are compared with the results of numerical calculations carried out with the MITgcm, in a global configuration with parameterized eddies, and in an idealized basin/re-entrant channel configuration with partially resolved eddies. The relationship between ACC adjustment, the global pycnocline and the AMOC, as well as the possibility of exciting seiching modes between the different basins, will also be discussed.

  13. SPIN-UP and Preparing Undergraduate Physics Majors for Careers in Industry

    Science.gov (United States)

    Howes, Ruth

    2011-03-01

    Seven years ago, the Strategic Programs for Innovations in Undergraduate Physics (SPIN-UP) Report produced by the National Task Force on Undergraduate Physics identified several key characteristics of thriving undergraduate physics departments including steps these departments had taken to prepare students better for careers in industry. Today statistical data from AIP shows that almost 40% of students graduating with a degree in physics seek employment as soon as they graduate. Successful undergraduate physics programs have taken steps to adapt their rigorous physics programs to ensure that graduating seniors have the skills they need to enter the industrial workplace as well as to go on to graduate school in physics. Typical strategies noted during a series of SPIN-UP workshops funded by a grant from NSF to APS, AAPT, and AIP include flexible curricula, early introduction of undergraduates to research techniques, revised laboratory experiences that provide students with skills they need to move directly into jobs, and increased emphasis on ``soft'' skills such as communication and team work. Despite significant success, undergraduate programs face continuing challenges in preparing students to work in industry, most significantly the fact that there is no job called ``physicist'' at the undergraduate level. supported by grant NSF DUE-0741560.

  14. Physical Environment of Accreting Neutron Stars

    Directory of Open Access Journals (Sweden)

    J. Wang

    2016-01-01

    Full Text Available Neutron stars (NSs powered by accretion, which are known as accretion-powered NSs, always are located in binary systems and manifest themselves as X-ray sources. Physical processes taking place during the accretion of material from their companions form a challenging and appealing topic, because of the strong magnetic field of NSs. In this paper, we review the physical process of accretion onto magnetized NS in X-ray binary systems. We, firstly, give an introduction to accretion-powered NSs and review the accretion mechanism in X-ray binaries. This review is mostly focused on accretion-induced evolution of NSs, which includes scenario of NSs both in high-mass binaries and in low-mass systems.

  15. Spin-up behavior and effects of initial conditions for an integrated hydrologic model

    Science.gov (United States)

    Seck, Alimatou; Welty, Claire; Maxwell, Reed M.

    2015-04-01

    Initial conditions have been shown to have a strong effect on outputs of surface water models, but their impact on integrated hydrologic models is not well documented. We investigated the effects of initial conditions on an integrated hydrologic model of a 5632 km2 domain in the northeastern U.S. Simulations were run for the year 1980 using four initial conditions spanning a range of average depth to water table, including 1 m ("wet"), 3m, 5m, and 7 m ("dry") below land surface. Model outputs showed significant effects of initial conditions on basin-averaged variables such as subsurface storage, surface storage, and surface runoff, with the greatest impact observed on surface storage and runoff. Effects of initial conditions were related to meteorological conditions, with precipitation reducing the effects of initial conditions on surface storage and runoff. Additionally, feedbacks between soil moisture and land-energy fluxes affected the impacts of initial conditions: higher temperatures magnified the differences in storage, recharge, and discharge among the four initial-condition scenarios. Ten year recursive runs were conducted for the wet and dry scenarios. Spin-up times varied by model components and were considerably smaller for land-surface states and fluxes. Spin-up for dry initial conditions was slower than for wet initial conditions, indicating longer system memory for dry initial conditions. These variations in persistence of initial conditions should be taken into consideration when designing model initialization approaches. More broadly, this behavior is indicative of increased persistence of the effects of dry years as opposed to wet years in hydrologic systems.

  16. NuSTAR discovery of a cyclotron line in the accreting X-ray pulsar IGR J16393-4643

    CERN Document Server

    Bodaghee, Arash; Fornasini, Francesca A; Krivonos, Roman; Stern, Daniel; Mori, Kaya; Rahoui, Farid; Boggs, Steven E; Christensen, Finn E; Craig, William W; Hailey, Charles J; Harrison, Fiona A; Zhang, William W

    2016-01-01

    The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50 ks. We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3) keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s per year, or a frequency derivative of nu' = 3e-14 Hz/s ). In the power density spectrum, a break appears at the pulse frequency which separates the zero slope at low frequency from the steeper slope at high frequency. This addition of angular momentum to the neutron star could be due to the accretion of a quasi-spherical wind, or it could be caused by the transient appearance of a prograde accretion disk that is nearly i...

  17. Accretion Discs in Blazars

    OpenAIRE

    Jolley, E. J. D.; Kuncic, Z.; Bicknell, G. V.; Wagner, S.(Max-Planck-Institut für Kernphysik, 69117, Heidelberg, Germany)

    2009-01-01

    The characteristic properties of blazars (rapid variability, strong polarization, high brightness) are widely attributed to a powerful relativistic jet oriented close to our line of sight. Despite the spectral energy distributions (SEDs) being strongly jet-dominated, a "big blue bump" has been recently detected in sources known as flat spectrum radio quasars (FSRQs). These new data provide a unique opportunity to observationally test coupled jet-disc accretion models in these extreme sources....

  18. The Orbital Period of the Accreting Pulsar GX1+4

    CERN Document Server

    Pereira, M G; Jablonski, F J; Pereira, Marildo G.; Braga, Joao; Jablonski, Francisco J.

    1999-01-01

    We report strong evidence for a ~304-day periodicity in the spin history of the accretion-powered pulsar GX1+4 that is most probably associated with the orbital period of the system. We have used data from the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory to show a clear periodic modulation of the pulsar frequency from 1991 to date, in excellent agreement with the ephemeris proposed by Cutler, Dennis & Dolan (1986). Our results indicate that the orbital period of GX1+4 is 303.8 +- 1.1 days, making it the widest known low-mass X-ray binary system by more than one order of magnitude and putting this long-standing question to rest. A likely scenario for this system is an elliptical orbit in which the neutron star decreases its spin-down rate (or even exhibits a momentary spin-up behavior) at periastron passages due to the higher torque exerted by the accretion disk onto the magnetosphere of the neutron star. These results are not inconsistent with both the X-ray pulsed flux light...

  19. Nanosatellite spin-up using magnetic actuators: ESTCube-1 flight results

    Science.gov (United States)

    Ehrpais, Hendrik; Kütt, Johan; Sünter, Indrek; Kulu, Erik; Slavinskis, Andris; Noorma, Mart

    2016-11-01

    This paper presents the in-orbit performance of the ESTCube-1 attitude control system that used electromagnetic actuators to achieve a high angular velocity. ESTCube-1 is a one-unit CubeSat that aimed to perform the first electric solar wind sail experiment. The attitude control system was designed to provide enough centrifugal force by spinning up the satellite to deploy a 10 m long tether. The required spin rate was a minimum of one rotation per second. The actuators used were three electromagnetic coils, each able to produce a magnetic moment of up to 0.1 A m2. In this paper, we describe the design of the attitude control system, implementation of the spin controller and the in-orbit performance of the system. In addition we describe the effect that a residual magnetic moment had on the attitude control of the satellite and the measures taken to overcome this issue. During testing of the satellite, ESTCube-1 achieved the highest known spin rate of 841°/s for small scale satellites. The satellite ended its operations on the 19th of May, 2015 after 2 years in orbit.

  20. HATS-18 b: An Extreme Short--Period Massive Transiting Planet Spinning Up Its Star

    CERN Document Server

    Penev, Dr Kaloyan M; Bakos, Dr Gaspar A; Ciceri, Ms Simona; Brahm, Dr Rafael; Bayliss, Dr Daniel; Bento, Joao; Jord'an, Andr'es; Csubry, Mr Zoltan; Bhatti, W; de Val-Borro, Miguel; Espinoza, Mr Néstor; Zhou, Dr George; Mancini, Dr Luigi; Rabus, Dr Markus; Suc, Vincent; Henning, Thomas; Schmidt, Prof Brian P; Noyes, Dr Robert W; L'az'ar, J; Papp, Istvan; S'ari, P

    2016-01-01

    We report the discovery by the HATSouth network of HATS-18 b: a 1.980 +/- 0.077 Mj, 1.337 +0.102 -0.049 Rj planet in a 0.8378 day orbit, around a solar analog star (mass 1.037 +/- 0.047 Msun, and radius 1.020 +0.057 -0.031 Rsun) with V=14.067 +/- 0.040 mag. The high planet mass, combined with its short orbital period, implies strong tidal coupling between the planetary orbit and the star. In fact, given its inferred age, HATS-18 shows evidence of significant tidal spin up, which together with WASP-19 (a very similar system) allows us to constrain the tidal quality factor for Sun-like stars to be in the range 6.5 <= lg(Q*/k_2) <= 7 even after allowing for extremely pessimistic model uncertainties. In addition, the HATS-18 system is among the best systems (and often the best system) for testing a multitude of star--planet interactions, be they gravitational, magnetic or radiative, as well as planet formation and migration theories.

  1. Empirical evidence for tidal spin-up in transiting planetary systems

    CERN Document Server

    Pont, Frederic

    2008-01-01

    Most transiting planets orbit very close to their parent star, causing strong tidal forces between the two bodies. Tidal interaction can modify the dynamics of the system through orbital alignment, circularisation and synchronisation, and orbital decay by exchange of angular moment. Evidence for tidal circularisation in close-in giant planet is well-known. Here we review the evidence for tidal spin-up of the parent stars due to the pull of tidal forces towards spin-orbit synchronisation. We find suggestive empirical evidence for such a process in the present sample of transiting planetary systems. The corresponding angular momentum exchange would imply that some planets have spiralled towards their star by substantial amounts since the dissipation of the protoplanetary disc. We show that this could quantitatively account for the observed mass-period relation of close-in gas giants. Finally, we discuss how this scenario can be further tested and point out some consequences for theoretical studies of tidal inte...

  2. Failure modes and conditions of a cohesive, spherical body due to YORP spin-up

    CERN Document Server

    Hirabayashi, Masatoshi

    2015-01-01

    This paper presents transition of the failure mode of a cohesive, spherical body due to YORP spin-up. On the assumption that the distribution of materials in the body is homogeneous, failed regions first appearing in the body at different spin rates are predicted by comparing the yield condition of an elastic stress in the body. It is found that as the spin rate increases, the locations of the failed regions move from the equatorial surface to the central region. To avoid such failure modes, the body should have higher cohesive strength. The results by this model are consistent with those by a plastic finite element model. Then, this model and a two-layered-cohesive model first proposed by Hirabayashi et al. are used to classify possible evolution and disruption of a spherical body. There are three possible pathways to disruption. First, because of a strong structure, failure of the central region is dominant and eventually leads to a breakup into multiple components. Second, a weak surface and a weak interio...

  3. HATS-18b: An Extreme Short-period Massive Transiting Planet Spinning Up Its Star

    Science.gov (United States)

    Penev, K.; Hartman, J. D.; Bakos, G. Á.; Ciceri, S.; Brahm, R.; Bayliss, D.; Bento, J.; Jordán, A.; Csubry, Z.; Bhatti, W.; de Val-Borro, M.; Espinoza, N.; Zhou, G.; Mancini, L.; Rabus, M.; Suc, V.; Henning, T.; Schmidt, B.; Noyes, R. W.; Lázár, J.; Papp, I.; Sári, P.

    2016-11-01

    We report the discovery by the HATSouth network of HATS-18b: a 1.980+/- 0.077 {M}{{J}}, {1.337}-0.049+0.102 {R}{{J}} planet in a 0.8378 day orbit, around a solar analog star (mass 1.037+/- 0.047 {M}⊙ and radius {1.020}-0.031+0.057 {R}⊙ ) with V=14.067+/- 0.040 mag. The high planet mass, combined with its short orbital period, implies strong tidal coupling between the planetary orbit and the star. In fact, given its inferred age, HATS-18 shows evidence of significant tidal spin up, which together with WASP-19 (a very similar system) allows us to constrain the tidal quality factor for Sun-like stars to be in the range of 6.5≲ {{log}}10({Q}* /{k}2)≲ 7 even after allowing for extremely pessimistic model uncertainties. In addition, the HATS-18 system is among the best systems (and often the best system) for testing a multitude of star-planet interactions, be they gravitational, magnetic, or radiative, as well as planet formation and migration theories. The HATSouth network is operated by a collaboration consisting of Princeton University (PU), the Max Planck Institute für Astronomie (MPIA), the Australian National University (ANU), and the Pontificia Universidad Católica de Chile (PUC). The station at Las Campanas Observatory (LCO) of the Carnegie Institute is operated by PU in conjunction with PUC, the station at the High Energy Spectroscopic Survey (H.E.S.S.) site is operated in conjunction with MPIA, and the station at Siding Spring Observatory (SSO) is operated jointly with ANU. This paper includes data gathered with the MPG 2.2 m telescope at the ESO Observatory in La Silla. This paper uses observations obtained with facilities of the Las Cumbres Observatory Global Telescope.

  4. Extremes of the jet-accretion power relation of blazars, as explored by NuSTAR

    DEFF Research Database (Denmark)

    Sbarrato, T.; Ghisellini, G.; Tagliaferri, G.

    2016-01-01

    Hard X-ray observations are crucial to study the non-thermal jet emission from high-redshift, powerful blazars. We observed two bright z > 2 flat-spectrum radio quasars (FSRQs) in hard X-rays to explore the details of their relativistic jets and their possible variability. S5 0014+81 (at z = 3.36...

  5. Measurements accounting for the impediment of ion spin-up in rotating magnetic field driven field reversed configurations

    Energy Technology Data Exchange (ETDEWEB)

    Deards, C. L. [Lockheed Martin, 1011 Lockheed Way, Palmdale, California 93599 (United States); Hoffman, A. L.; Steinhauer, L. C. [Redmond Plasma Physics Laboratory, University of Washington, 14700 NE 95th Street, Suite 100, Redmond, Washington 98052 (United States)

    2011-11-15

    Improved vacuum hygiene, wall conditioning, and reduced recycling in the rotating magnetic field (RMF) driven translation, confinement, and sustainment-upgrade (TCSU) field reversed configuration experiment have made possible a more accurate assessment of the forces affecting ion spin-up. This issue is critical in plasmas sustained by RMFs, such as TCSU since ion spin-up can substantially reduce or cancel the RMF current drive effect. Several diagnostics are brought to bear, including a 3-axis translatable magnetic probe allowing the first experimental measurement of the end shorting effect. These results show that the ion rotation is determined by a balance between electron-ion friction, the end shorting effect, and ion drag against neutrals.

  6. Measurements accounting for the impediment of ion spin-up in rotating magnetic field driven field reversed configurations

    Science.gov (United States)

    Deards, C. L.; Hoffman, A. L.; Steinhauer, L. C.

    2011-11-01

    Improved vacuum hygiene, wall conditioning, and reduced recycling in the rotating magnetic field (RMF) driven translation, confinement, and sustainment-upgrade (TCSU) field reversed configuration experiment have made possible a more accurate assessment of the forces affecting ion spin-up. This issue is critical in plasmas sustained by RMFs, such as TCSU since ion spin-up can substantially reduce or cancel the RMF current drive effect. Several diagnostics are brought to bear, including a 3-axis translatable magnetic probe allowing the first experimental measurement of the end shorting effect. These results show that the ion rotation is determined by a balance between electron-ion friction, the end shorting effect, and ion drag against neutrals.

  7. Rejuvenation of the Innocent Bystander: Testing Spin-Up in Dwarf Carbon Stars

    Science.gov (United States)

    Green, Paul

    2013-09-01

    Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dCs are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

  8. Rejuvenation of the Innocent Bystander: Testing Spin-Up in a Dwarf Carbon Star Sample

    Science.gov (United States)

    Green, Paul

    2014-09-01

    Carbon stars (C>O) were long assumed to all be giants, because only AGB stars dredge up significant carbon into their atmospheres. We now know that dwarf carbon (dC) stars are actually far more common than C giants. These dC stars are hypothesized to have accreted C-rich envelope material from an AGB companion, in systems that have likely undergone a planetary nebula phase, eventually yielding a white dwarf and a dC star that has gained both significant mass and angular momentum. To test whether the X-ray emission strength and spectral properties are consistent with a rejuvenated dynamo, we propose a Chandra pilot study of dCs selected from the SDSS; some have hot white dwarf companions (indicating more recent mass transfer), and all show Balmer emission lines (a sign of activity).

  9. The influence of soil type, vegetation cover and soil moisture on spin up behaviour of a land surface model in a monsoonal region

    Science.gov (United States)

    Bhattacharya, Anwesha; Mandal, Manabottam

    2015-04-01

    Model spin-up is the process through which the model is adequately equilibrated to ensure balance between the mass fields and velocity fields. In this study, an offline one dimensional Noah land surface model is integrated recursively for three years to assess its spin-up behavior at different sites over the Indian Monsoon domain. Several numerical experiments are performed to investigate the impact of soil category, vegetation cover, initial soil moisture and subsequent dry or wet condition on model spin-up. These include simulations with the dominant soil and vegetation covers of this region, different initial soil moisture content (observed soil moisture; dry soil; moderately wet soil; saturated soil), simulations initialized at different rain conditions (no rain; infrequent rain; continuous rain) and different seasons (Winter, Spring, Summer/Pre-Monsoon, Monsoon and Autumn). It is seen that the spin-up behavior of the model depends on the soil type and vegetation cover with soil characteristics having the larger influence. Over India, the model has the longest spin-up in the case of simulations with loamy soil covered with mixed-shrub. It is noted that the model has a significantly longer spin-up when initialized with very low initial soil moisture content than with higher soil moisture content. It is also seen that in general, simulations initialized just before a continuous rainfall event have the least spin-up time. This observation is reinforced by the results from the simulations initialized in different seasons. It is seen that for monsoonal region, the model spin-up time is least for simulations initialized just before the Monsoon. Model initialized during the Monsoon rain episodes has a longer spin-up than that initialized in any other season. Furthermore, it is seen that the model has a shorter spin-up if it reaches the equilibrium state predominantly via drying process and could be as low as two months under quasi-equilibrium condition depending on

  10. The impact of soil moisture on the spin up of 1-D Noah land surface model at a site in monsoonal region

    Science.gov (United States)

    Bhattacharya, A.; Mandal, M.

    2014-12-01

    Model spin-up is the process through which the model is adequately equilibrated to ensure balance between the mass fields and velocity fields. In this study, an offline 1-D Noah land surface model (LSM) has been used to investigate the impact of soil moisture on the model spin up at Kharagpur, India which is a site in monsoonal region. The model is integrated recursively for 3-years to assess its spin-up behavior. Several numerical experiments are performed to investigate the impact of initial soil moisture and subsequent dry or wet condition on model spin-up. These include simulations with different initial soil moisture content (observed soil moisture; dry soil; moderately wet soil; saturated soil), simulations initialized before different rain conditions (no rain; infrequent rain; continuous rain) and simulations initialized in different seasons (Winter, Spring, Summer/Pre-Monsoon, Monsoon and Autumn). It is noted that the model has significantly longer spin-up when initialized with very low initial soil moisture content than with higher soil moisture content. It is also seen that in general, simulations initialized just before a continuous rainfall event have the least spin-up time. In a region affected by the monsoon, such as Kharagpur, this observation is reinforced by the results from the simulations initialized in different seasons. It is seen that for monsoonal region, the model spin-up time is least for simulations initialized during Summer/Pre-monsoon. Model initialized during the Monsoon has a longer spin-up than that initialized in any other season. It appears that the model has shorter spin-up if it reaches the equilibrium state predominantly via drying process. It is also observed that the spin-up of offline 1-D Noah LSM may be as low as two months under quasi-equilibrium condition if the initial soil moisture content and time of start of simulations are chosen carefully.

  11. A decade of timing an accretion-powered millisecond pulsar: The continuing spin down and orbital evolution of SAX J1808.4-3658

    CERN Document Server

    Hartman, J M; Chakraborty, D; Markwardt, C B; Morgan, E H; Van der Klis, M; Wijnands, R

    2009-01-01

    The Rossi X-ray Timing Explorer has observed five outbursts from the transient 2.5 ms accretion-powered pulsar SAX J1808.4-3658 during 1998-2008. We present a pulse timing study of the most recent outburst and compare it with the previous timing solutions. The spin frequency of the source continues to decrease at a rate of (-5.5+/-1.2)x10^-18 Hz/s, which is consistent with the previously determined spin derivative. The spin-down occurs mostly during quiescence, and it is most likely due to the magnetic dipole torque from a B = 1.5x10^8 G dipolar field at the neutron star surface. We also find that the 2 hr binary orbital period is increasing at a rate of (3.80+/-0.06)x10^-12 s/s, also consistent with previous measurements. It remains uncertain whether this orbital change reflects secular evolution or short-term variability.

  12. Assessing the impact of model spin-up on surface water-groundwater interactions using an integrated hydrologic model

    KAUST Repository

    Ajami, Hoori

    2014-03-01

    Integrated land surface-groundwater models are valuable tools in simulating the terrestrial hydrologic cycle as a continuous system and exploring the extent of land surface-subsurface interactions from catchment to regional scales. However, the fidelity of model simulations is impacted not only by the vegetation and subsurface parameterizations, but also by the antecedent condition of model state variables, such as the initial soil moisture, depth to groundwater, and ground temperature. In land surface modeling, a given model is often run repeatedly over a single year of forcing data until it reaches an equilibrium state: the point at which there is minimal artificial drift in the model state or prognostic variables (most often the soil moisture). For more complex coupled and integrated systems, where there is an increased computational cost of simulation and the number of variables sensitive to initialization is greater than in traditional uncoupled land surface modeling schemes, the challenge is to minimize the impact of initialization while using the smallest spin-up time possible. In this study, multicriteria analysis was performed to assess the spin-up behavior of the ParFlow.CLM integrated groundwater-surface water-land surface model over a 208 km2 subcatchment of the Ringkobing Fjord catchment in Denmark. Various measures of spin-up performance were computed for model state variables such as the soil moisture and groundwater storage, as well as for diagnostic variables such as the latent and sensible heat fluxes. The impacts of initial conditions on surface water-groundwater interactions were then explored. Our analysis illustrates that the determination of an equilibrium state depends strongly on the variable and performance measure used. Choosing an improper initialization of the model can generate simulations that lead to a misinterpretation of land surface-subsurface feedback processes and result in large biases in simulated discharge. Estimated spin-up

  13. Atmospheric Ice Accretion Measurement Techniques

    Directory of Open Access Journals (Sweden)

    M Virk

    2016-09-01

    Full Text Available Atmospheric icing on structures has proven to be an area of concern in many cold climate geographical regions like arctic and alpine. Difficulties encountered by the communication, construction and power industries in these areas are the subject of intense investigations for researchers from decades. Three main methods of investigation are generally employed by researchers to study atmospheric ice accretion on structures: a continuous field measurements, b lab based simulations using icing wind tunnel & c numerical modelling. This paper presents a brief review study of various techniques to understand and measure the atmospheric ice accretion on structures, anti/de icing techniques and important parameters for numerical modelling of atmospheric ice accretion.

  14. Spatial spin-up of fine scales in a regional climate model simulation driven by low-resolution boundary conditions

    Science.gov (United States)

    Matte, Dominic; Laprise, René; Thériault, Julie M.; Lucas-Picher, Philippe

    2017-07-01

    In regional climate modelling, it is well known that domains should be neither too large to avoid a large departure from the driving data, nor too small to provide a sufficient distance from the lateral inflow boundary to allow the full development of the small-scale (SS) features permitted by the finer resolution. Although most practitioners of dynamical downscaling are well aware that the jump of resolution between the lateral boundary condition (LBC) driving data and the nested regional climate model affects the simulated climate, this issue has not been fully investigated. In principle, as the jump of resolution becomes larger, the region of interest in the limited-area domain should be located further away from the lateral inflow boundary to allow the full development of the SS features. A careless choice of domain might result in a suboptimal use of the full finer resolution potential to develop fine-scale features. To address this issue, regional climate model (RCM) simulations using various resolution driving data are compared following the perfect-prognostic Big-Brother protocol. Several experiments were carried out to evaluate the width of the spin-up region (i.e. the distance between the lateral inflow boundary and the domain of interest required for the full development of SS transient eddies) as a function of the RCM and LBC resolutions, as well as the resolution jump. The spin-up distance turns out to be a function of the LBC resolution only, independent of the RCM resolution. When varying the RCM resolution for a given resolution jump, it is found that the spin-up distance corresponds to a fixed number of RCM grid points that is a function of resolution jump only. These findings can serve a useful purpose to guide the choice of domain and RCM configuration for an optimal development of the small scales allowed by the increased resolution of the nested model.

  15. Spatial spin-up of fine scales in a regional climate model simulation driven by low-resolution boundary conditions

    Science.gov (United States)

    Matte, Dominic; Laprise, René; Thériault, Julie M.; Lucas-Picher, Philippe

    2016-09-01

    In regional climate modelling, it is well known that domains should be neither too large to avoid a large departure from the driving data, nor too small to provide a sufficient distance from the lateral inflow boundary to allow the full development of the small-scale (SS) features permitted by the finer resolution. Although most practitioners of dynamical downscaling are well aware that the jump of resolution between the lateral boundary condition (LBC) driving data and the nested regional climate model affects the simulated climate, this issue has not been fully investigated. In principle, as the jump of resolution becomes larger, the region of interest in the limited-area domain should be located further away from the lateral inflow boundary to allow the full development of the SS features. A careless choice of domain might result in a suboptimal use of the full finer resolution potential to develop fine-scale features. To address this issue, regional climate model (RCM) simulations using various resolution driving data are compared following the perfect-prognostic Big-Brother protocol. Several experiments were carried out to evaluate the width of the spin-up region (i.e. the distance between the lateral inflow boundary and the domain of interest required for the full development of SS transient eddies) as a function of the RCM and LBC resolutions, as well as the resolution jump. The spin-up distance turns out to be a function of the LBC resolution only, independent of the RCM resolution. When varying the RCM resolution for a given resolution jump, it is found that the spin-up distance corresponds to a fixed number of RCM grid points that is a function of resolution jump only. These findings can serve a useful purpose to guide the choice of domain and RCM configuration for an optimal development of the small scales allowed by the increased resolution of the nested model.

  16. The Accreting Millisecond X-ray Pulsar IGR J00291+5934: Evidence for a Long Timescale Spin Evolution

    CERN Document Server

    Patruno, Alessandro

    2010-01-01

    Accreting Millisecond X-ray Pulsars like IGR J00291+5934 are important because it is possible to test theories of pulsar formation and evolution. They give also the possibility to constrain gravitational wave emission theories and the equation of state of ultra dense matter. Particularly crucial to our understanding is the measurement of the long term spin evolution of the accreting neutron star. An open question is whether these accreting pulsars are spinning up during an outburst and spinning down in quiescence as predicted by the recycling scenario. Until now it has been very difficult to measure torques, due to the presence of fluctuations in the pulse phases that compromise their measurements with standard coherent timing techniques. By applying a new method, I am now able to measure a spin up during an outburst and a spin down during quiescence. I ascribe the spin up (Fdot=5.1(3)x10^{-13}\\Hz/s) to accretion torques and the spin down (Fdot=-3.0(8)x10^{-15} Hz/s) to magneto dipole torques, as those observ...

  17. Plasma physics of accreting neutron stars

    Science.gov (United States)

    Ghosh, Pranab; Lamb, Frederick K.

    1991-01-01

    Plasma concepts and phenomena that are needed to understand X- and gamma-ray sources are discussed. The capture of material from the wind or from the atmosphere or envelope of a binary companion star is described and the resulting types of accretion flows discussed. The reasons for the formation of a magnetosphere around the neutron star are explained. The qualitative features of the magnetospheres of accreting neutron stars are then described and compared with the qualitative features of the geomagnetosphere. The conditions for stable flow and for angular and linear momentum conservation are explained in the context of accretion by magnetic neutron stars and applied to obtain rough estimates of the scale of the magnetosphere. Accretion from Keplerian disks is then considered in some detail. The radial structure of geometrically thin disk flows, the interaction of disk flows with the neutron star magnetosphere, and models of steady accretion from Keplerian disks are described. Accretion torques and the resulting changes in the spin frequencies of rotating neutron stars are considered. The predicted behavior is then compared with observations of accretion-powered pulsars. Magnetospheric processes that may accelerate particles to very high energies, producing GeV and, perhaps, TeV gamma-rays are discussed. Finally, the mechanisms that decelerate and eventually stop accreting plasma at the surfaces of strongly magnetic neutron stars are described.

  18. Perturbation growth in accreting filaments

    Science.gov (United States)

    Clarke, S. D.; Whitworth, A. P.; Hubber, D. A.

    2016-05-01

    We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long filaments as they form and grow by accretion. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length-scale which is roughly four times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multiwavelength density power spectrum, there exists a clear preferred core separation equal to the largest peak in the dispersion relation. Our results allow one to estimate a minimum age for a filament which is breaking up into regularly spaced fragments, as well as an average accretion rate. We apply the model to observations of filaments in Taurus by Tafalla & Hacar and find accretion rates consistent with those estimated by Palmeirim et al.

  19. Wind accretion in symbiotic X-ray binaries

    CERN Document Server

    Postnov, K; González-Galán, A; Kuulkers, E; Kretschmar, P; Larsson, S; Finger, M H; Kochetkova, A; Lü, G; Yungelson, L

    2011-01-01

    The properties of wind accretion in symbiotic X-ray binaries (SyXBs) consisting of red-giant and magnetized neutron star (NS) are discussed. The spin-up/spin-down torques applied to NS are derived based on a hydrodynamic theory of quasi-spherical accretion onto magnetized NSs. In this model, a settling subsonic accretion proceeds through a hot shell formed around the NS magnetosphere. The accretion rate onto the NS is determined by the ability of the plasma to enter the magnetosphere.Due to large Reynolds numbers in the shell, the interaction of the rotating magnetosphere with plasma initiates a subsonic turbulence. The convective motions are capable of carrying the angular momentum through the shell. We carry out a population synthesis of SyXBs in the Galaxy with account for the spin evolution of magnetized NS. The Galactic number of SyXBs with bright (M_v<1) low-mass red-giant companion is found to be from \\sim 40 to 120, and their birthrate is \\sim 5\\times 10^{-5}-10^{-4} per year. According to our mode...

  20. Can neutron stars have auroras ? : electromagnetic coupling process between neutron star and magnetized accretion disk

    Science.gov (United States)

    Kimura, T.; Iwakiri, W. B.; Enoto, T.; Wada, T.; Tao, C.

    2015-12-01

    In the binary neutron star system, angular momentum transfer from accretion disk to a star is essential process for spin-up/down of stars. The angular momentum transfer has been well formulated for the accretion disk strongly magnetized by the neutron star [e.g., Ghosh and Lamb, 1978, 1979a, b]. However, the electromagnetic (EM) coupling between the neutron star and accretion disk has not been self-consistently solved in the previous studies although the magnetic field lines from the star are strongly tied with the accretion disk. In this study, we applied the planet-magnetosphere coupling process established for Jupiter [Hill, 1979] to the binary neutron star system. Angular momentum distribution is solved based on the torque balance between the neutron star's surface and accretion disk coupled by the magnetic field tensions. We found the EM coupling can transfer significantly larger fraction of the angular momentum from the magnetized accretion disk to the star than the unmagnetized case. The resultant spin-up rate is estimated to ~10^-14 [sec/sec] for the nominal binary system parameters, which is comparable with or larger than the other common spin-down/up processes: e.g., the magnetic dipole radiation spin-down. The Joule heating energy dissipated in the EM coupling is estimated to be up to ~10^36 [erg/sec] for the nominal binary system parameters. The release is comparable to that of gravitation energy directly caused by the matters accreting onto the neutron star. This suggests the EM coupling at the neutron star can accompany the observable radiation as auroras with a similar manner to those at the rotating planetary magnetospheres like Jupiter, Saturn, and other gas giants.

  1. Impact of Spin-up Forcing on Vegetation States Simulated by a Dynamic Global Vegetation Model Coupledwith a Land Surface Model

    Institute of Scientific and Technical Information of China (English)

    LI Fang; ZENG Xiaodong; SONG Xiang; TIAN Dongxiao; SHAO Pu; ZHANG Dongling

    2011-01-01

    A dynamic global vegetation model (DGVM) coupled with a land surface model (LSM) is generally initialized using a spin-up process to derive a physically-consistent initial condition. Spin-up forcing, which is the atmospheric forcing used to drive the coupled model to equilibrium solutions in the spin-up process,varies across earlier studies. In the present study, the impact of the spin-up forcing in the initialization stage on the fractional coverages (FCs) of plant functional type (PFT) in the subsequent simulation stage are assessed in seven classic climate regions by a modified Community Land Model's Dynamic Global Vegetation Model (CLM-DGVM). Results show that the impact of spin-up forcing is considerable in all regions except the tropical rainforest climate region (TR) and the wet temperate climate region (WM). In the tropical monsoon climate region (TM), the TR and TM transition region (TR-TM), the dry temperate climate region (DM), the highland climate region (H), and the boreal forest climate region (BF), where FCs are affected by climate non-negligibly, the discrepancies in initial FCs, which represent long-term cumulative response of vegetation to different climate anomalies, are large. Moreover, the large discrepancies in initial FCs usually decay slowly because there are trees or shrubs in the five regions. The intrinsic growth timescales of FCs for tree PFTs and shrub PFTs are long, and the variation of FCs of tree PFTs or shrub PFTs can affect that of grass PFTs.

  2. Perturbation growth in accreting filaments

    CERN Document Server

    Clarke, Seamus D; Hubber, David A

    2016-01-01

    We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long, initially sub-critical but accreting filaments. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length scale which is roughly 4 times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multi-wavelength density power spectrum there exists a clear preferred core separation equal to the largest peak in the dispe...

  3. Poynting Jets from Accretion Disks

    CERN Document Server

    Lovelace, R V E; Ustyugova, G V; Romanova, M M; Colgate, S A

    2002-01-01

    The powerful narrow jets observed to emanate from many compact accreting objects may arise from the twisting of a magnetic field threading a differentially rotating accretion disk which acts to magnetically extract angular momentum and energy from the disk. Two main regimes have been discussed, {\\it hydromagnetic outflows}, which have a significant mass flux and have energy and angular momentum carried by both the matter and the electromagnetic field and, Poynting outflows, where the mass flux is negligible and energy and angular momentum are carried predominantly by the electromagnetic field. Here we consider a Keplerian disk initially threaded by a dipole-like magnetic field and we present solutions of the force-free Grad-Shafranov equation for the coronal plasma. We find solutions with Poynting jets where there is a continuous outflow of energy and toroidal magnetic flux from the disk into the external space. This behavior contradicts the commonly accepted ``theorem'' of Solar plasma physics that the motio...

  4. Harnessing the full power of the widest Chandra field: average accretion rates of black holes in SDSS galaxies through X-ray stacking

    Science.gov (United States)

    Goulding, Andy D.; Greene, Jenny E.; Hickox, Ryan C.; Alexander, David M.; Forman, William R.; Jones, Christine; Lehmer, Bret

    2017-08-01

    Galaxy-scale bars are expected to provide an effective means for driving material towards the central region in spiral galaxies, and possibly feeding supermassive black holes (BHs). I will present our latest results on a statistically-complete study of the effect of bars on average BH accretion. From a well-selected sample of over 50,000 spiral galaxies extracted from the Sloan Digital Sky Survey, we separate those sources considered to contain galaxy-scale bars from those that do not. Using the first 16 years worth of data taken by the Chandra X-ray Observatory, we identify X-ray luminous AGN and perform the widest-area X-ray stacking analysis to date on the remaining X-ray undetected sources. Through our X-ray stacking, we derive a time-averaged look at accretion for galaxies at fixed stellar mass and star formation rate, finding that the average nuclear accretion rates of galaxies with bar structures are fully consistent with those lacking bars, and robustly concluding that large-scale bars have little or no effect on the average growth of BHs in nearby (z < 0.15) galaxies over gigayear timescales.

  5. Coronal Neutrino Emission in Hypercritical Accretion Flows

    CERN Document Server

    Kawabata, R; Kawanaka, N

    2007-01-01

    Hypercritical accretion flows onto stellar mass black holes (BHs) are commonly considered as a promising model of central engines of gamma-ray bursts (GRBs). In this model a certain fraction of gravitational binding energy of accreting matter is deposited to the energy of relativistic jets via neutrino annihilation and/or magnetic fields. However, some recent studies have indicated that the energy deposition rate by neutrino annihilation is somewhat smaller than that needed to power a GRB. To overcome this difficulty, Ramirez-Ruiz & Socrates (2005) proposed that high energy neutrinos from hot corona above the accretion disk might enhance the efficiency of energy deposition. We elucidate the disk corona model in the context of hypercritical accretion flows. From the energy balance in the disk and the corona, we can calculate the disk and coronal temperature, Td and Tc, and neutrino spectra, taking into account the neutrino cooling processes by neutrino-electron scatterings and neutrino pair productions. Th...

  6. A semi-analytical solution to accelerate spin-up of a coupled carbon and nitrogen land model to steady state

    Directory of Open Access Journals (Sweden)

    J. Xia

    2012-04-01

    Full Text Available The spin-up of land models to steady state of coupled carbon-nitrogen processes is computationally so costly that it becomes a~bottleneck issue for global analysis. In this study, we introduced a semi-analytical solution (SAS for the spin-up issue. SAS is fundamentally based on the analytic solution to a set of equations that describe carbon transfers within ecosystems over time. SAS is implemented by three steps: (1 having an initial spin-up with prior pool-size values until net primary productivity (NPP reaches steady state, (2 calculating quasi steady-state pool sizes by letting fluxes of the equations equal zero, and (3 having a final spin-up to meet the criterion of steady state. Step 2 is enabled by averaged time-varying variables over one period of repeated driving forcings. SAS was applied to both site-level and global scale spin-up of the Australian Community Atmosphere Biosphere Land Exchange (CABLE model. For the carbon-cycle-only simulations, SAS saved 95.7% and 92.4% of computational time for site-level and global spin-up, respectively, in comparison with the traditional method. For the carbon-nitrogen-coupled simulations, SAS reduced computational cost by 84.5% and 86.6% for site-level and global spin-up, respectively. The estimated steady-state pool sizes represent the ecosystem carbon storage capacity, which was 12.1 kg C m−2 with the coupled carbon-nitrogen global model, 14.6% lower than that with the carbon-only model. The nitrogen down-regulation in modeled carbon storage is partly due to the 4.6% decrease in carbon influx (i.e., net primary productivity and partly due to the 10.5% reduction in residence times. This steady-state analysis accelerated by the SAS method can facilitate comparative studies of structural differences in determining the ecosystem carbon storage capacity among biogeochemical models. Overall, the computational efficiency of SAS potentially permits many global analyses that are impossible

  7. Chapter 31 Sensitivity and spin-up times of cohesive sediment transport models used to simulate bathymetric change

    Science.gov (United States)

    Schoellhamer, D.H.; Ganju, N.K.; Mineart, P.R.; Lionberger, M.A.; ,

    2008-01-01

    box model was calibrated to bathymetric change data and shows rapidly evolving bathymetry in the first 10-20 years, though sediment supply and hydrodynamic forcing did not vary greatly. This initial burst of bathymetric change is believed to be model adjustment to initial conditions, and suggests a spin-up time of greater than 10 years. These three diverse modeling approaches reinforce the sensitivity of cohesive sediment transport models to initial conditions and model parameters, and highlight the importance of appropriate calibration data. Adequate spin-up time of the order of years is required to initialize models, otherwise the solution will contain bathymetric change that is not due to environmental forcings, but rather improper specification of initial conditions and model parameters. Temporally intensive bathymetric change data can assist in determining initial conditions and parameters, provided they are available. Computational effort may be reduced by selectively updating hydrodynamics and bathymetry, thereby allowing time for spin-up periods. ?? 2008 Elsevier B.V. All rights reserved.

  8. Magnetically controlled accretion onto a black hole

    CERN Document Server

    Ikhsanov, N R; Beskrovnaya, N G; 10.1088/1742-6596/372/1/012062

    2012-01-01

    An accretion scenario in which the material captured by a black hole from its environment is assumed to be magnetized (\\beta ~ 1) is discussed. We show that the accretion picture in this case is strongly affected by the magnetic field of the flow itself. The accretion power within this Magnetically Controlled Accretion (MCA) scenario is converted predominantly into the magnetic energy of the accretion flow. The rapidly amplified field prevents the accretion flow from forming a homogeneous Keplerian disk. Instead, the flow is decelerated by its own magnetic field at a large distance (Shvartsman radius) from the black hole and switches into a non-Keplerian dense magnetized slab. The material in the slab is confined by the magnetic field and moves towards the black hole on the time scale of the magnetic field annihilation. The basic parameters of the slab are evaluated. Interchange instabilities in the slab may lead to a formation of Z-pinch type configuration of the magnetic field over the slab in which the acc...

  9. Spin-Up and Tuning of the Global Carbon Cycle Model Inside the GISS ModelE2 GCM

    Science.gov (United States)

    Aleinov, Igor; Kiang, Nancy Y.; Romanou, Anastasia

    2015-01-01

    Planetary carbon cycle involves multiple phenomena, acting at variety of temporal and spacial scales. The typical times range from minutes for leaf stomata physiology to centuries for passive soil carbon pools and deep ocean layers. So, finding a satisfactory equilibrium state becomes a challenging and computationally expensive task. Here we present the spin-up processes for different configurations of the GISS Carbon Cycle model from the model forced with MODIS observed Leaf Area Index (LAI) and prescribed ocean to the prognostic LAI and to the model fully coupled to the dynamic ocean and ocean biology. We investigate the time it takes the model to reach the equilibrium and discuss the ways to speed up this process. NASA Goddard Institute for Space Studies General Circulation Model (GISS ModelE2) is currently equipped with all major algorithms necessary for the simulation of the Global Carbon Cycle. The terrestrial part is presented by Ent Terrestrial Biosphere Model (Ent TBM), which includes leaf biophysics, prognostic phenology and soil biogeochemistry module (based on Carnegie-Ames-Stanford model). The ocean part is based on the NASA Ocean Biogeochemistry Model (NOBM). The transport of atmospheric CO2 is performed by the atmospheric part of ModelE2, which employs quadratic upstream algorithm for this purpose.

  10. Black hole accretion discs

    CERN Document Server

    Lasota, Jean-Pierre

    2015-01-01

    This is an introduction to models of accretion discs around black holes. After a presentation of the non-relativistic equations describing the structure and evolution of geometrically thin accretion discs we discuss their steady-state solutions and compare them to observation. Next we describe in detail the thermal-viscous disc instability model and its application to dwarf novae for which it was designed and its X-ray irradiated-disc version which explains the soft X--ray transients, i.e. outbursting black-hole low-mass X-ray binaries. We then turn to the role of advection in accretion flow onto black holes illustrating its action and importance with a toy model describing both ADAFs and slim discs. We conclude with a presentation of the general-relativistic formalism describing accretion discs in the Kerr space-time.

  11. The application of Jacobian-free Newton–Krylov methods to reduce the spin-up time of ocean general circulation models

    NARCIS (Netherlands)

    Bernsen, Erik; Dijkstra, Henk A.; Thies, Jonas; Wubs, Fred W.

    2010-01-01

    In present-day forward time stepping ocean-climate models, capturing both the wind-driven and thermohaline components, a substantial amount of CPU time is needed in a so-called spin-up simulation to determine an equilibrium solution. In this paper, we present methodology based on Jacobian-Free

  12. The application of Jacobian-free Newton-Krylov methods to reduce the spin-up time of ocean general circulation models

    NARCIS (Netherlands)

    Bernsen, Erik; Dijkstra, Henk A.; Thies, Jonas; Wubs, Fred W.

    2010-01-01

    In present-day forward time stepping ocean-climate models, capturing both the wind-driven and thermohaline components, a substantial amount of CPU time is needed in a so-called spin-up simulation to determine an equilibrium solution. In this paper, we present methodology based on Jacobian-Free

  13. Mapping the QCD Phase Transition with Accreting Compact Stars

    CERN Document Server

    Blaschke, David; Grigorian, Hovik

    2008-01-01

    We discuss an idea for how accreting millisecond pulsars could contribute to the understanding of the QCD phase transition in the high-density nuclear matter equation of state (EoS). It is based on two ingredients, the first one being a ``phase diagram'' of rapidly rotating compact star configurations in the plane of spin frequency and mass, determined with state-of-the-art hybrid equations of state, allowing for a transition to color superconducting quark matter. The second is the study of spin-up and accretion evolution in this phase diagram. We show that the quark matter phase transition leads to a characteristic line in the Omega-M plane, the phase border between neutron stars and hybrid stars with a quark matter core. Along this line a change in the pulsar's moment of inertia entails a waiting point phenomenon in the accreting millisecond X-ray pulsar (AMXP) evolution: most of these objects should therefore be found along the phase border in the Omega-M plane, which may be viewed as the AMXP analog of th...

  14. Jet Luminosity from Neutrino-Dominated Accretion Flows in GRBs

    CERN Document Server

    Kawanaka, Norita

    2013-01-01

    A hyperaccretion disk around a stellar-mass black hole is a plausible model for the central engine that powers gamma-ray bursts (GRBs). We estimate the luminosity of a jet driven by magnetohydrodynamic processes such as the Blandford-Znajek (BZ) mechanism as a function of mass accretion rate, the black hole mass, and other accretion parameters. We show that the jet is most efficient when the accretion flow is cooled via optically-thin neutrino emission, and that its luminosity is much larger than the energy deposition rate through neutrino annihilation provided that the black hole is spinning rapidly enough. Also, we find a significant jump in the jet luminosity at the transition mass accretion rate between the advection dominated accretion flow (ADAF) regime and the neutrino-dominated accretion flow (NDAF) regime. This may cause the large variability observed in the prompt emission of GRBs.

  15. The Magnetic Field Evolution of ULX NuSTAR J095551+6940.8 in M82--A Legacy of Accreting Magnetar

    CERN Document Server

    Pan, Y Y; Zhang, C M; Tong, H

    2015-01-01

    Ultra luminous X-ray sources (ULXs) are usually believed to be black holes with mass about 10^{2--3}M_{sun}. However, the recent discovery of ULX NuSTAR J095551+6940.8 in M82 with the spin period P=1.37s and period derivation P_{dot}=-2*10^{-10} ss^{-1} provides a strong evidence that some ULXs are accreting neutron stars (NSs). To investigate such a particular accreting neutron star, we ascribe it as an evolved magnetar in the accretion binary system. By means of the model of accretion induced the NS magnetic evolution and standard spinup torque, we calculate the magnetic field decay and spin-up of M82 X-2, and show that its magnetic field is now 4.5*10^{12} G, which is evolved from a magnetar in a high mass Xray binary system (HMXB) with the initial values of magnetic field B~10^{14.5} G and spin period P~100 s by accreting ~10^{-3}M_{sun}, while the mass accretion rate for spin-up is set as 5.0*10^{18} gs^{-1}. The evolutionary track of magnetic field and spin period of M82 X-2 is simulated and plotted in ...

  16. The magnetic field evolution of ULX NuSTAR J095551+6940.8 in M82 - a legacy of accreting magnetar

    Science.gov (United States)

    Pan, Y. Y.; Song, L. M.; Zhang, C. M.; Tong, H.

    2016-09-01

    Ultraluminous X-ray sources are usually believed to be black holes with mass about 102-3 M⊙. However, the recent discovery of NuSTAR J095551+6940.8 in M82 by Bachetti et al. shows that it holds the spin period P = 1.37 s and period derivative dot{P}≈ -2× 10^{-10} s s^{-1}, which provides a strong evidence that some ultraluminous X-ray sources could be neutron stars. We obtain that the source may be an evolved magnetar according to our simulation by employing the model of accretion induced the polar magnetic field decay and standard spin-up torque of an accreting neutron star. The results show that NuSTAR J095551+6940.8 is still in the spin-up process, and the polar magnetic field decays to about 4.5 × 1012 G after accreting ˜10-2.5 M⊙, while the strong magnetic field exists in the out-polar region, which could be responsible for the observed low field magnetar. The ultra luminosity of the source can be explained by the beaming effect and two kinds of accretion-radial random accretion and disc accretion. Since the birth rate of magnetars is about ten per cent of the normal neutron stars, we guess that several ultraluminous X-ray sources should share the similar properties to that of NuSTAR J095551+6940.8.

  17. Orbital-Phase-Resolved Study of OAO 1657-415 in Spin-Up and Spin-Down

    Science.gov (United States)

    Chakrabarty, Deepto

    2003-01-01

    We observed the 38-s X-ray pulsar OAO 1657-415 in a series of daily snapshots spanning its 10-day binary orbit, on three different occasions corresponding to different accretion torque states. The goal was to see if the previously observed drastic variability in X-ray spectral properties showed a systematic pattern with respect to orbital phase or torque state. Our study failed to identify any systematic patterns, although considerably stochastic variability was observed. Overall, our results were inconclusive. We chose not to prepare a journal paper on this study, although the data were shared with several other groups interested in monitoring the properties of this source.

  18. Gas accretion onto galaxies

    CERN Document Server

    Davé, Romeel

    2017-01-01

    This edited volume presents the current state of gas accretion studies from both observational and theoretical perspectives, and charts our progress towards answering the fundamental yet elusive question of how galaxies get their gas. Understanding how galaxies form and evolve has been a central focus in astronomy for over a century. These studies have accelerated in the new millennium, driven by two key advances: the establishment of a firm concordance cosmological model that provides the backbone on which galaxies form and grow, and the recognition that galaxies grow not in isolation but within a “cosmic ecosystem” that includes the vast reservoir of gas filling intergalactic space. This latter aspect in which galaxies continually exchange matter with the intergalactic medium via inflows and outflows has been dubbed the “baryon cycle”. The topic of this book is directly related to the baryon cycle, in particular its least well constrained aspect, namely gas accretion. Accretion is a rare area of ast...

  19. Supermassive blackholes without super Eddington accretion

    Science.gov (United States)

    Christian, Damian Joseph; Kim, Matt I.; Garofalo, David; D'Avanzo, Jaclyn; Torres, John

    2017-08-01

    We explore the X-ray luminosity function at high redshift for active galactic nuclei using an albeit simplified model for mass build-up using a combination of mergers and mass accretion in the gap paradigm (Garofalo et al. 2010). Using a retrograde-dominated configuration we find an interesting low probability channel for the growth of one billion solar mass black holes within hundreds of millions of years of the big bang without appealing to super Eddington accretion (Kim et al. 2016). This result is made more compelling by the connection between this channel and an end product involving active galaxies with FRI radio morphology but weaker jet powers in mildly sub-Eddington accretion regimes. We will discuss our connection between the unexplained paucity of a given family of AGNs and the rapid growth of supermassive black holes, two heretofore seemingly unrelated aspects of the physics of AGNs that will help further understand their properties and evolution.

  20. Magnetohydrodynamic Origin of Jets from Accretion Disks

    CERN Document Server

    Lovelace, R V E; Koldoba, A V

    1999-01-01

    A review is made of recent magnetohydrodynamic (MHD) theory and simulations of origin of jets from accretion disks. Many compact astrophysical objects emit powerful, highly-collimated, oppositely directed jets. Included are the extra galactic radio jets of active galaxies and quasars, and old compact stars in binaries, and emission line jets in young stellar objects. It is widely thought that these different jets arise from rotating, conducting accretion disks threaded by an ordered magnetic field. The twisting of the magnetic field by the rotation of the disk drives the jets by magnetically extracting matter, angular momentum, and energy from the accretion disk. Two main regimes have been discussed theoretically, hydromagnetic winds which have a significant mass flux, and Poynting flux jets where the mass flux is negligible. Over the past several years, exciting new developments on models of jets have come from progress in MHD simulations which now allow the study of the origin - the acceleration and collima...

  1. Black hole accretion discs

    OpenAIRE

    Lasota, Jean-Pierre

    2015-01-01

    This is an introduction to models of accretion discs around black holes. After a presentation of the non-relativistic equations describing the structure and evolution of geometrically thin accretion discs we discuss their steady-state solutions and compare them to observation. Next we describe in detail the thermal-viscous disc instability model and its application to dwarf novae for which it was designed and its X-ray irradiated-disc version which explains the soft X--ray transients, i.e. ou...

  2. MHD of accretion-disk flows

    Science.gov (United States)

    Yankova, Krasimira

    2015-01-01

    Accretion is one of the most important problems of astrophysics concerning the transfer of matter and the transformation of energy into space. Process represents a falling of the substance on a cosmic object from the surrounding area and is a powerful gravitational mechanism for the production of radiation. Accretion disc effectively converts the mass of the substance by viscous friction and released potential energy transformed into radiation by particle collisions. Accretion onto compact object shows high energy efficiency and temporal variability in a broad class of observational data in all ranges. In the disks of these objects are developed a series instabilities and structures that govern the distribution of the energy. They are expressed in many variety non-stationary phenomena that we observe. That is why we propose generalized model of magnetized accretion disk with advection, which preserves the nonlinearity of the problem. We study interaction of the plasmas flow with the magnetic field, and how this affects the self-organizing disk. The aim of the work is to describe the accretion flow in detail, in his quality of the open astrophysical system, to investigate the evolution and to reveal the mechanisms of the structuring the disk-corona system for to interpret correctly the high energy behavior of such sources.

  3. Bondi accretion in the early universe

    CERN Document Server

    Ricotti, Massimo

    2007-01-01

    This paper presents a study of quasi-steady spherical accretion in the early Universe, before the formation of the first stars and galaxies. The main motivation is to derive the basic formulas that will be used in a companion paper to calculate the accretion luminosity of primordial black holes and their effect on the cosmic ionization history. The following cosmological effects are investigated: the coupling of the gas to the CMB photon fluid (i.e., Compton drag), Hubble expansion, and the growth of the dark matter halo seeded by the gravitational potential of the central point mass. The gas equations of motion are solved assuming either a polytropic or an isothermal equation of state. We consider the cases in which the accreting object is a point mass or a spherical dark matter halo with power-law density profile, as predicted by the theory of "secondary infall''. Analytical solutions for the sonic radius and fitting formulas for the accretion rate are provided. Different accretion regimes exist depending o...

  4. Dynamos in accretion discs

    OpenAIRE

    Brandenburg, A.; von Rekowski, B.

    2007-01-01

    It is argued that accretion discs in young stellar objects may have hot coronae that are heated by magnetic reconnection. This is a consequence of the magneto-rotational instability driving turbulence in the disc. Magnetic reconnection away from the midplane leads to heating of the corona which, in turn, contributes to driving disc winds.

  5. Viscosity in accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Katz, J.I.

    1980-01-01

    Both HerX-1 and SS433 may contain accretion disks slaved to a precessing companion star. If so, it is possible to bound the effective viscosity in these disks. The results, in terms of the disk parameter alpha, are lower bounds of 0.01 for HerX-1 and of 0.1 for SS433.

  6. Constraining the dipolar magnetic field of M82 X-2 by the accretion model

    CERN Document Server

    Chen, Wen-Cong

    2016-01-01

    Recently, ultraluminous X-ray source (ULX) M82 X-2 has been identified to be an accreting neutron star, which has a $P=1.37$ s spin period, and is spinning up at a rate $\\dot{P}=-2.0\\times 10^{-10}~\\rm s\\,s^{-1}$. Interestingly, its isotropic X-ray luminosity $L_{\\rm iso}=1.8\\times 10^{40}~\\rm erg\\,s^{-1}$ during outbursts is 100 times the Eddington limit for a $1.4~\\rm M_{\\odot}$ neutron star. In this Letter, based on the standard accretion model we attempt to constrain the dipolar magnetic field of the pulsar in ULX M82 X-2. Our calculations indicate that the accretion rate at the magnetospheric radius must be super-Eddington during outbursts. To support such a super-Eddington accretion, a relatively high multipole field ($\\ga 10^{13}$ G) near the surface of the accretor is invoked to produce an accreting gas column. However, our constraint shows that the surface dipolar magnetic field of the pulsar should be in the range of $1.0-3.5\\times 10^{12}$ G. Therefore, our model supports that the neutron star in U...

  7. Helium accreting CO white dwarfs with rotation: helium novae instead of double detonation

    CERN Document Server

    Yoon, S C

    2004-01-01

    We present evolutionary models of helium accreting carbon-oxygen white dwarfs in which we include the effects of the spin-up of the accreting star induced by angular momentum accretion, rotationally induced chemical mixing and rotational energy dissipation. Initial masses of 0.6 Msun and 0.8 Msun and constant accretion rates of a few times 10^{-8} Msun/yr of helium rich matter have been considered, which is typical for the sub-Chandrasekhar mass progenitor scenario for Type Ia supernovae. It is found that the helium envelope in an accreting white dwarf is heated efficiently by friction in the differentially rotating spun-up layers. As a result, helium ignites much earlier and under much less degenerate conditions compared to the corresponding non-rotating case. Consequently, a helium detonation may be avoided, which questions the sub-Chandrasekhar mass progenitor scenario for Type Ia supernovae. We discuss implications of our results for the evolution of helium star plus white dwarf binary systems as possible...

  8. Probing General Relativity with Accreting Black Holes

    CERN Document Server

    Fabian, A C

    2012-01-01

    Most of the X-ray emission from luminous accreting black holes emerges from within 20 gravitational radii. The effective emission radius is several times smaller if the black hole is rapidly spinning. General Relativistic effects can then be very important. Large spacetime curvature causes strong lightbending and large gravitational redshifts. The hard X-ray, power-law-emitting corona irradiates the accretion disc generating an X-ray reflection component. Atomic features in the reflection spectrum allow gravitational redshifts to be measured. Time delays between observed variations in the power-law and the reflection spectrum (reverberation) enable the physical scale of the reflecting region to be determined. The relative strength of the reflection and power-law continuum depends on light bending. All of these observed effects enable the immediate environment of the black hole where the effects of General Relativity are on display to be probed and explored.

  9. Accretion discs trapped near corotation

    NARCIS (Netherlands)

    D'Angelo, C.R.; Spruit, H.C.

    2012-01-01

    We show that discs accreting on to the magnetosphere of a rotating star can end up in a trapped state, in which the inner edge of the disc stays near the corotation radius, even at low and varying accretion rates. The accretion in these trapped states can be steady or cyclic; we explore these states

  10. Retrograde accretion discs in high-mass Be/X-ray binaries

    Science.gov (United States)

    Christodoulou, D. M.; Laycock, S. G. T.; Kazanas, D.

    2017-09-01

    We have compiled a comprehensive library of all X-ray observations of Magellanic pulsars carried out by XMM-Newton, Chandra and RXTE in the period 1997-2014. In this work, we use the data from 53 high-mass Be/X-ray binaries in the Small Magellanic Cloud to demonstrate that the distribution of spin-period derivatives versus spin periods of spinning-down pulsars is not at all different from that of the accreting spinning-up pulsars. The inescapable conclusion is that the up and down samples were drawn from the same continuous parent population; therefore, Be/X-ray pulsars that are spinning down over periods spanning 18 yr are, in fact, accreting from retrograde discs. The presence of prograde and retrograde discs in roughly equal numbers supports a new evolutionary scenario for Be/X-ray pulsars in their spin period-period derivative diagram.

  11. Massive star formation by accretion. I. Disc accretion

    Science.gov (United States)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  12. Accretion of the Earth.

    Science.gov (United States)

    Canup, Robin M

    2008-11-28

    The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 45 degrees and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon.

  13. Massive thin accretion discs: Pt. 3; Comparison with the observations

    Energy Technology Data Exchange (ETDEWEB)

    Laor, A. (Tel Aviv Univ. (Israel). Wise Observatory)

    1990-10-01

    This paper presents a detailed comparison of theoretical thin accretion disc models with the spectra of 101 bright AGN. The models have been described previously; they take into account all relativistic effects and use an improved approximation for the local spectrum. The comparison is made by fitting the observation with a two-component model, a thin 'bare' accretion disc and an underlying power law. (author).

  14. Evolution and precession of accretion disk in tidal disruption events

    Directory of Open Access Journals (Sweden)

    Matzner C.D.

    2012-12-01

    Full Text Available In a supermassive black hole (BH tidal disruption event (TDE, the tidally disrupted star feeds the BH via an accretion disk. Most often it is assumed that the accretion rate history, hence the emission light curve, tracks the rate at which new debris mass falls back onto the disk, notably the t−5/3 power law. But this is not the case when the disk evolution due to viscous spreading - the driving force for accretion - is carefully considered. We construct a simple analytical model that comprehensively describes the accretion rate history across 4 different phases of the disk evolution, in the presence of mass fallback and disk wind loss. Accretion rate evolves differently in those phases which are governed by how the disk heat energy is carried away, early on by advection and later by radiation. The accretion rate can decline as steeply as t−5/3 only if copious disk wind loss is present during the early advection-cooled phase. Later, the accretion rate history is t−8/7 or shallower. These have great implications on the TDE flare light curve. A TDE accretion disk is most likely misaligned with the equatorial plane of the spinning BH. Moreover, in the TDE the accretion rate is super- or near-Eddington thus the disk is geometrically thick, for which case the BH’s frame dragging effect may cause the disk precess as a solid body, which may manifest itself as quasi-periodic signal in the TDE light curve. Our disk evolution model predicts the disk precession period increases with time, typically as ∝ t. The results are applied to the recently jetted TDE flare Swift transient J1644 + 57 which shows numerous, quasi-periodic dips in its long-term X-ray light curve. As the current TDE sample increases, the identification of the disk precession signature provides a unique way of measuring BH spin and studying BH accretion physics.

  15. Photon Bubbles in Accretion Discs

    OpenAIRE

    Gammie, Charles F.

    1998-01-01

    We show that radiation dominated accretion discs are likely to suffer from a ``photon bubble'' instability similar to that described by Arons in the context of accretion onto neutron star polar caps. The instability requires a magnetic field for its existence. In an asymptotic regime appropriate to accretion discs, we find that the overstable modes obey the remarkably simple dispersion relation \\omega^2 = -i g k F(B,k). Here g is the vertical gravitational acceleration, B the magnetic field, ...

  16. The fundamental plane of accretion onto black holes with dynamical masses

    NARCIS (Netherlands)

    K. Gültekin; E.M. Cackett; J.M. Miller; T. Di Matteo; S. Markoff; D.O. Richstone

    2009-01-01

    Black hole accretion and jet production are areas of intensive study in astrophysics. Recent work has found a relation between radio luminosity, X-ray luminosity, and black hole mass. With the assumption that radio and X-ray luminosities are suitable proxies for jet power and accretion power, respec

  17. Strong C+ emission in galaxies at z~1-2: Evidence for cold flow accretion powered star formation in the early Universe

    CERN Document Server

    Brisbin, Drew; Nikola, Thomas; Parshley, Stephen; Stacey, Gordon J; Spoon, Henrik; Hailey-Dunsheath, Steven; Verma, Aprajita

    2014-01-01

    We have recently detected the [CII] 157.7 micron line in eight star forming galaxies at redshifts 1 to 2 using the redshift(z) Early Universe Spectrometer (ZEUS). Our sample targets star formation dominant sources detected in PAH emission. This represents a significant addition to [CII] observations during the epoch of peak star formation. We have augmented this survey with observations of the [OI] 63 micron line and far infrared photometry from the PACS and SPIRE Herschel instruments as well as Spitzer IRS spectra from the literature showing PAH features. Our sources exhibit above average gas heating efficiency, many with both [OI]/FIR and [CII]/FIR ~1% or more. The relatively strong [CII] emission is consistent with our sources being dominated by star formation powered PDRs, extending to kpc scales. We suggest that the star formation mode in these systems follows a Schmidt-Kennicutt law similar to local systems, but at a much higher rate due to molecular gas surface densities 10 to 100 times that of local s...

  18. Massive star formation by accretion I. Disc accretion

    CERN Document Server

    Haemmerlé, Lionel; Meynet, Georges; Maeder, André; Charbonnel, Corinne

    2016-01-01

    Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the HR diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the Hertzsprung-Russell (HR) diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the acc...

  19. Strong C+ Emission in Galaxies at z ~ 1-2: Evidence for Cold Flow Accretion Powered Star Formation in the Early Universe

    Science.gov (United States)

    Brisbin, Drew; Ferkinhoff, Carl; Nikola, Thomas; Parshley, Stephen; Stacey, Gordon J.; Spoon, Henrik; Hailey-Dunsheath, Steven; Verma, Aprajita

    2015-01-01

    We have recently detected the [C II] 157.7 μm line in eight star-forming galaxies at redshifts 1 to 2 using the redshift (z) Early Universe Spectrometer (ZEUS). Our sample targets star formation dominant sources detected in PAH emission. This represents a significant addition to [C II] observations during the epoch of peak star formation. We have augmented this survey with observations of the [O I] 63 μm line and far infrared photometry from the PACS and SPIRE Herschel instruments as well as Spitzer IRS spectra from the literature showing PAH features. Our sources exhibit above average gas heating efficiency, many with both [O I]/FIR and [C II]/FIR of ~1% or more. The relatively strong [C II] emission is consistent with our sources being dominated by star formation powered photo-dissociation regions, extending to kiloparsec scales. We suggest that the star formation mode in these systems follows a Schmidt-Kennicutt law similar to local systems, but at a much higher rate due to molecular gas surface densities 10-100 times that of local star-forming systems. The source of the high molecular gas surface densities may be the infall of neutral gas from the cosmic web. In addition to the high [C II]/FIR values, we also find high [C II]/PAH ratios and, in at least one source, a cool dust temperature. This source, SWIRE 4-5, bears a resemblance in these diagnostics to shocked regions of Stephan's Quintet, suggesting that another mode of [C II] excitation in addition to normal photoelectric heating may be contributing to the observed [C II] line.

  20. Relativistic Accretion Mediated by Turbulent Comptonization

    CERN Document Server

    Socrates, Aristotle

    2008-01-01

    Black hole and neutron star accretion flows display unusually high levels of hard coronal emission in comparison to all other optically thick, gravitationally bound, turbulent astrophysical systems. Since these flows sit in deep relativistic gravitational potentials, their random bulk motions approach the speed of light, therefore allowing turbulent Comptonization to be an important effect. We show that the inevitable production of hard X-ray photons results from turbulent Comptonization in the limit where the turbulence is trans-sonic and the accretion power approaches the Eddington Limit. In this regime, the turbulent Compton y-parameter approaches unity and the turbulent Compton temperature is a significant fraction of the electron rest mass energy, in agreement with the observed phenomena.

  1. FITDisk: Cataclysmic Variable Accretion Disk Demonstration Tool

    Science.gov (United States)

    Wood, Matthew A.; Dolence, J.

    2013-05-01

    FITDisk models accretion disk phenomena using a fully three-dimensional hydrodynamics calculation, and data can either be visualized as they are computed or stored to hard drive for later playback at a fast frame rate. Simulations are visualized using OpenGL graphics and the viewing angle can be changed interactively. Pseudo light curves of simulated systems can be plotted along with the associated Fourier amplitude spectrum. It provides an easy to use graphical user interface as well as 3-D interactive graphics. The code computes the evolution of a CV accretion disk, visualizes results in real time, records and plays back simulations, and generates and plots pseudo light curves and associated power spectra.

  2. WIND-ACCRETION DISKS IN WIDE BINARIES, SECOND-GENERATION PROTOPLANETARY DISKS, AND ACCRETION ONTO WHITE DWARFS

    Energy Technology Data Exchange (ETDEWEB)

    Perets, Hagai B. [Technion-Israel Institute of Technology, Haifa (Israel); Kenyon, Scott J., E-mail: hperets@physics.technion.ac.il [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

    2013-02-20

    Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long-term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind accretion leads to long-lived stable disks over the lifetime of the asymptotic giant branch donor star. The disks have masses of a few times 10{sup -5}-10{sup -3} M {sub Sun }, with surface density and temperature profiles that follow broken power laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly, 50%-80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M {sub Sun }. When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks, suggesting that coordinated observational programs might improve our understanding of uncertain disk physics.

  3. Constraints on Slim Accretion Discs

    Institute of Scientific and Technical Information of China (English)

    CAI Zhen-Yi; GU Wei-Min; LU Ju-Fu

    2008-01-01

    @@ We show that when the gravitational force in the vertical direction is correctly calculated, the well-known Sshaped sequence of thermal equilibrium solutions can be constructed only for small radii of black hole accretion flows, such that slim accretion discs can possibly exist only in the inner regions of these flows.

  4. Protostellar accretion traced with chemistry

    DEFF Research Database (Denmark)

    Frimann, Søren; Jørgensen, Jes K.; Padoan, Paolo;

    2016-01-01

    Context. Understanding how protostars accrete their mass is a centralquestion of star formation. One aspect of this is trying to understandwhether the time evolution of accretion rates in deeply embedded objectsis best characterised by a smooth decline from early to late stages orby intermittent ...

  5. Binary interactions with high accretion rates onto main sequence stars

    Science.gov (United States)

    Shiber, Sagiv; Schreier, Ron; Soker, Noam

    2016-07-01

    Energetic outflows from main sequence stars accreting mass at very high rates might account for the powering of some eruptive objects, such as merging main sequence stars, major eruptions of luminous blue variables, e.g., the Great Eruption of Eta Carinae, and other intermediate luminosity optical transients (ILOTs; red novae; red transients). These powerful outflows could potentially also supply the extra energy required in the common envelope process and in the grazing envelope evolution of binary systems. We propose that a massive outflow/jets mediated by magnetic fields might remove energy and angular momentum from the accretion disk to allow such high accretion rate flows. By examining the possible activity of the magnetic fields of accretion disks, we conclude that indeed main sequence stars might accrete mass at very high rates, up to ≈ 10-2 M ⊙ yr-1 for solar type stars, and up to ≈ 1 M ⊙ yr-1 for very massive stars. We speculate that magnetic fields amplified in such extreme conditions might lead to the formation of massive bipolar outflows that can remove most of the disk's energy and angular momentum. It is this energy and angular momentum removal that allows the very high mass accretion rate onto main sequence stars.

  6. Cold, clumpy accretion onto an active supermassive black hole

    CERN Document Server

    Tremblay, Grant R; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P; Baum, Stefi A; Voit, G Mark; Donahue, Megan; McNamara, Brian R; Davis, Timothy A; McDonald, Michael A; Edge, Alastair C; Clarke, Tracy E; Galván-Madrid, Roberto; Bremer, Malcolm N; Edwards, Louise O V; Fabian, Andrew C; Hamer, Stephen L; Li, Yuan; Maury, Anaëlle; Russell, Helen R; Quillen, Alice C; Urry, C Megan; Sanders, Jeremy S; Wise, Michael

    2016-01-01

    Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds - a departure from the "hot mode" accretion model - although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z=0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities can precipitate from this hot gas, producing a rain of c...

  7. Images of accretion discs. 1. The eclipse mapping method

    Energy Technology Data Exchange (ETDEWEB)

    Horne, K.

    1985-03-01

    A method of mapping the surface brightness distributions of accretion discs in eclipsing cataclysmic binaries is described and tested with synthetic eclipse data. Accurate synthetic light curves are computed by numerical simulation of the accretion disc eclipse, and images of the disc are reconstructed by maximum entropy methods. The conventional definition of entropy leads to a distorted image of the disc. A modified form of entropy, sensitive to the aximuthal structure of the image but not to its radial profile, suppresses azimuthal structure but correctly recovers the radial structure of the accretion disc. This eclipse mapping method permits powerful tests of accretion disc theory by deriving the spatial structure of discs from observational data with a minimum of model-dependent assumptions.

  8. Snow accretion on overhead wires

    Energy Technology Data Exchange (ETDEWEB)

    Sakamoto, Y. [Meteorological Research Inst. for Technology Co. Ltd., Tokyo (Japan); Tachizaki, S.; Sudo, N. [Tohoku Electric Power Co. Ltd., Miyagi (Japan)

    2005-07-01

    Wet snow accretion can cause extensive damage to transmission systems. This paper reviewed some of the difficulties faced by researchers in the study of wet snow accretion on overhead lines in Japan. The study of snow accretion phenomena is complicated by the range of phase changes in water. Snowflakes produced in an upper atmospheric layer with a temperature below freezing do not melt when they go through a lower atmospheric layer with a temperature above freezing, but are in a mixed state of solid and liquid due to the latent heat of melting. The complicated properties of water make studies of snow accretion difficult, as well as the fact that snow changes its physical properties rapidly, due to the effects of ambient temperature, rainfall, and solar radiation. The adhesive forces that cause snow accretion include freezing; bonding through freezing; sintering; condensation and freezing of vapor in the air; mechanical intertwining of snowflakes; capillary action due to liquids; coherent forces between ice particles and water formed through the metamorphosis of snowflakes. In addition to these complexities, differences in laboratory room environments and natural snow environments can also pose difficulties for researchers. Equations describing the relationship between the density of accreted snow and the meteorological parameters involved were presented, as well as empirical equations which suggested that snow accretion efficiency has a dependency on air temperature. An empirical model for estimating snow loads in Japan was outlined, as well as various experiments observing show shedding. Correlations for wet snow accretion included precipitation intensity; duration of precipitation; air temperature; wind speed and wind direction in relation to the overhead line. Issues concerning topography and wet snow accretion were reviewed. It was concluded that studies of snow accretion will benefit by the collection of data in each matrix of the relevant parameters. 12 refs

  9. Accretion by the Galaxy

    Directory of Open Access Journals (Sweden)

    Binney J.

    2012-02-01

    Full Text Available Cosmology requires at least half of the baryons in the Universe to be in the intergalactic medium, much of which is believed to form hot coronae around galaxies. Star-forming galaxies must be accreting from their coronae. Hi observations of external galaxies show that they have Hi halos associated with star formation. These halos are naturally modelled as ensembles of clouds driven up by supernova bubbles. These models can fit the data successfully only if clouds exchange mass and momentum with the corona. As a cloud orbits, it is ablated and forms a turbulent wake where cold high-metallicity gas mixes with hot coronal gas causing the prompt cooling of the latter. As a consequence the total mass of Hi increases. This model has recently been used to model the Leiden-Argentina-Bonn survey of Galactic Hi. The values of the model’s parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a remarkable degree of consistency, despite the very different natures of the two external galaxies and the dramatic difference in the nature of the data for our Galaxy and the external galaxies. The parameter values are also consistent with hydrodynamical simulations of the ablation of individual clouds. The model predicts that a galaxy that loses its cool-gas disc for instance through a major merger cannot reform it from its corona; it can return to steady star formation only if it can capture a large body of cool gas, for example by accreting a gas-rich dwarf. Thus the model explains how major mergers can make galaxies “red and dead.”

  10. Interbasin exchanges and their roles in global ocean circulation:A study based on 1 400 years’ spin up of MOM4p1

    Institute of Scientific and Technical Information of China (English)

    ZHU Yaohua; WEI Zexun; FANG Guohong; WANG Yonggang; GUAN Yuping

    2014-01-01

    A global prognostic model based on MOM4p1, which is a primitive equation nonBoussinesq numerical model, has been integrated with 1 400 years from the state of rest based on the realistic topography to study the long-term pattern of combined wind-driven and thermodynamically-driven general circulation. The model is driven by monthly climatological mean forces and includes 192×189 horizontal grids and 31 pressure-based vertical levels. The main objective is to investigate the mass and heat transports at inter-basin passages and their compensations and roles in the global ocean circulation under equilibrium state of long-term spin up. The kinetic energy analysis divides the spin up process into three stages:the quasi-stable state of wind driven current, the growing phase of thermodynamical circulation and the equilibrium state of thermohaline circulation. It is essential to spin up over a thousand years in order to reach the thermohaline equilibrium state from a state of rest. The Arctic Throughflow from the Bering Strait to the Greenland Sea and the Indonesian Throughflow (ITF) are captured and examined with their compensations and existing data. Analysis reveals that the slope structures of sea surface height are the dynamical driving mechanism of the Pacific-Arctic-Atlantic throughflow and ITF. The analysis denotes, in spite of O (1.4×106 m3/s) of the southward volume transport in the northern Atlantic, that there is still O (1 PW ) of heat transported north-ward since the northward currents in the upper layer carry much higher temperature water than the south-ward flowing northern Atlantic deep water (NADW ). Meridional volume and heat transports are focused on the contributions to NADW renewals and Atlantic meridional overturning circulation (AMOC). Quantitative descriptions of the interbasin exchanges are explained by meridional compensations and supported by pre-vious observations and numerical modeling results. Analysis indicates that the volume and heat

  11. The rigid shell component for superrotation in planetary atmospheres: Angular momentum budget, mechanical analog and simulation of the spin up process

    Science.gov (United States)

    Mayr, H. G.; Harris, I.

    1981-01-01

    An analysis of superrotation in the atmosphere of planets, with rotation axis perpendicular to the orbital plane is presented. As the atmosphere expands, Hadley cells develop producing a redistribution of mass and angular momentum. A three dimensional thermally driven zonally symmetric spectral model and Laplace transformation simulate the time evolution of a fluid leading from corotation under globally uniform heating to superrotation under globally nonuniform heating. For high viscosities the rigid shell component of atmospheric superrotation can be understood in analogy with a pirouette. During spin up angular momentum is transferred to the planet. For low iscosities, the process is reversed. A tendency toward geostrophy, combined with increase of surface pressure toward the poles (due to meridional mass transport), induces the atmosphere to subrotate temporarily at lower altitudes. Resultant viscous shear near the surface permits angular momentum to flow from the planet into the atmosphere propagating upwards to produce high altitude superrotation rates.

  12. Spherical Accretion in a Uniformly Expanding Universe

    Science.gov (United States)

    Colpi, Monica; Shapiro, Stuart L.; Wasserman, Ira

    1996-10-01

    We consider spherically symmetric accretion of material from an initially homogeneous, uniformly expanding medium onto a Newtonian point mass M. The gas is assumed to evolve adiabatically with a constant adiabatic index F, which we vary over the range Γ ɛ [1, 5/3]. We use a one-dimensional Lagrangian code to follow the spherical infall of material as a function of time. Outflowing shells gravitationally bound to the point mass fall back, giving rise to a inflow rate that, after a rapid rise, declines as a power law in time. If there were no outflow initially, Bondi accretion would result, with a characteristic accretion time-scale ta,0. For gas initially expanding at a uniform rate, with a radial velocity U = R/t0 at radius R, the behavior of the flow at all subsequent times is determined by ta,0/t0. If ta,0/t0 ≫ 1, the gas has no time to respond to pressure forces, so the fluid motion is nearly collisionless. In this case, only loosely bound shells are influenced by pressure gradients and are pushed outward. The late-time evolution of the mass accretion rate Mdot is close to the result for pure dust, and we develop a semianalytic model that accurately accounts for the small effect of pressure gradients in this limit. In the opposite regime, ta,0/t0 ≪ 1, pressure forces significantly affect the motion of the gas. At sufficiently early times, t ≤ ttr, the flow evolved along a sequence of quasi-stationary, Bondi-like states, with a time-dependent Mdot determined by the slowly varying gas density at large distances. However, at later times, t ≥ ttr, the fluid flow enters a dustllke regime; ttr is the time when the instantaneous Bondi accretion radius reaches the marginally bound radius. The transition time ttr depends sensitively on ta,0/t0 for a given Γ and can greatly exceed t0. We show that there exists a critical value Γ = 11/9, below which the transition from fluid to ballistic motion disappears. As one application of our calculations, we consider the

  13. AGN Accretion Physics: Insights from K2

    Science.gov (United States)

    Vogeley, Michael

    between the stochastic model parameters and physical parameters will provide new methods for classification of AGN from their variability and aid in identification of AGN from photometric surveys. (5) Results of this empirical study will serve as a guide to theorists who study the complex astrophysics of accretion. (6) Sensitivity analysis of these statistics to cadence will allow us to make forecasts for detection and classification of AGN from future surveys such as LSST. Results of this research will include new constraints on models of accretion physics, more powerful methods for classifying AGN by their variability, and forecasts that guide design of future time-domain surveys of AGN. Products of this program will include a catalog of precision light curves of AGN, variability model parameters for every light curve, and a software package for time series statistical analysis. Thus, the proposed research directly addresses a key NASA goal in astrophysics in the area of Cosmic Origins (understanding the formation of black holes and their impact on galaxies). This research pursues several science themes identified by the 2010 Decadal Survey: Discovery (Opening the Time Domain); Origins (Origin of black holes); and Frontiers of Knowledge (Nature of Compact Objects). The research team for this project combines expertise in time series analysis, variability studies of AGN, statistical analysis of large surveys, Kepler photometry, and multi-wavelength observations of quasars and AGN. This work is timely because observations from the K2 campaigns now include a large set of high-cadence light curves with very high-quality photometry and we have fully tested our methods and analysis software.

  14. Dynamics of core accretion

    Science.gov (United States)

    Nelson, Andrew F.; Ruffert, Maximilian

    2013-02-01

    We perform three-dimensional hydrodynamic simulations of gas flowing around a planetary core of mass Mpl = 10M⊕ embedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behaviour following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate-temperature background disc with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the `Piecewise Parabolic Method' with as many as six fixed nested grids, providing spatial resolution on the finest grid comparable to the present-day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static envelope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disc. It is sensitive to the thermodynamic treatment of the gas, as modelled by prescribed equations of state (either `locally isothermal' or `locally isentropic') and the temperature of the background disc material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influences the magnitude and character of hydrodynamic feedback of the small-scale flow on the background, and we conclude that accurate modelling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling however, as defined by locally isothermal or

  15. Accretion discs trapped near corotation

    OpenAIRE

    D'Angelo, C.R.; Spruit, H.C.

    2012-01-01

    We show that discs accreting on to the magnetosphere of a rotating star can end up in a trapped state, in which the inner edge of the disc stays near the corotation radius, even at low and varying accretion rates. The accretion in these trapped states can be steady or cyclic; we explore these states over a wide range of parameter space. We find two distinct regions of instability: one related to the buildup and release of mass in the disc outside corotation, and the other to mass storage with...

  16. He-Accreting WDs: accretion regimes and final outcomes

    CERN Document Server

    Piersanti, L; Yungelson, L R

    2014-01-01

    The behaviour of carbon-oxygen white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyze the thermal response of the accreting WD to mass deposition at different time scales. The analysis has been performed for initial WDs masses and accretion rates in the range (0.60 - 1.02) Msun and 1.e-9 - 1.e-5 Msun/yr, respectively. Thermal regimes in the parameters space M_{WD} - dot{M}_{He}, leading to formation of red-giant-like structure, steady burning of He, mild, strong and dynamical flashes have been identified and the transition between those regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated in order to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building-up of a He-rich layer via H-burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomi...

  17. Contrasting Behaviour from Two Be/X-ray Binary Pulsars: Insights into Differing Neutron Star Accretion Modes

    Science.gov (United States)

    Townsend, L. J.; Drave, S. P.; Hill, A. B.; Coe, M. J.; Corbet, R. H. D.; Bird, A. J.

    2013-01-01

    In this paper we present the identification of two periodic X-ray signals coming from the direction of the Small Magellanic Cloud (SMC). On detection with the Rossi X-ray Timing Explorer (RXTE), the 175.4 s and 85.4 s pulsations were considered to originate from new Be/X-ray binary (BeXRB) pulsars with unknown locations. Using rapid follow-up INTEGRAL and XMM-Newton observations, we show the first pulsar (designated SXP175) to be coincident with a candidate high-mass X-ray binary (HMXB) in the northern bar region of the SMC undergoing a small Type II outburst. The orbital period (87d) and spectral class (B0-B0.5IIIe) of this system are determined and presented here for the first time. The second pulsar is shown not to be new at all, but is consistent with being SXP91.1 - a pulsar discovered at the very beginning of the 13 year long RXTE key monitoring programme of the SMC. Whilst it is theoretically possible for accreting neutron stars to change spin period so dramatically over such a short time, the X-ray and optical data available for this source suggest this spin-up is continuous during long phases of X-ray quiescence, where accretion driven spin-up of the neutron star should be minimal.

  18. Dynamics of continental accretion.

    Science.gov (United States)

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

  19. Slim accretion discs with different viscosity prescriptions

    Energy Technology Data Exchange (ETDEWEB)

    Szuszkiewicz, E. (Max-Planck-Institut fuer Physik und Astrophysik, Garching (Germany, F.R.). Inst. fuer Astrophysik)

    1990-05-15

    The variability of X-ray sources powered by accretion may be connected to thermal instabilities in the innermost parts of slim discs. The time-scales of variability predicted by the theory with the standard {alpha}-viscosity prescription agree with those observed in a wide range of sources. The amplitudes (3-4 orders of magnitude in luminosity) are correctly predicted for X-ray transient sources, but in general are too big for quasars, Seyferts, galactic black hole candidates and LMXBs. We show here that a slight modification of the viscosity prescription can offer a much better agreement with observations. (author).

  20. Analyzing the Spectra of Accreting X-Ray Pulsars

    Science.gov (United States)

    Wolff, Michael

    This proposal seeks funding for the analysis of accretion-powered X-ray pulsar spectra from NASA/ HEASARC archived X-ray data. Spectral modeling of accreting X-ray pulsars can tell us a great deal about the physical conditions in and near high mass X-ray binary systems. Such systems have accretion flows where plasma is initially channeled from an accretion disk by the strong neutron star magnetic field, eventually falling onto the magnetic polar cap of the neutron star compact object. Many of these accreting X-ray pulsars have X-ray spectra that consist of broad power-law continua with superposed cyclotron resonant scattering features indicating magnetic field strengths above 10^12 G. The energies of these cyclotron line features have recently been shown to vary with X-ray luminosity in a number of sources such as Her X-1 and V 0332+53, a phenomenon not well understood. Another recent development is the relatively new analytic model for the spectral continuum formation in accretion-powered pulsar systems developed by Becker & Wolff. In their formalism the accretion flows are assumed to go through radiation- dominated radiative shocks and settle onto the neutron star surface. The radiation field consists of strongly Comptonized bremsstrahlung emission from the entire plasma, Comptonized cyclotron emission from the de-excitations of Landau-excited electrons in the neutron star magnetic field, and Comptonized black-body emission from a thermal mound near the neutron star surface. We seek to develop the data analysis tools to apply this model framework to the X-ray data from a wide set of sources to make progress characterizing the basic accretion properties (e.g., magnetic field strength, plasma temperatures, polar cap size, accretion rate per unit area, dominance of bulk vs. thermal Comptonization) as well as understanding the variations of the cyclotron line energies with X-ray luminosity. The three major goals of our proposed work are as follows: In the first year

  1. The Magnetospheres of (Accreting Neutron Stars

    Directory of Open Access Journals (Sweden)

    Wilms J.

    2014-01-01

    Full Text Available I give an overview of the most important observational tools to study the magnetospheres of accreting neutron stars, with a focus on accreting neutron stars in high mass X-ray binary systems. Topics covered are the different types of accretion onto neutron stars and the structure of the accretion column, and how models for these can be tested with observations.

  2. A Hot and Massive Accretion Disk around the High-mass Protostar IRAS 20126+4104

    Science.gov (United States)

    Chen, Huei-Ru Vivien; Keto, Eric; Zhang, Qizhou; Sridharan, T. K.; Liu, Sheng-Yuan; Su, Yu-Nung

    2016-06-01

    We present new spectral line observations of the CH3CN molecule in the accretion disk around the massive protostar IRAS 20126+4104 with the Submillimeter Array, which, for the first time, measure the disk density, temperature, and rotational velocity with sufficient resolution (0.″37, equivalent to ˜600 au) to assess the gravitational stability of the disk through the Toomre-Q parameter. Our observations resolve the central 2000 au region that shows steeper velocity gradients with increasing upper state energy, indicating an increase in the rotational velocity of the hotter gas nearer the star. Such spin-up motions are characteristics of an accretion flow in a rotationally supported disk. We compare the observed data with synthetic image cubes produced by three-dimensional radiative transfer models describing a thin flared disk in Keplerian motion enveloped within the centrifugal radius of an angular-momentum-conserving accretion flow. Given a luminosity of 1.3 × 104 L ⊙, the optimized model gives a disk mass of 1.5 M ⊙ and a radius of 858 au rotating about a 12.0 M ⊙ protostar with a disk mass accretion rate of 3.9 × 10-5 M ⊙ yr-1. Our study finds that, in contrast to some theoretical expectations, the disk is hot and stable to fragmentation with Q > 2.8 at all radii which permits a smooth accretion flow. These results put forward the first constraints on gravitational instabilities in massive protostellar disks, which are closely connected to the formation of companion stars and planetary systems by fragmentation.

  3. Eclipse mapping of accretion discs

    OpenAIRE

    Baptista, Raymundo

    2000-01-01

    The eclipse mapping method is an inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs. In this review I present the basics of the method and discuss its different implementations. I summarize the most important results obtained to date and discuss how they have helped to improve our understanding of accretion physics, from testing the theoretical radial brightness temperature dist...

  4. Self-gravitating accretion discs

    OpenAIRE

    Lodato, G.

    2008-01-01

    I review recent progresses in the dynamics and the evolution of self-gravitating accretion discs. Accretion discs are a fundamental component of several astrophysical systems on very diverse scales, and can be found around supermassive black holes in Active Galactic Nuclei (AGN), and also in our Galaxy around stellar mass compact objects and around young stars. Notwithstanding the specific differences arising from such diversity in physical extent, all these systems share a common feature whe...

  5. The quiescent state of the accreting X-ray pulsar SAX J2103.5+4545

    CERN Document Server

    Reig, P; Zezas, A

    2014-01-01

    We present an X-ray timing and spectral analysis of the Be/X-ray binary SAX J2103.5+4545 at a time when the Be star's circumstellar disk had disappeared and thus the main reservoir of material available for accretion had extinguished. In this very low optical state, pulsed X-ray emission was detected at a level of L_X~10^{33} erg/s. This is the lowest luminosity at which pulsations have ever been detected in an accreting pulsar. The derived spin period is 351.13 s, consistent with previous observations. The source continues its overall long-term spin-up, which reduced the spin period by 7.5 s since its discovery in 1997. The X-ray emission is consistent with a purely thermal spectrum, represented by a blackbody with kT=1 keV. We discuss possible scenarios to explain the observed quiescent luminosity and conclude that the most likely mechanism is direct emission resulting from the cooling of the polar caps, heated either during the most recent outburst or via intermittent accretion in quiescence.

  6. A Hot and Massive Accretion Disk around the High-Mass Protostar IRAS 20126+4104

    CERN Document Server

    Chen, Huei-Ru Vivien; Zhang, Qizhou; Sridharan, T K; Liu, Sheng-Yuan; Su, Yu-Nung

    2016-01-01

    We present new spectral line observations of the CH3CN molecule in the accretion disk around the massive protostar IRAS 20126+4104 with the Submillimeter Array that for the first time measure the disk density, temperature, and rotational velocity with sufficient resolution (0.37", equivalent to ~600 AU) to assess the gravitational stability of the disk through the Toomre-Q parameter. Our observations resolve the central 2000 AU region that shows steeper velocity gradients with increasing upper state energy, indicating an increase in the rotational velocity of the hotter gas nearer the star. Such spin-up motions are characteristics of an accretion flow in a rotationally supported disk. We compare the observed data with synthetic image cubes produced by three-dimensional radiative transfer models describing a thin flared disk in Keplerian motion enveloped within the centrifugal radius of an angular-momentum-conserving accretion flow. Given a luminosity of 1.3x10^4 Lsun, the optimized model gives a disk mass of ...

  7. Migration of accreting giant planets

    Science.gov (United States)

    Crida, A.; Bitsch, B.; Raibaldi, A.

    2016-12-01

    We present the results of 2D hydro simulations of giant planets in proto-planetary discs, which accrete gas at a more or less high rate. First, starting from a solid core of 20 Earth masses, we show that as soon as the runaway accretion of gas turns on, the planet is saved from type I migration : the gap opening mass is reached before the planet is lost into its host star. Furthermore, gas accretion helps opening the gap in low mass discs. Consequently, if the accretion rate is limited to the disc supply, then the planet is already inside a gap and in type II migration. We further show that the type II migration of a Jupiter mass planet actually depends on its accretion rate. Only when the accretion is high do we retrieve the classical picture where no gas crosses the gap and the planet follows the disc spreading. These results impact our understanding of planet migration and planet population synthesis models. The e-poster presenting these results in French can be found here: L'e-poster présentant ces résultats en français est disponible à cette adresse: http://sf2a.eu/semaine-sf2a/2016/posterpdfs/156_179_49.pdf.

  8. How do accretion discs break?

    Science.gov (United States)

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  9. NuSTAR discovery of an unusually steady long-term spin-up of the Be binary 2RXP J130159.6-635806

    DEFF Research Database (Denmark)

    Krivonos, Roman A.; Tsygankov, Sergey S.; Lutovinov, Alexander A.

    2015-01-01

    We present spectral and timing analysis of NuSTAR observations of the accreting X-ray pulsar 2RXP J130159.6-635806. The source was serendipitously observed during a campaign focused on the gamma-ray binary PSR B1259-63 and was later targeted for a dedicated observation. The spectrum has a typical...

  10. Terrestrial Planets Accreted Dry

    Science.gov (United States)

    Albarede, F.; Blichert-Toft, J.

    2007-12-01

    Plate tectonics shaped the Earth, whereas the Moon is a dry and inactive desert. Mars probably came to rest within the first billion years of its history, and Venus, although internally very active, has a dry inferno for its surface. The strong gravity field of a large planet allows for an enormous amount of gravitational energy to be released, causing the outer part of the planetary body to melt (magma ocean), helps retain water on the planet, and increases the pressure gradient. The weak gravity field and anhydrous conditions prevailing on the Moon stabilized, on top of its magma ocean, a thick buoyant plagioclase lithosphere, which insulated the molten interior. On Earth, the buoyant hydrous phases (serpentines) produced by reactions between the terrestrial magma ocean and the wet impactors received from the outer Solar System isolated the magma and kept it molten for some few tens of million years. The elemental distributions and the range of condensation temperatures show that the planets from the inner Solar System accreted dry. The interior of planets that lost up to 95% of their K cannot contain much water. Foundering of their wet surface material softened the terrestrial mantle and set the scene for the onset of plate tectonics. This very same process may have removed all the water from the surface of Venus 500 My ago and added enough water to its mantle to make its internal dynamics very strong and keep the surface very young. Because of a radius smaller than that of the Earth, not enough water could be drawn into the Martian mantle before it was lost to space and Martian plate tectonics never began. The radius of a planet therefore is the key parameter controlling most of its evolutional features.

  11. The radiative efficiency of a radiatively inefficient accretion flow

    NARCIS (Netherlands)

    D'Angelo, C.R.; Fridriksson, J.K.; Messenger, C.; Patruno, A.

    2015-01-01

    A recent joint XMM-Newton/Nuclear Spectroscopic Telescope Array (NuSTAR) observation of the accreting neutron star Cen X-4 (LX ∼ 1033 erg s−1) revealed a hard power-law component (Γ ∼ 1-1.5) with a relatively low cut-off energy (∼10 keV), suggesting bremsstrahlung emission. The physical requirements

  12. Inner disk radius, accretion and the propeller effect in the spin-down phase of neutron stars

    CERN Document Server

    Ertan, Unal

    2015-01-01

    We have investigated the critical conditions required for an efficient steady propeller mechanism in the spin-down phases of magnetized neutron stars with optically thick accretion disks. We have shown through simple analytical calculations that: (1) the strength of the dipole field at the Alfven radius is not sufficient to sustain an efficient mass-outflow even when the magnetic dipole field lines rotate much faster than the escape speed, (2) in the spin-down phase, mass accretion onto the star could persist above a minimum disk mass-flow rate that is orders of magnitude lower than the rate corresponding to the transition between the spin-up and the spin-down states, (3) below this critical mass-flow rate, a steady propeller state could be established with a maximum inner disk radius about 25 times smaller than the Alfven radius. Our results indicate that only for spherical accretion, the inner disk radius is likely to approach the Alfven radius, and for all realistic cases, the accretion-propeller transitio...

  13. The diffusion of contaminant through an accretion disc

    Science.gov (United States)

    Clarke, C. J.; Pringle, J. E.

    1988-01-01

    The manner in which a trace contaminant diffuses through an accretion disk is calculated. For a steady disk in which the ratio of diffusivity to viscosity, zeta, is taken to be constant through the disk and for which the surface density distribution is taken to be a power law, analytic solutions to the time-dependent equations governing contaminant evolution are presented. The amount of contaminant which can diffuse its way outwards against the bulk inward accretion flow is calculated, and it is shown that this is a sensitive function of a and zeta.

  14. Bulk Comptonization by Turbulence in Accretion Disks

    CERN Document Server

    Kaufman, J

    2016-01-01

    Radiation pressure dominated accretion discs around compact objects may have turbulent velocities that greatly exceed the electron thermal velocities within the disc. Bulk Comptonization by the turbulence may therefore dominate over thermal Comptonization in determining the emergent spectrum. Bulk Comptonization by divergenceless turbulence is due to radiation viscous dissipation only. It can be treated as thermal Comptonization by solving the Kompaneets equation with an equivalent "wave" temperature, which is a weighted sum over the power present at each scale in the turbulent cascade. Bulk Comptonization by turbulence with non-zero divergence is due to both pressure work and radiation viscous dissipation. Pressure work has negligible effect on photon spectra in the limit of optically thin turbulence, and in this limit radiation viscous dissipation alone can be treated as thermal Comptonization with a temperature equivalent to the full turbulent power. In the limit of extremely optically thick turbulence, ra...

  15. Externally Fed Accretion onto Protostars

    CERN Document Server

    Dalba, Paul A

    2012-01-01

    The asymmetric molecular emission lines from dense cores reveal slow, inward motion in the clouds' outer regions. This motion is present both before and after the formation of a central star. Motivated by these observations, we revisit the classic problem of steady, spherical accretion of gas onto a gravitating point mass, but now include self-gravity of the gas and impose a finite, subsonic velocity as the outer boundary condition. We find that the accretion rate onto the protostar is lower than values obtained for isolated, collapsing clouds, by a factor that is the Mach number of the outer flow. Moreover, the region of infall surrounding the protostar spreads out more slowly, at a speed close to the subsonic, incoming velocity. Our calculation, while highly idealized, provides insight into two longstanding problems -- the surprisingly low accretion luminosities of even the most deeply embedded stellar sources, and the failure so far to detect spatially extended, supersonic infall within their parent dense ...

  16. Eclipse mapping of accretion discs

    CERN Document Server

    Baptista, R

    2000-01-01

    The eclipse mapping method is an inversion technique that makes use of the information contained in eclipse light curves to probe the structure, the spectrum and the time evolution of accretion discs. In this review I present the basics of the method and discuss its different implementations. I summarize the most important results obtained to date and discuss how they have helped to improve our understanding of accretion physics, from testing the theoretical radial brightness temperature distribution and measuring mass accretion rates to showing the evolution of the structure of a dwarf novae disc through its outburst cycle, from isolating the spectrum of a disc wind to revealing the geometry of disc spiral shocks. I end with an outline of the future prospects.

  17. Black hole accretion disc impacts

    CERN Document Server

    Pihajoki, Pauli

    2015-01-01

    We present an analytic model for computing the luminosity and spectral evolution of flares caused by a supermassive black hole impacting the accretion disc of another supermassive black hole. Our model includes photon diffusion, emission from optically thin regions and relativistic corrections to the observed spectrum and time-scales. We test the observability of the impact scenario with a simulated population of quasars hosting supermassive black hole binaries. The results indicate that for a moderate binary mass ratio of 0.3, and impact distances of 100 primary Schwarzschild radii, the accretion disc impacts can be expected to equal or exceed the host quasar in brightness at observed wavelength {\\lambda} = 510 nm up to z = 0.6. We conclude that accretion disc impacts may function as an independent probe for supermassive black hole binaries. We release the code used for computing the model light curves to the community.

  18. Accretion Disks and Dynamos: Toward a Unified Mean Field Theory

    CERN Document Server

    Blackman, Eric G

    2012-01-01

    Conversion of gravitational energy into radiation near stars and compact objects in accretion disks the origin of large scale magnetic fields in astrophysical rotators have long been distinct topics of active research in astrophysics. In semi-analytic work on both problems it has been useful to presume large scale symmetries, which necessarily results in mean field theories; magnetohydrodynamic turbulence makes the underlying systems locally asymmetric and highly nonlinear. Synergy between theory and simulations should aim for the development of practical, semi-analytic mean field models that capture the essential physics and can be used for observational modeling. Mean field dynamo (MFD) theory and alpha-viscosity accretion disc theory have exemplified such distinct pursuits. Both are presently incomplete, but 21st century MFD theory has nonlinear predictive power compared to 20th century MFD. in contrast, alpha-viscosity accretion theory is still in a 20th century state. In fact, insights from MFD theory ar...

  19. The influence of outflow in supercritical accretion flows

    CERN Document Server

    Zeraatgari, Fatemeh Zahra; Mosallanezhad, Amin

    2016-01-01

    We solve the radiation-hydrodynamic (RHD) equations of supercritical accretion flows in the presence of radiation force and outflow by using self similar solutions. Compare with the pioneer works, in this paper we consider power-law function for mass inflow rate as $ \\dot{M} \\propto r^{s} $. We found that $ s = 1 $ when the radiative cooling term is included in the energy equation. Correspondingly, the effective temperature profile with respect to the radius was obtained as $ T_{\\text{eff}} \\varpropto r^{-1/2} $. In addition, we investigated the influence of the outflow on the dynamics of the accretion flow. We also calculated the continuum spectrum emitted from the disk surface as well as the bolometric luminosity of the accretion flow. Furthermore, our results show that the advection parameter, $ f $, strongly depends on mass inflow rate.

  20. Peculiarities of the Accretion Flow in the System HL CMa

    CERN Document Server

    Semena, Andrey; Buckley, David; Lutovinov, Alexander; Breytenbach, Hannes

    2016-01-01

    The properties of the aperiodic brightness variability for the dwarf nova HL CMa are considered. The variability of the system HL CMa is shown to be suppressed at frequencies above $7\\times10^{-3}$Hz. Different variability suppression mechanisms related to the radiation reprocessing time, partial disk evaporation, and characteristic variability formation time are proposed. It has been found that the variability suppression frequency does not change when the system passes from the quiescent state to the outburst one, suggesting that the accretion flow geometry is invariable. It is concluded from the optical and X-ray luminosities of the system that the boundary layer on the white dwarf surface is optically thick in both quiescent and outburst states. The latter implies that the optically thick part of the accretion flow (disk) reaches the white dwarf surface. The accretion rate in the system, the flow geometry and temperature have been estimated from the variability power spectra and spectral characteristics i...

  1. The Influence of Black Hole Mass and Accretion Rate on the FRI/FRII Radio Galaxy Dichotomy

    Science.gov (United States)

    Wold, M.; Lacy, M.; Armus, L.

    We use medium resolution optical spectra of 3CR radio galaxies to estimate their black hole masses and accretion rates. Black hole masses are found from central stellar velocity dispersions, and accretion rates are derived from narrow emission-line luminosities. The sample covers both Fanaroff-Riley (FR) classes; the more powerful FRIIs and the less powerful FRIs. We find that FRIs and FRIIs separate in diagrams of radio luminosity and narrow-line luminosity versus black hole mass. This suggests that, at a given black hole mass, the FRIIs accrete more efficiently, or accrete more matter, than FRIs.

  2. Dissecting accretion and outflows in accreting white dwarf binaries

    CERN Document Server

    de Martino, D; Balman, S; Bernardini, F; Bianchini, A; Bode, M; Bonnet-Bidaud, J -M; Falanga, M; Greiner, J; Groot, P; Hernanz, M; Israel, G; Jose, J; Motch, C; Mouchet, M; Norton, A J; Nucita, A; Orio, M; Osborne, J; Ramsay, G; Rodriguez-Gil, P; Scaringi, S; Schwope, A; Traulsen, I; Tamburini, F

    2015-01-01

    This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of accreting white dwarfs. For a summary, we refer to the paper.

  3. Elliptical Accretion and Low Luminosity from High Accretion Rate Stellar Tidal Disruption Events

    Science.gov (United States)

    Svirski, Gilad; Piran, Tsvi; Krolik, Julian

    2017-01-01

    Models for tidal disruption events (TDEs) in which a supermassive black hole disrupts a star commonly assume that the highly eccentric streams of bound stellar debris promptly form a circular accretion disk at the pericenter scale. However, the bolometric peak luminosity of most TDE candidates, ˜ 10^{44} {erg s^{-1}}, implies that we observe only ˜1% of the energy expected from radiatively efficient accretion. Even the energy that must be lost to circularize the returning tidal flow is larger than the observed energy. Recently, Piran et al. (2015) suggested that the observed optical TDE emission is powered by shocks at the apocenter between freshly infalling material and earlier arriving matter. This model explains the small radiated energy, the low temperature, and the large radius implied by the observations as well as the t-5/3 light curve. However the question of the system's low bolometric efficiency remains unanswered. We suggest that the high orbital energy and low angular momentum of the flow make it possible for magnetic stresses to reduce the matter's already small angular momentum to the point at which it can fall ballistically into the SMBH before circularization. As a result, the efficiency is only ˜1-10% of a standard accretion disk's efficiency. Thus, the intrinsically high eccentricity of the tidal debris naturally explains why most TDE candidates are fainter than expected.

  4. Constraining the Accretion Mode in LINER 1.9s

    Science.gov (United States)

    Sabra, Bassem; Der Sahaguian, Elias; Badr, Elie

    2016-01-01

    The accretion mode and the dominant power source in low-ionization nuclear emission-line regions (LINERs), a class of active galactic nuclei (AGN), are still elusive. We focus on a sample of 22 LINER 1.9s (Ho et al. 1997), a subclass of LINERs that show broad Halpha lines, a signature of blackhole-powered accretion, to test the hypothesis that the ionizing continuum emitted by a radiatively inefficient accretion flow (RIAF) could lead to the LINER ultraviolet (UV) emission-line ratios. Optical line-ratio diagrams are a weak diagnostic tool in distinguishing between possible power sources (Sabra et al. 2003). We search the Mikulski Archive for Space Telescopes (MAST) for UV spectra of the objects in the above sample and also perform photoionization simulations using CLOUDY (Ferland et al. 2013). Unfortunately, only one object (NGC 1052; Gabel et al. 2000) of the 22 LINER 1.9s has UV spectra that cover many emission lines; the rest of the objects either do not have any UV spectra, the spectral coverage is in-adequate, or the spectra have very low signal-to-noise ratios. Our photoionization simulations set up two identical grids of clouds with a range of densities and ionization parameters. We illuminate one grid with radiation emitted by a thin accretion disk (AD) and we illuminate the other grid with radiation from a RIAF. We overplot the UV emission-line ratio predictions for AD and RIAF illumination, together with the available line ratios for NGC 1052. Initial results show that UV lines could be used as diagnostics for the accretion mode in AGN. More UV spectral coverage of LINER 1.9s is needed in order to more fully utilize the diagnostic powers of UV emission line ratios.

  5. Accretion physics: It's not U, it's B

    Science.gov (United States)

    Miller, Jon

    2017-03-01

    Black holes grow by accreting mass, but the process is messy and redistributes gas and energy into their environments. New evidence shows that magnetic processes mediate both the accretion and ejection of matter.

  6. Slim Discs with Varying Accretion Rates

    Institute of Scientific and Technical Information of China (English)

    JIAO Cheng-Liang; LU Ju-Fu

    2009-01-01

    @@ It was revealed in our previous studies that there exists a maximal possible accretion rate for slim discs with constant accretion rates because the correctly calculated vertical gravitational force can only gather some limited amount of accreted matter. Here we show that when the accretion rate is not constant and instead decreases with decreasing radius because of outflows, such that the amount of accreted matter is adjusted to be within the allowed limit, global slim disc solutions can be constructed even for the case that accretion rates at large radii apparently exceed the maximal possible value. This result further demonstrates that outflows seem to be unavoidable for accretion flows with large accretion rates at large radii.

  7. Outflow from Hot Accretion Flows

    CERN Document Server

    Yuan, Feng; Wu, Maochun

    2012-01-01

    Numerical simulations of hot accretion flows have shown that the mass accretion rate decreases with decreasing radius. Two models have been proposed to explain this result. In the adiabatic inflow-outflow solution (ADIOS), it is thought to be due to the loss of gas in outflows. In the convection-dominated accretion flow (CDAF) model, it is explained as because that the gas is locked in convective eddies. In this paper we use hydrodynamical (HD) and magnetohydrodynamical (MHD) simulations to investigate which one is physical. We calculate and compare various properties of inflow (gas with an inward velocity) and outflow (gas with an outward velocity). Systematic and significant differences are found. For example, for HD flows, the temperature of outflow is higher than inflow; while for MHD flows, the specific angular momentum of outflow is much higher than inflow. We have also analyzed the convective stability of MHD accretion flow and found that they are stable. These results suggest that systematic inward an...

  8. The Black Hole Accretion Code

    CERN Document Server

    Porth, Oliver; Mizuno, Yosuke; Younsi, Ziri; Rezzolla, Luciano; Moscibrodzka, Monika; Falcke, Heino; Kramer, Michael

    2016-01-01

    We present the black hole accretion code (BHAC), a new multidimensional general-relativistic magnetohydrodynamics module for the MPI-AMRVAC framework. BHAC has been designed to solve the equations of ideal general-relativistic magnetohydrodynamics in arbitrary spacetimes and exploits adaptive mesh refinement techniques with an efficient block-based approach. Several spacetimes have already been implemented and tested. We demonstrate the validity of BHAC by means of various one-, two-, and three-dimensional test problems, as well as through a close comparison with the HARM3D code in the case of a torus accreting onto a black hole. The convergence of a turbulent accretion scenario is investigated with several diagnostics and we find accretion rates and horizon-penetrating fluxes to be convergent to within a few percent when the problem is run in three dimensions. Our analysis also involves the study of the corresponding thermal synchrotron emission, which is performed by means of a new general-relativistic radi...

  9. Fundamental Ice Crystal Accretion Physics Studies

    Science.gov (United States)

    Struk, Peter M.; Broeren, Andy P.; Tsao, Jen-Ching; Vargas, Mario; Wright, William B.; Currie, Tom; Knezevici, Danny; Fuleki, Dan

    2012-01-01

    Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. In November 2010, two weeks of testing occurred at the NRC Research Altitude Facility utilizing a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations. Icing tests were performed at Mach numbers of 0.2 to 0.3, total pressures from 93 to 45 kPa, and total temperatures from 5 to 15 C. Ice and liquid water contents ranged up to 20 and 3 g/m3, respectively. The ice appeared well adhered to the surface in the lowest pressure tests (45 kPa) and, in a particular case, showed continuous leading-edge ice growth to a thickness greater than 15 mm in 3 min. Such widespread deposits were not observed in the highest pressure tests, where the accretions were limited to a small area around the leading edge. The suction surface was typically ice-free in the tests at high pressure, but not at low pressure. The icing behavior at high and low pressure appeared to be correlated with the wet-bulb temperature, which was estimated to be above 0 C in tests at 93 kPa and below 0 C in tests at lower pressure, the latter enhanced by more evaporative cooling of water. The authors believe that the large ice accretions observed in the low pressure tests would undoubtedly cause the aerodynamic performance of a compressor component

  10. Accretion onto a Kiselev black hole

    Energy Technology Data Exchange (ETDEWEB)

    Jiao, Lei [Hebei University, College of Physical Science and Technology, Baoding (China); Yang, Rongjia [Hebei University, College of Physical Science and Technology, Baoding (China); Hebei University, Hebei Key Lab of Optic-Electronic Information and Materials, Baoding (China)

    2017-05-15

    We consider accretion onto a Kiselev black hole. We obtain the fundamental equations for accretion without the back-reaction. We determine the general analytic expressions for the critical points and the mass accretion rate and find the physical conditions the critical points should fulfill. The case of a polytropic gas are discussed in detail. It turns out that the quintessence parameter plays an important role in the accretion process. (orig.)

  11. SMBH Spherically Symmetric Accretion Regulated by Violent Star Formation Feedback

    CERN Document Server

    Silich, S; Tenorio-Tagle, G

    2008-01-01

    The mounting evidence for violent nuclear star formation in Seyfert galaxies has led us to consider the hydrodynamics of the matter reinserted by massive stars through strong stellar winds and supernovae, under the presence of a central massive BH. We show that in all cases there is a bimodal solution strongly weighted by the location of the stagnation radius (Rst), which splits the star cluster into two different zones. Matter reinserted within the stagnation volume is to be accreted by the BH while its outer counterpart would composed a star cluster wind. The mechanical power of the latter, ensures that there is no accretion of the ISM into the BH and thus the BH accretion and its luminosity is regulated by the star formation feedback. The location of the stagnation radius is a function of three parameters: the BH mass, the mechanical power (or mass) of the star formation event and the size of the star forming region. Here we present our self-consistent, stationary solution, discuss the accretion rates and ...

  12. On rapid migration and accretion within disks around supermassive black holes

    CERN Document Server

    McKernan, B; Lyra, W; Perets, H B; Winter, L M; Yaqoob, T

    2011-01-01

    Galactic nuclei should contain a cluster of stars and compact objects in the vicinity of the central supermassive black hole due to stellar evolution, minor mergers and gravitational dynamical friction. By analogy with protoplanetary migration, nuclear cluster objects (NCOs) can migrate in the accretion disks that power active galactic nuclei by exchanging angular momentum with disk gas. Here we show that an individual NCO undergoing runaway outward migration comparable to Type III protoplanetary migration can generate an accretion rate corresponding to Seyfert AGN or quasar luminosities. Multiple migrating NCOs in an AGN disk can dominate traditional viscous disk accretion and at large disk radii, ensemble NCO migration and accretion could provide sufficient heating to prevent the gravitational instability from consuming disk gas in star formation. The magnitude and energy of the X-ray soft excess observed at ~0.1-1keV in Seyfert AGN could be explained by a small population of ~10^{2}-10^{3} accreting stella...

  13. Structures of magnetized thin accretion disks

    Institute of Scientific and Technical Information of China (English)

    LI; xiaoqing(李晓卿); JI; Haisheng(季海生)

    2002-01-01

    We investigate the magnetohydrodynamic (MHD) process in thin accretion disks. Therelevant momentum as well as magnetic reduction equations in the thin disk approximation areincluded. On the basis of these equations, we examine numerically the stationary structures, includingdistributions of the surface mass density, temperature and flow velocities of a disk around a youngstellar object (YSO). The numerical results are as follows: (i) There should be an upper limit to themagnitude of magnetic field, such an upper limit corresponds to the equipartition field. For relevantmagnitude of magnetic field of the disk's interior the disk remains approximately Keplerian. (ii) Thedistribution of effective temperature T(r) is a smoothly decreasing function of radius with power 1 corresponding to the observed radiation flux density, provided that the magnetic fieldindex γ= -1/2,is suitably chosen.

  14. Accretion Discs Show Their True Colours

    Science.gov (United States)

    2008-07-01

    Quasars are the brilliant cores of remote galaxies, at the hearts of which lie supermassive black holes that can generate enough power to outshine the Sun a trillion times. These mighty power sources are fuelled by interstellar gas, thought to be sucked into the hole from a surrounding 'accretion disc'. A paper in this week's issue of the journal Nature, partly based on observations collected with ESO's Very Large Telescope, verifies a long-standing prediction about the intensely luminous radiation emitted by these accretion discs. Uncovering the disc ESO PR Photo 21/08 Uncovering the inner disc "Astronomers were puzzled by the fact that the best models of these discs couldn't quite be reconciled with some of the observations, in particular, with the fact that these discs did not appear as blue as they should be," explains lead-author Makoto Kishimoto. Such a discrepancy could be the signal that there was something very wrong with the models. With his colleagues, he investigated this discrepancy by studying the polarised light from six quasars. This enabled them to demonstrate that the disc spectrum is as blue as predicted. "The crucial observational difficulty here has been that the disc is surrounded by a much larger torus containing hot dust, whose light partly outshines that of the disc," says Kishimoto. "Because the light coming from the disc is scattered in the disc vicinity and thus polarised, by observing only polarised light from the quasars, one can uncover the buried light from the disc." In a similar way that a fisherman would wear polarised sunglasses to help get rid of the glare from the water surface and allow him to see more clearly under the water, the filter on the telescope allowed the astronomers to see beyond surrounding clouds of dust and gas to the blue colour of the disc in infrared light. The observations were done with the FORS and ISAAC instruments on one of the 8.2-m Unit Telescopes of ESO's Very Large Telescope, located in the Atacama

  15. Revealing accretion onto black holes through X-ray reflection

    Science.gov (United States)

    Plant, D.; Fender, R.; Ponti, G.; Munoz-Darias, T.; Coriat, M.

    2014-07-01

    Understanding the dynamics behind black hole state transitions and the changes they reflect in outbursts has become long-standing problem. The X-ray reflection spectrum describes the interaction between the hard X-ray source (the power-law continuum) and the cool accretion disc it illuminates, and thus permits an indirect view of how the two evolve. We present a systematic analysis of the reflection spectrum throughout three outbursts (500+ RXTE observations) of the black hole binary GX 339-4, representing the largest study applying a self-consistent treatment of reflection to date. Particular attention is payed to the coincident evolution of the power-law and reflection, which can be used to determine the accretion geometry. The hard state is found to be distinctly reflection weak, however the ratio of reflection to power-law gradually increases as the source luminosity rises. In contrast the reflection is found dominate the power-law throughout most of the soft state, with increasing supremacy as the source decays. Using results from archival and AO-12 observations of GX 339-4 with XMM-Newton we reveal the dynamics driving this evolution and the nature of accretion onto black holes in outburst.

  16. Ringed accretion disks: equilibrium configurations

    CERN Document Server

    Pugliese, D

    2015-01-01

    We investigate a model of ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the General Relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can be then determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We pr...

  17. Dynamics of warped accretion discs

    OpenAIRE

    Tremaine, Scott; Davis, Shane W.

    2013-01-01

    Accretion discs are present around both stellar-mass black holes in X-ray binaries and supermassive black holes in active galactic nuclei. A wide variety of circumstantial evidence implies that many of these discs are warped. The standard Bardeen--Petterson model attributes the shape of the warp to the competition between Lense--Thirring torque from the central black hole and viscous angular-momentum transport within the disc. We show that this description is incomplete, and that torques from...

  18. Counter-Rotating Accretion Discs

    CERN Document Server

    Dyda, Sergei; Ustyugova, Galina V; Romanova, Marina M; Koldoba, Alexander V

    2014-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud onto the surface of an existing co-rotating disc or from the counter-rotating gas moving radially inward to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc center. We discuss high-resolution axisymmetric hydrodynamic simulations of a viscous counter-rotating disc for cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic $\\alpha-$viscosity including all terms in the viscous stress tensor. For the vertically separated components a shear layer forms between them. The middle of this layer free-falls to the disk center. The accretion rates are increased by factors $\\sim 10^2-10^4$ over that of a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dep...

  19. The genesis of Typhoon Nuri as observed during the Tropical Cyclone Structure 2008 (TCS-08 field experiment – Part 3: Dynamics of low-level spin-up during the genesis

    Directory of Open Access Journals (Sweden)

    L. L. Lussier III

    2013-10-01

    Full Text Available Aircraft reconnaissance data collected during the Tropical Cyclone Structure 2008 field campaign are used to examine further kinematical, dynamical and thermodynamical aspects of the genesis of Typhoon Nuri. Data from the first two missions into the pre-Nuri disturbance document the transition from a tropical wave to a tropical depression. Dropwindsonde-derived tangential wind profiles at several radii from the low-level circulation center indicate that the magnitude of low-level circulation increases and that the corresponding tangential velocity maximum moves inward from the first to second reconnaissance mission. To compliment these findings, a three-dimensional variational analysis incorporating both dropwindsonde and aircraft Doppler radar data is conducted. These data are used to perform circulation tendency calculations at multiple distances from the low-level circulation center. The results demonstrate a net spin-up of the system-scale circulation in the low-levels near the center and in the outer regions of the recirculating Kelvin cat's eye circulation. In these regions, the spin-up tendency due to the influx of cyclonic absolute vorticity exceeds the frictional spin-down tendency for both Nuri missions. The system-scale spin up is found to be accompanied by areas of low-level vorticity concentration through vortex-tube stretching associated with cumulus convection. The areal coverage and intensity of these high-vorticity regions increase between the first and second Nuri missions. The findings of this study are consistent in some respects to the Nuri observational analysis carried out by Raymond and Lopez (2011, but differ in their suggested key result and related scientific implication that the pre-Nuri disturbance was spinning down on the first day of observations. The findings herein strongly support a recent tropical cyclogenesis model positing that the Kelvin cat's eye circulation of the parent wave-like disturbance provides a

  20. A Simple test for the existence of two accretion modes in active galactic nuclei

    Energy Technology Data Exchange (ETDEWEB)

    Jester, Sebastian; /Fermilab

    2005-02-01

    By analogy to the different accretion states observed in black-hole X-ray binaries (BHXBs), it appears plausible that accretion disks in active galactic nuclei (AGN) undergo a state transition between a radiatively efficient and inefficient accretion flow. If the radiative efficiency changes at some critical accretion rate, there will be a change in the distribution of black hole masses and bolometric luminosities at the corresponding transition luminosity. To test this prediction, the author considers the joint distribution of AGN black hole masses and bolometric luminosities for a sample taken from the literature. The small number of objects with low Eddington-scaled accretion rates m < 0.01 and black hole masses M{sub BH} < 10{sup 9} M{sub {circle_dot}} constitutes tentative evidence for the existence of such a transition in AGN. Selection effects, in particular those associated with flux-limited samples, systematically exclude objects in particular regions of the (M{sub BH}, L{sub bol}) plane. Therefore, they require particular attention in the analysis of distributions of black hole mass, bolometric luminosity, and derived quantities like the accretion rate. The author suggests further observational tests of the BHXB-AGN unification scheme which are based on the jet domination of the energy output of BHXBs in the hard state, and on the possible equivalence of BHXB in the very high (or steep power-law) state showing ejections and efficiently accreting quasars and radio galaxies with powerful radio jets.

  1. Radio Loud AGN Unification: Connecting Jets and Accretion

    Directory of Open Access Journals (Sweden)

    Meyer Eileen T.

    2013-12-01

    Full Text Available While only a fraction of Active Galactic Nuclei are observed to host a powerful relativistic jet, a cohesive picture is emerging that radio-loud AGN may represent an important phase in the evolution of galaxies and the growth of the central super-massive black hole. I will review my own recent observational work in radio-loud AGN unification in the context of understanding how and why jets form and their the connection to different kinds of accretion and growing the black hole, along with a brief discussion of possible connections to recent modeling work in jet formation. Starting from the significant observational advances in our understanding of jetted AGN as a population over the last decade thanks to new, more sensitive instruments such as Fermi and Swift as well as all-sky surveys at all frequencies, I will lay out the case for a dichotomy in the jetted AGN population connected to accretion mode onto the black hole. In recent work, we have identified two sub-populations of radio-loud AGN which appear to be distinguished by jet structure, where low-efficiency accreting systems produce ‘weak’ jets which decelerate more rapidly than the ’strong’ jets of black holes accreting near the Eddington limit. The two classes are comprised of: (1The weak jet sources, corresponding to the less collimated, edge-darkened FR Is, with a decelerating or spine-sheath jet with velocity gradients, and (2 The strong jet sources, having fast, collimated jets, and typically displaying strong emission lines. The dichotomy in the vp-Lp plane can be understood as a "broken power sequence" in which jets exist on one branch or the other based on the particular accretion mode (Georganopolous 2011.We suggest that the intrinsic kinetic power (as measured by low-frequency, isotropic radio emission, the orientation, and the accretion rate of the SMBH system are the the fundamental axes needed for unification of radio-loud AGN by studying a well-characterized sample

  2. Accretion disc onto a static non-baryonic compact object

    OpenAIRE

    Torres, Diego F.

    2002-01-01

    We study the emissivity properties of a geometrically thin, optically thick, steady accretion disc about a static boson star. Starting from a numerical computation of the metric potentials and the rotational velocities of the particles in the vicinity of the compact object, we obtain the power per unit area, the temperature of the disc, and the spectrum of the emitted radiation. In order to see if different central objects could be actually distinguished, all these results are compared with t...

  3. Initial sizes of planetesimals and accretion of the asteroids

    Science.gov (United States)

    Weidenschilling, S. J.

    2011-08-01

    The present size frequency distribution (SFD) of bodies in the asteroid belt appears to have preserved some record of the primordial population, with an excess of bodies of diameter D ˜ 100 km relative to a simple power law. The survival of Vesta's basaltic crust also implies that the early SFD had a shallow slope in the range ˜10-100 km. (Morbidelli, A., Bottke, W.F., Nesvorny, D., Levison, H.F. [2009]. Icarus 204, 558-573) were unable to produce these features by accretion from an initial population of km-sized planetesimals. They concluded that bodies with sizes in the range ˜100-1000 km and a SFD similar to the current population were produced directly from solid particles of sub-meter scale, without experiencing accretion through intermediate sizes. We present results of new accretion simulations in the primordial asteroid region. The requisite SFD can be produced from an initial population of planetesimals of sizes ≲0.1 km, smaller than the usual assumption of km-sized bodies. The bump at D ˜ 100 km is produced by a transition from dispersion-dominated runaway growth to a regime dominated by Keplerian shear, before the formation of large protoplanetary embryos. Thus, accretion of the asteroids from an initial population of small (sub-km) planetesimals cannot be ruled out.

  4. The influence of accretion rate and metallicity on thermonuclear bursts: predictions from KEPLER models

    CERN Document Server

    Lampe, Nathanael; Galloway, Duncan K

    2015-01-01

    Using the KEPLER hydrodynamics code, 464 models of thermonuclear X-ray bursters were performed across a range of accretion rates and compositions. We present the library of simulated burst profiles from this sample, and examine variations in the simulated lightcurve for different model conditions. We find that the recurrence time varies as a power law against accretion rate, and measure its slope while mixed H/He burning is occurring for a range of metallicities, finding the power law gradient to vary from $\\eta = 1.1$ to $1.24$. We also identify the accretion rates at which mixed H/He burning stops and a transition occurs to different burning regimes. We also explore how varying the accretion rate and metallicity affects burst morphology in both the rise and tail.

  5. Black-hole masses, accretion rates and hot- and cold-mode accretion in radio galaxies at z~1

    CERN Document Server

    Fernandes, Cristina A C; Martínez-Sansigre, Alejo; Rawlings, Steve; Afonso, José; Hardcastle, Martin J; Lacy, Mark; Stevens, Jason A; Vardoulaki, Eleni

    2014-01-01

    Understanding the evolution of accretion activity is fundamental to our understanding of how galaxies form and evolve over the history of the Universe. We analyse a complete sample of 27 radio galaxies which includes both high-excitation (HEGs) and low excitation galaxies (LEGs), spanning a narrow redshift range of 0.9 < z < 1.1 and covering a factor of ~1000 in radio luminosity. Using data from the Spitzer Space Telescope combined with ground-based optical and near-infrared imaging, we show that the host galaxies have masses in the range of 10.7 < log (M /M_sun) < 12.0 with HEGs and LEGs exhibiting no difference in their mass distributions. We also find that HEGs accrete at significantly higher rates than LEGs, with the HEG/LEG division lying at an Eddington ratio of ~0.04, which is in excellent agreement with theoretical predictions of where the accretion rate becomes radiatively inefficient, thus supporting the idea of HEGs and LEGs being powered by different modes of accretion. Our study also ...

  6. AGN Variability: Probing Black Hole Accretion

    Science.gov (United States)

    Moreno, Jackeline; O'Brien, Jack; Vogeley, Michael S.; Richards, Gordon T.; Kasliwal, Vishal P.

    2017-01-01

    We combine the long temporal baseline of Sloan Digital Sky Survey (SDSS) for quasars in Stripe 82 with the high precision photometry of the Kepler/K2 Satellite to study the physics of optical variability in the accretion disk and supermassive black hole engine. We model the lightcurves directly as Continuous-time Auto Regressive Moving Average processes (C-ARMA) with the Kali analysis package (Kasliwal et al. 2016). These models are extremely robust to irregular sampling and can capture aperiodic variability structure on various timescales. We also estimate the power spectral density and structure function of both the model family and the data. A Green's function kernel may also be estimated for the resulting C-ARMA parameter fit, which may be interpreted as the response to driving impulses such as hotspots in the accretion disk. We also examine available spectra for our AGN sample to relate observed and modelled behavior to spectral properties. The objective of this work is twofold: to explore the proper physical interpretation of different families of C-ARMA models applied to AGN optical flux variability and to relate empirical characteristic timescales of our AGN sample to physical theory or to properties estimated from spectra or simulations like the disk viscosity and temperature. We find that AGN with strong variability features on timescales resolved by K2 are well modelled by a low order C-ARMA family while K2 lightcurves with weak amplitude variability are dominated by outliers and measurement errors which force higher order model fits. This work explores a novel approach to combining SDSS and K2 data sets and presents recovered characteristic timescales of AGN variability.

  7. Black hole feedback from thick accretion discs

    CERN Document Server

    Sadowski, Aleksander; Abramowicz, Marek A; Narayan, Ramesh

    2015-01-01

    We study energy flows in geometrically thick accretion discs, both optically thick and thin, using general relativistic, three-dimensional simulations of black hole accretion flows. We find that for non-rotating black holes the efficiency of the total feedback from thick accretion discs is $3\\%$ - roughly half of the thin disc efficiency. This amount of energy is ultimately distributed between outflow and radiation, the latter scaling weakly with the accretion rate for super-critical accretion rates, and returned to the interstellar medium. Accretion on to rotating black holes is more efficient because of the additional extraction of rotational energy. However, the jet component is collimated and likely to interact only weakly with the environment, whereas the outflow and radiation components cover a wide solid angle.

  8. Interpreting MAD within multiple accretion regimes

    CERN Document Server

    Mocz, Philip

    2014-01-01

    General relativistic magnetohydrodynamic (GRMHD) simulations of accreting black holes in the radiatively inefficient regime show that systems with sufficient magnetic poloidal flux become magnetically arrested disc (MAD) systems, with a well-defined relationship between the magnetic flux and the mass accretion rate. Recently, Zamaninasab (2014) report that the jet magnetic flux and accretion disc luminosity are tightly correlated over 7 orders of magnitude for a sample of 76 radio-loud active galaxies, concluding that the data are explained by the MAD mode of accretion. Their analysis assumes radiatively efficient accretion, and their sample consists primarily of radiatively efficient sources, while GRMHD simulations of MAD thus far have been carried out in the radiatively inefficient regime. We propose a model to interpret MAD systems in the context of multiple accretion regimes, and apply it to the sample in Zamaninasab (2014), along with additional radiatively inefficient sources from archival data. We sho...

  9. Power

    DEFF Research Database (Denmark)

    Elmholdt, Claus Westergård; Fogsgaard, Morten

    2016-01-01

    In this chapter, we will explore the dynamics of power in processes of creativity, and show its paradoxical nature as both a bridge and a barrier to creativity in organisations. Recent social psychological experimental research (Slighte, de Dreu & Nijstad, 2011) on the relation between power...... and creativity suggests that when managers give people the opportunity to gain power and explicate that there is reason to be more creative, people will show a boost in creative behaviour. Moreover, this process works best in unstable power hierarchies, which implies that power is treated as a negotiable...... and floating source for empowering people in the organisation. We will explore and discuss here the potentials, challenges and pitfalls of power in relation to creativity in the life of organisations today. The aim is to demonstrate that power struggles may be utilised as constructive sources of creativity...

  10. Theory of Disk Accretion onto Magnetic Stars

    Directory of Open Access Journals (Sweden)

    Lai Dong

    2014-01-01

    Full Text Available Disk accretion onto magnetic stars occurs in a variety of systems, including accreting neutron stars (with both high and low magnetic fields, white dwarfs, and protostars. We review some of the key physical processes in magnetosphere-disk interaction, highlighting the theoretical uncertainties. We also discuss some applications to the observations of accreting neutron star and protostellar systems, as well as possible connections to protoplanetary disks and exoplanets.

  11. Theory of Disk Accretion onto Magnetic Stars

    CERN Document Server

    Lai, Dong

    2014-01-01

    Disk accretion onto magnetic stars occurs in a variety of systems, including accreting neutron stars (with both high and low magnetic fields), white dwarfs, and protostars. We review some of the key physical processes in magnetosphere-disk interaction, highlighting the theoretical uncertainties. We also discuss some applications to the observations of accreting neutron stars and protostellar systems, as well as possible connections to protoplanetary disks and exoplanets.

  12. Lithium synthesis in microquasar accretion.

    Science.gov (United States)

    Iocco, Fabio; Pato, Miguel

    2012-07-13

    We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes 6Li and 7Li can be produced, the exact figures varying with the characteristics of the torus and reaching as much as 10(-2) M⊙ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis.

  13. Protostellar accretion traced with chemistry

    DEFF Research Database (Denmark)

    Frimann, Søren; Jørgensen, Jes Kristian; Dunham, Michael M.

    2017-01-01

    . Our aim is to characterise protostellar accretion histories towards individual sources by utilising sublimation and freeze-out chemistry of CO. Methods. A sample of 24 embedded protostars are observed with the Submillimeter Array (SMA) in context of the large program "Mass Assembly of Stellar Systems...... and their Evolution with the SMA" (MASSES). The size of the C18O-emitting region, where CO has sublimated into the gas-phase, is measured towards each source and compared to the expected size of the region given the current luminosity. The SMA observations also include 1.3 mm continuum data, which are used...

  14. Counter-Rotating Accretion Discs

    OpenAIRE

    Dyda, Sergei; Lovelace, Richard V. E.; Ustyugova, Galina V.; Romanova, Marina M.; Koldoba, Alexander V.

    2014-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud onto the surface of an existing co-rotating disc or from the counter-rotating gas moving radially inward to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc center. We discuss high-resolution axisymmetric hydrodynamic simulations of a viscous counter-rotating disc for cases where the tw...

  15. Cold, clumpy accretion onto an active supermassive black hole.

    Science.gov (United States)

    Tremblay, Grant R; Oonk, J B Raymond; Combes, Françoise; Salomé, Philippe; O'Dea, Christopher P; Baum, Stefi A; Voit, G Mark; Donahue, Megan; McNamara, Brian R; Davis, Timothy A; McDonald, Michael A; Edge, Alastair C; Clarke, Tracy E; Galván-Madrid, Roberto; Bremer, Malcolm N; Edwards, Louise O V; Fabian, Andrew C; Hamer, Stephen; Li, Yuan; Maury, Anaëlle; Russell, Helen R; Quillen, Alice C; Urry, C Megan; Sanders, Jeremy S; Wise, Michael W

    2016-06-09

    Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds--a departure from the 'hot mode' accretion model--although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities produce a rain of cold clouds that fall towards the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that is found at its core. The observations show that these cold clouds also fuel black hole accretion, revealing 'shadows' cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

  16. Black Hole Accretion in Gamma Ray Bursts

    Directory of Open Access Journals (Sweden)

    Agnieszka Janiuk

    2017-02-01

    Full Text Available We study the structure and evolution of the hyperaccreting disks and outflows in the gamma ray bursts central engines. The torus around a stellar mass black hole is composed of free nucleons, Helium, electron-positron pairs, and is cooled by neutrino emission. Accretion of matter powers the relativistic jets, responsible for the gamma ray prompt emission. The significant number density of neutrons in the disk and outflowing material will cause subsequent formation of heavier nuclei. We study the process of nucleosynthesis and its possible observational consequences. We also apply our scenario to the recent observation of the gravitational wave signal, detected on 14 September 2015 by the two Advanced LIGO detectors, and related to an inspiral and merger of a binary black hole system. A gamma ray burst that could possibly be related with the GW150914 event was observed by the Fermi satellite. It had a duration of about 1 s and appeared about 0.4 s after the gravitational-wave signal. We propose that a collapsing massive star and a black hole in a close binary could lead to the event. The gamma ray burst was powered by a weak neutrino flux produced in the star remnant’s matter. Low spin and kick velocity of the merged black hole are reproduced in our simulations. Coincident gravitational-wave emission originates from the merger of the collapsed core and the companion black hole.

  17. RINGED ACCRETION DISKS: EQUILIBRIUM CONFIGURATIONS

    Energy Technology Data Exchange (ETDEWEB)

    Pugliese, D.; Stuchlík, Z., E-mail: d.pugliese.physics@gmail.com, E-mail: zdenek.stuchlik@physics.cz [Institute of Physics and Research Centre of Theoretical Physics and Astrophysics, Faculty of Philosophy and Science, Silesian University in Opava, Bezručovo náměstí 13, CZ-74601 Opava (Czech Republic)

    2015-12-15

    We investigate a model of a ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the general relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can then be determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We present also a perturbation analysis based on evolution of the oscillating components of the ringed disk. The dynamics of the unstable phases of the ringed disk evolution seems to be promising in relation to high-energy phenomena demonstrated in active galactic nuclei.

  18. Relativistic reverberation in the accretion flow of a tidal disruption event.

    Science.gov (United States)

    Kara, Erin; Miller, Jon M; Reynolds, Chris; Dai, Lixin

    2016-07-21

    Our current understanding of the curved space-time around supermassive black holes is based on actively accreting black holes, which make up only ten per cent or less of the overall population. X-ray observations of that small fraction reveal strong gravitational redshifts that indicate that many of these black holes are rapidly rotating; however, selection biases suggest that these results are not necessarily reflective of the majority of black holes in the Universe. Tidal disruption events, where a star orbiting an otherwise dormant black hole gets tidally shredded and accreted onto the black hole, can provide a short, unbiased glimpse at the space-time around the other ninety per cent of black holes. Observations of tidal disruptions have hitherto revealed the formation of an accretion disk and the onset of an accretion-powered jet, but have failed to reveal emission from the inner accretion flow, which enables the measurement of black hole spin. Here we report observations of reverberation arising from gravitationally redshifted iron Kα photons reflected off the inner accretion flow in the tidal disruption event Swift J1644+57. From the reverberation timescale, we estimate the mass of the black hole to be a few million solar masses, suggesting an accretion rate of 100 times the Eddington limit or more. The detection of reverberation from the relativistic depths of this rare super-Eddington event demonstrates that the X-rays do not arise from the relativistically moving regions of a jet, as previously thought.

  19. LMXB X-ray Transients: Revealing Basic Accretion Parameters in Non-stationary Regimes

    Science.gov (United States)

    Yu, Wenfei; Yan, Zhen; Zhang, Hui; Zhang, Wenda

    2014-08-01

    X-ray observations of low mass X-ray binaries(LMXBs), especially those black hole transient systems, have been very important in shaping up our understanding of black hole accretion and testing accretion theory in a broad range of accretion regimes. We show strong evidence for non-stationary accretion regimes in the X-ray observations of spectral states and multi-wavelength observations of disk-jet coupling in more than 100 outbursts of 36 black hole and neutron star transients in the past decade or so. The occurrence of spectral state transitions and the peak episodic jet power during the rising phase of transient outbursts are found correlated with rate-of-increase of the X-ray luminosity, indicating the rate-of-change of the mass accretion rate, in addition to the mass accretion rate, must be considered when interpreting observations of spectral state transitions and disk-jet coupling in these X-ray transients. This is supported by observations since the increase of the mass accretion rate due to its rate-of-change on the observational time scale of interest is significant during outbursts.

  20. Unification of Radio Galaxies and their Accretion Jet Properties

    Indian Academy of Sciences (India)

    Qingwen Wu; Ya-Di Xu; Xinwu Cao

    2011-03-01

    We investigate the relation between black hole mass, bh, and jet power, jet, for a sample of BL Lacs and radio quasars. We find that BL Lacs are separated from radio quasars by the FR I/II dividing line in bh-jet plane, which strongly supports the unification scheme of FR I/BL Lac and FR II/radio quasar. The Eddington ratio distribution of BL Lacs and radio quasars exhibits a bimodal nature with a rough division at bol/Edd ∼ 0.01, which imply that they may have different accretion modes. We calculate the jet power extracted from advection-dominated accretion flow (ADAF), and find that it requires dimensionless angular momentum of black hole ≃ 0.9 - 0.99 to reproduce the dividing line between FR I/II or BL Lac/radio quasar if dimensionless accretion rate $\\dot{m} = 0.01$ is adopted, which is required by the above bimodal distribution of Eddington ratios. Our results suggest that black holes in radio galaxies are rapidly spinning.

  1. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni.

    Science.gov (United States)

    Muñoz-Darias, T; Casares, J; Mata Sánchez, D; Fender, R P; Armas Padilla, M; Linares, M; Ponti, G; Charles, P A; Mooley, K P; Rodriguez, J

    2016-06-02

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10(-8) solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.

  2. Toward a New Paradigm for the Unification of Radio Loud AGN and its Connection to Accretion

    Science.gov (United States)

    Georganpoulos, Markos; Meyer, Eileen T.; Fossati, Giovanni; Lister, Matthew L.

    2012-01-01

    We recently argued [21J that the collective properties. of radio loud active galactic nuclei point to the existence of two families of sources, one of powerful sources with single velocity jets and one of weaker jets with significant velocity gradients in the radiating plasma. These families also correspond to different accretion modes and therefore different thermal and emission line intrinsic properties: powerful sources have radiatively efficient accretion disks, while in weak sources accretion must be radiatively inefficient. Here, after we briefly review of our recent work, we present the following findings that support our unification scheme: (i) along the broken sequence of aligned objects, the jet kinetic power increases. (ii) in the powerful branch of the sequence of aligned objects the fraction of BLLs decreases with increasing jet power. (iii) for powerful sources, the fraction of BLLs increases for more un-aligned objects, as measured by the core to extended radio emission. Our results are also compatible with the possibility that a given accretion power produces jets of comparable kinetic power.

  3. WARPING AND PRECESSION IN EXTRAGALACTIC MASER ACCRETION DISCS

    Directory of Open Access Journals (Sweden)

    A. Caproni

    2009-01-01

    Full Text Available Interferometric maser observations have been used to probe the physical conditions of extragalactic accretion discs at sub-parsec scales. The inferred kinematic of the water maser spots presents small deviations from Keplerian motions, which have been attributed to the warping and twisting of the parsec-scale disc. However, their physical origin is still a matter of debate in the literature. Motivated by this, we analyzed the general relativistic Bardeen-Petterson e ect, driven by a Kerr black hole, as the potential physical mechanism responsible for the disc warping and precession in the nearby Seyfert 2 galaxies NGC 1068 and NGC 4258. Assuming a power-law accretion disc, whose parameters were constrained by the observational data, we derived the basic quantities concerning the Bardeen-Petterson e ect for both sources. Some consequences from this peculiar relativistic mechanism are also presented in this work.

  4. Retrograde versus Prograde Models of Accreting Black Holes

    Directory of Open Access Journals (Sweden)

    David Garofalo

    2013-01-01

    Full Text Available There is a general consensus that magnetic fields, accretion disks, and rotating black holes are instrumental in the generation of the most powerful sources of energy in the known universe. Nonetheless, because magnetized accretion onto rotating black holes involves both the complications of nonlinear magnetohydrodynamics that currently cannot fully be treated numerically, and uncertainties about the origin of magnetic fields that at present are part of the input, the space of possible solutions remains less constrained. Consequently, the literature still bears witness to the proliferation of rather different black hole engine models. But the accumulated wealth of observational data is now sufficient to meaningfully distinguish between them. It is in this light that this critical paper compares the recent retrograde framework with standard “spin paradigm” prograde models.

  5. Foundations of Black Hole Accretion Disk Theory.

    Science.gov (United States)

    Abramowicz, Marek A; Fragile, P Chris

    2013-01-01

    This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).

  6. Accretion, primordial black holes and standard cosmology

    Indian Academy of Sciences (India)

    B Nayak; P Singh

    2011-01-01

    Primordial black holes evaporate due to Hawking radiation. We find that the evaporation times of primordial black holes increase when accretion of radiation is included. Thus, depending on accretion efficiency, more primordial black holes are existing today, which strengthens the conjecture that the primordial black holes are the proper candidates for dark matter.

  7. State Transitions of Black Hole Accretion Flows

    Institute of Scientific and Technical Information of China (English)

    卢炬甫; 潘刘彬

    2001-01-01

    We show that the thermal instability-triggered transition from the state of the Shakura-Sunyaev disc to the state of the advection-dominated accretion flow is possible for black hole accretion flows composed of two-temperature plasma with bremsstrahlung and synchrotron radiation and Comptonization.

  8. Critical condition for the propeller effect in systems with magnetized neutron stars accreting from geometrically thin accretion disks

    Science.gov (United States)

    Ertan, Unal

    2016-07-01

    The inner disk radius around a magnetized neutron star in the spin-down phase is usually assumed to be close to the radius at which the viscous and magnetic stresses are balanced. With different assumptions, this radius is estimated to be very close the Alfven radius. Furthermore, it is commonly assumed that the propeller mechanism can expel the matter from the system when this radius is found to be greater than the co-rotation radius. In the present work, we have shown with simple analytical calculations from the first principles that a steady-state propeller mechanism cannot be established at the radius where the viscous and the magnetic torques are balanced. We have found that a steady-state propeller phase can be built up with an inner disk radius that is at least ~10 - 30 times smaller than the Alfven radius depending on the current mass-flow rate of the disk, the field strength and the rotational period of the source. This result also indicates that the critical accretion rate for the accretion-propeller transition is orders of magnitude smaller than the rate found by equating the Alfven and the co-rotation radii. Our results are consistent with the properties of recently discovered transitional millisecond pulsars which show transitions between the rotational powered radio pulsar and the accretion powered X-ray pulsar states.

  9. Accretion by a Neutron Star Moving at a High Kick Velocity in the Supernova Ejecta

    Institute of Scientific and Technical Information of China (English)

    Xu Zhang; Ye Lu; Yong-Heng Zhao

    2007-01-01

    We suggest a two-dimensional time dependent analytic model to describe the accretion of matter onto a neutron star moving at a high speed across the ejecta left in the aftermath of a supernova explosion. The formation of a strange star resulting from the accretion is also addressed. The newborn neutron star is assumed to move outward at a kick velocity of vns ~ 103 km s-1, and the accretion flow is treated as a dust flow. When the neutron star travels across the ejecta with high speed, it sweeps up material, and when the accreted mass has reached a critical value, the neutron star will undergo a phase transition,for instance, to become a strange star. Our results show that the accretion rate decreases in a complicated way in time, not just a power law dependence: it drops much faster than the power law derived by Colpi et al. We also found that the total accreted mass and the phase transition of the neutron star depend sensitively on the velocity of supernova ejecta.

  10. Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AA Tau

    Science.gov (United States)

    Donati, J.-F.; Skelly, M. B.; Bouvier, J.; Gregory, S. G.; Grankin, K. N.; Jardine, M. M.; Hussain, G. A. J.; Ménard, F.; Dougados, C.; Unruh, Y.; Mohanty, S.; Aurière, M.; Morin, J.; Farès, R.; MAPP Collaboration

    2010-12-01

    From observations collected with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope (CFHT) and with the NARVAL spectropolarimeter at the Télescope Bernard Lyot (TBL), we report the detection of Zeeman signatures on the prototypical classical T Tauri star AA Tau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AA Tau. We find that AA Tau hosts a 2-3 kG magnetic dipole tilted at ≃20° to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AA Tau host large cool spots at photospheric level and accretion regions at chromospheric level. The accretion rate at the surface of AA Tau at the time of our observations (estimated from the emission in the He I D3 line mainly) is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2 in average (in logarithmic scale and in M⊙ yr-1); this is an order of magnitude smaller than the disc accretion rate at which the magnetic truncation radius (below which the disc is disrupted by the stellar magnetic field) matches the corotation radius (where the Keplerian period equals the stellar rotation period) - a necessary condition for accretion to occur. It suggests that AA Tau is largely in the propeller regime, with most of the accreting material in the inner disc regions being expelled outwards and only a small fraction accreted towards the surface of the star. The strong variability in the observed surface mass accretion rate and the systematic time-lag of optical occultations (by the warped accretion disc) with respect to magnetic and accretion-powered emission maxima also support this conclusion. Our results imply that AA Tau is being actively spun-down by the star/disc magnetic coupling and appears as an ideal laboratory for studying angular momentum losses of forming suns

  11. Rapid growth of seed black holes in the early universe by supra-exponential accretion.

    Science.gov (United States)

    Alexander, Tal; Natarajan, Priyamvada

    2014-09-12

    Mass accretion by black holes (BHs) is typically capped at the Eddington rate, when radiation's push balances gravity's pull. However, even exponential growth at the Eddington-limited e-folding time t(E) ~ few × 0.01 billion years is too slow to grow stellar-mass BH seeds into the supermassive luminous quasars that are observed when the universe is 1 billion years old. We propose a dynamical mechanism that can trigger supra-exponential accretion in the early universe, when a BH seed is bound in a star cluster fed by the ubiquitous dense cold gas flows. The high gas opacity traps the accretion radiation, while the low-mass BH's random motions suppress the formation of a slowly draining accretion disk. Supra-exponential growth can thus explain the puzzling emergence of supermassive BHs that power luminous quasars so soon after the Big Bang.

  12. Rapid growth of seed black holes in the early universe by supra-exponential accretion

    CERN Document Server

    Alexander, Tal

    2014-01-01

    Mass accretion by black holes (BHs) is typically capped at the Eddington rate, when radiation's push balances gravity's pull. However, even exponential growth at the Eddington-limited e-folding time t_E ~ few x 0.01 Gyr, is too slow to grow stellar-mass BH seeds into the supermassive luminous quasars that are observed when the universe is 1 Gyr old. We propose a dynamical mechanism that can trigger supra-exponential accretion in the early universe, when a BH seed is trapped in a star cluster fed by the ubiquitous dense cold gas flows. The high gas opacity traps the accretion radiation, while the low-mass BH's random motions suppress the formation of a slowly-draining accretion disk. Supra-exponential growth can thus explain the puzzling emergence of supermassive BHs that power luminous quasars so soon after the Big Bang.

  13. Pycnonuclear burning and accreting neutron stars

    CERN Document Server

    Yakovlev, D G

    2002-01-01

    We outline the phenomenon of deep crustal heating in transiently accreting neutron stars. It is produced by nuclear transformations (mostly, by pycnonuclear reactions) in accreted matter while this matter sinks to densities rho > 10^{10} g/cc under the weight of freshly accreted material. We consider then thermal states of transiently accreting neutron stars (with mean mass accretion rates \\dot{M}=(10^{-14}-10^{-9}) M_\\odot/yr) determined by deep crustal heating. In a simplified fashion we study how the thermal flux emergent from such stars depends on the properties of superdense matter in stellar interiors. We analyze the most important regulators of the thermal flux: strong superfluidity in the cores of low-mass stars and fast neutrino emission (in nucleon, pion-condensed, kaon-condensed, or quark phases of dense matter) in the cores of high-mass stars. We compare the results with observations of soft X-ray transients in quiescent states.

  14. Accretion and Outflow Activity in Brown Dwarfs

    CERN Document Server

    Riaz, B

    2013-01-01

    An investigation of the magnetospheric accretion and outflow signatures in sub-stellar objects is a natural extension of similar studies conducted on classical T Tauri stars (CTTS), and helps understand if brown dwarfs (BDs) follow the same formation mechanism as stars. Over the past decade, evidence for accretion in very low-mass stars (VLMs) and BDs has been accumulated using various techniques, which indicates that the overall accretion characteristics are continuous across the sub-stellar boundary. Outflow activity in VLMs and BDs has been confirmed based on spectro-astrometry of forbidden emission lines observed in the optical, and in millimetre continuum images of CO J=2-1 emission. This review summarizes the past and current state of observational work on accretion and outflow activity in VLMs and BDs, particularly with the advent of new instruments such as VLT/X-Shooter which has allowed the study of several accretion and outflow indicators over a wider wavelength range.

  15. Spiral shocks and accretion in discs

    Energy Technology Data Exchange (ETDEWEB)

    Spruit, H.C.; Matsuda, T.; Inoue, M.; Sawada, K.

    1987-12-01

    Recent numerical and analytical results on disc-like accretion with shock waves as the only dissipation mechanism are compared. The global properties of the process are similar to those of the viscous (..cap alpha..) disc model, but precise values of the effective ..cap alpha.. value as a function of the accretion rate can be calculated. At low values of the ratio of specific heats (..gamma.. < 1.45) accretion is possible without radiative losses. Such adiabatic accretion can occur in practice at high accretion rates on to low mass objects and may be important in the formation of planets. Following previous authors, it is pointed out that non-axisymmetric perturbations in the outer parts of a disc increase in amplitude as they propagate in and cause spiral shocks more easily in a disc than perturbations originating in the inner parts.

  16. Late Accretion and the Late Veneer

    CERN Document Server

    Morbidelli, Alessandro

    2014-01-01

    The concept of Late Veneer has been introduced by the geochemical community to explain the abundance of highly siderophile elements in the Earth's mantle and their chondritic proportions relative to each other. However, in the complex scenario of Earth accretion, involving both planetesimal bombardment and giant impacts from chondritic and differentiated projectiles, it is not obvious what the "Late Veneer" actually corresponds to. In fact, the process of differentiation of the Earth was probably intermittent and there was presumably no well-defined transition between an earlier phase where all metal sunk into the core and a later phase in which the core was a closed entity separated from the mantle. In addition, the modellers of Earth accretion have introduced the concept of "Late Accretion", which refers to the material accreted by our planet after the Moon-forming event. Characterising Late Veneer, Late Accretion and the relationship between the two is the major goal of this chapter.

  17. X-shooter study of accretion in Chamaeleon I. II. A steeper increase of accretion with stellar mass for very low-mass stars?

    Science.gov (United States)

    Manara, C. F.; Testi, L.; Herczeg, G. J.; Pascucci, I.; Alcalá, J. M.; Natta, A.; Antoniucci, S.; Fedele, D.; Mulders, G. D.; Henning, T.; Mohanty, S.; Prusti, T.; Rigliaco, E.

    2017-08-01

    The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star-forming region carried out using spectra taken with the ESO VLT/X-shooter spectrograph. The sample is nearly complete down to stellar masses (M⋆) 0.1 M⊙ for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broadband flux-calibrated medium resolution spectrum. The correlation between accretion luminosity to stellar luminosity, and of mass accretion rate to stellar mass in the logarithmic plane yields slopes of 1.9 ± 0.1 and 2.3 ± 0.3, respectively. These slopes and the accretion rates are consistent with previous results in various star-forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity lower than 0.45 L⊙ and for stellar masses lower than 0.3 M⊙ is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane that are empty of objects: one region at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second region is located just above the observational limits imposed by chromospheric emission, at M⋆ 0.3 - 0.4 M⊙. These are typical masses where photoevaporation is known to be effective. The mass accretion rates of this region are 10-10M⊙/yr, which is compatible with the value expected for photoevaporation to rapidly dissipate the inner disk. This work is

  18. 星载静电加速度计加转回路建模与分析%Modeling and analysis for a spin-up loop of electrostatic space accelerometer

    Institute of Scientific and Technical Information of China (English)

    韩丰田; 付荔; 贺晓霞

    2011-01-01

    Two rotating bodies with the same material but quite different spin angular momentum are utilized to conduct a free fall experiment for testing a novel equivalence principle in space. A conceptual differential electrostatic accelerometer with a variable-capacitance motor based spin-up loop is proposed to meet the demand of the scientific objective in this space mission. The analytical torque model is obtained by calculating the capacitance between the stator electrodes and rotor. The dynamics of the spin-up loop is developed by further estimating the effects of slide-film air damping and magnetic damping on the rotor in an effort to stimulate the start-up response. The simulated results indicate that it will experience a time of 9.8 days to spin up the rotor speed from 0 to 10 000 rpm. The proposed spin-up scheme behaves compact electrode structure by integrated in levitation electrode cylinder, and is superior to the traditional scheme in ground applications by eliminating inherent electromagnetic disturbance.%新型等效原理实验通过检验两个相同材料但自旋运动有显著差异的宏观物体的自由落体运动来验证等效原理可能存在的破坏.根据新型等效原理空间实验的科学目标,提出了一种星载差分静电加速度计方案,设计了基于可变电容式电机原理的静电加转回路;在分析了作用于转子上的静电加转力矩、残余气体阻尼及磁场阻尼的基础上,建立了转子加转回路的动力学模型并进行了仿真分析;仿真结果表明,启动过程使转子达到目标转速(10000 rpm)的启动时间为9.8天.静电加转方法可在电极筒上同时配置加转电极与悬浮电极,结构紧凑,同时避免了传统的异步电机加转产生的电磁干扰.

  19. Superhumps, resonances and accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Whitehurst, R.; King, A. (Leicester Univ. (UK). Dept. of Astronomy)

    1991-03-01

    The structure of accretion discs within binary systems is shown to be influenced by the excitation of resonances within the disc. Of particular importance is that near the 3:1 commensurability with the stars' orbit. This can be used to explain the superhump phenomenon of SU Ursae Majoris dwarf novae in superoutburst. This resonance can only appear for mass ratios which satisfy M{sub 2}/M{sub 1} < {approx equal} 0.25-0.33: for larger mass ratios the available resonances are weaker and of the wrong form to produce the superhump phenomenon. The mass-transfer stream is shown to be an important contributor to the growth rate of the resonance. (author).

  20. Protoplanetary Accretion by Collisional Fusion

    CERN Document Server

    Wettlaufer, J S

    2009-01-01

    The formation of a solar system is believed to have followed a multi-stage process around a protostar. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag; there is a ``bottleneck'' at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Thus, successful planetary accretion requires rapid planetesimal growth to km scale. A commonly accepted picture is that for collisional velocities $V_c$ above a certain threshold collisional velocity, ${V_{th}} \\sim$ 0.1-10 cm s$^{-1}$, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all collisions the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt explicitly with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter--p...

  1. Heating of protostellar accretion disks

    Science.gov (United States)

    de Campos, R. R.; Jatenco-Pereira, V.

    2017-07-01

    The magneto-rotational instability (MRI) is believed to be the mechanism responsible for a magneto-hydrodynamic turbulence that could lead to the accretion observed in protoplanetary disks. The need of a minimum amount of ionization in protostellar accretion disks is necessary for the MRI to take place. There are in the literature several studies that include the damping of Alfvén waves as an additional heating source besides the viscous heating mechanism in a geometrically thin and optically thick disk. The damping of the waves transfers energy to the disk increasing the temperature and consequently its ionization fraction, making possible the presence of the MRI in a large part of the disk. We analyzed the contribution of non-ideal effects such as Ohmic and ambipolar diffusion for the disk heating and compare these heating rates with those obtained by damping of Alfvén waves. In order to study these non-ideal effects, we have estimated the radiation emission of each effect through the energy conservation equation, and associated each emission with a black body radiation, which enabled us to assign a temperature contribution of each effect. Using the ATHENA code we were able to simulate the disk at different radial distances, and estimate the electric current density needed to calculate the radiation emission associated with each effect. Once we have those data, we were able to compare the results with other heating sources, like viscosity and Alfvén waves damping, and we concluded that the Ohmic and ambipolar diffusions do not heat the disk in any significant way.

  2. Accreting Binary Populations in the Earlier Universe

    Science.gov (United States)

    Hornschemeier, Ann

    2010-01-01

    It is now understood that X-ray binaries dominate the hard X-ray emission from normal star-forming galaxies. Thanks to the deepest (2-4 Ms) Chandra surveys, such galaxies are now being studied in X-rays out to z approximates 4. Interesting X-ray stacking results (based on 30+ galaxies per redshift bin) suggest that the mean rest-frame 2-10 keV luminosity from z=3-4 Lyman break galaxies (LBGs), is comparable to the most powerful starburst galaxies in the local Universe. This result possibly indicates a similar production mechanism for accreting binaries over large cosmological timescales. To understand and constrain better the production of X-ray binaries in high-redshift LBGs, we have utilized XMM-Newton observations of a small sample of z approximates 0.1 GALEX-selected Ultraviolet-Luminous Galaxies (UVLGs); local analogs to high-redshift LBGs. Our observations enable us to study the X-ray emission from LBG-like galaxies on an individual basis, thus allowing us to constrain object-to-object variances in this population. We supplement these results with X-ray stacking constraints using the new 3.2 Ms Chandra Deep Field-South (completed spring 2010) and LBG candidates selected from HST, Swift UVOT, and ground-based data. These measurements provide new X-ray constraints that sample well the entire z=0-4 baseline

  3. Glancing through the accretion column of EXO 2030+375

    Science.gov (United States)

    Ferrigno, Carlo; Pjanka, Patryk; Bozzo, Enrico; Klochkov, Dmitry; Ducci, Lorenzo; Zdziarski, Andrzej A.

    2016-09-01

    Context. The current generation of X-ray instruments progressively reveals more and more details about the complex magnetic field topology and the geometry of the accretion flows in highly magnetized accretion-powered pulsars. Aims: We took advantage of the large collecting area and good timing capabilities of the EPIC cameras onboard XMM-Newton to investigate the accretion geometry onto the magnetized neutron star hosted in the high-mass X-ray binary EXO 2030+375 during the rise of a source type I outburst in 2014. Methods: We carried out a timing and spectral analysis of the XMM-Newton observation as a function of the neutron star spin phase. We used a phenomenological spectral continuum model comprising the required fluorescence emission lines. Two neutral absorption components are present: one covering the source fully, one only partially. The same analysis was also carried out on two Suzaku observations of the source performed during outbursts in 2007 and 2012, to search for possible spectral variations at different luminosities. Results: The XMM-Newton data caught the source at an X-ray luminosity of 2 × 1036 erg s-1 and revealed a narrow dip-like feature in its pulse profile that was never reported before. The width of this feature corresponds to about one hundredth of the neutron star spin period. Based on the results of the phase-resolved spectral analysis we suggest that this feature can be ascribed to the self-obscuration of the accretion stream passing in front of the observer line of sight. We inferred from the Suzaku observation carried out in 2007 that the self-obscuration of the accretion stream might produce a significantly wider feature in the neutron star pulsed profile at higher luminosities (≳2 × 1037 erg s-1). Conclusions: This discovery allowed us to derive additional constraints on the physical properties of the accretion flow in this object at relatively small distances from the neutron star surface. The narrow dip-like feature in the

  4. Pulsed Accretion in a Variable Protostar

    CERN Document Server

    Muzerolle, James; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-01

    Periodic increases in luminosity arising from variable accretion rates have been predicted for some close pre-main sequence binary stars as they grow from circumbinary disks. The phenomenon is known as "pulsed accretion" and can affect the orbital evolution and mass distribution of young binaries, as well as the potential for planet formation in the circumbinary environment. Accretion variability is a common feature of young stars, with a large range of amplitudes and timescales as measured from multi-epoch observations at optical and infrared wavelengths. Periodic variations consistent with pulsed accretion have been seen in only a few young binaries via optical accretion tracers, albeit intermittently with accretion luminosity variations ranging from 0-50 percent from orbit to orbit. Here we report on a young protostar (age ~10^5 yr) that exhibits periodic variability in which the infrared luminosity increases by a factor of 10 in roughly one week every 25.34 days. We attribute this to pulsed accretion asso...

  5. AGN jets driven stochastic cold accretion in cluster cores

    CERN Document Server

    Prasad, Deovrat; Babul, Arif

    2016-01-01

    Several arguments suggest that stochastic condensation of cold gas and its accretion onto the central supermassive black hole (SMBH) is essential for active galactic nuclei (AGN) feedback to work in the most massive galaxies that lie at the centers of galaxy clusters. Our 3-D AGN jet-ICM (intracluster medium) simulations show that the angular momentum of the cold gas crossing $\\lesssim 1$ kpc is essentially isotropic. With almost equal mass in clockwise and counter-clockwise orientations, we expect a cancellation of angular momentum on roughly the dynamical time. This means that a compact accretion flow with a short viscous time ought to form, through which enough accretion power can be channeled into jet mechanical energy sufficiently quickly to prevent a cooling flow. The inherent stochasticity, expected in feedback cycles driven by cold gas condensation, gives rise to a large variation in the cold gas mass at the centers of galaxy clusters, for similar cluster and SMBH masses, in agreement with the observa...

  6. Low radiative efficiency accretion in the nuclei of elliptical galaxies

    CERN Document Server

    Matteo, T D; Allen, S W; Narayan, R; Fabian, A C

    1999-01-01

    The discovery of hard, power-law X-ray emission from a sample of six nearby elliptical galaxies, including the dominant galaxies of the Virgo,Fornax and Centaurus clusters (M87, NGC 1399 and NGC 4696, respectively), and NGC 4472, 4636 and 4649 in the Virgo cluster, has important implications for the study of quiescent supermassive black holes. We describe how the broad band spectral energy distributions of these galaxies, which accrete from their hot gaseous halos at rates comparable to their Bondi rates, can be explained by low-radiative efficiency accretion flows in which a significant fraction of the mass, angular momentum and energy is removed from the flows by winds. The observed suppression of the synchrotron component in the radio band and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low-radiative efficiencies. We briefly discuss an alternative model ...

  7. Observational evidence for matter propagation in accretion flows

    CERN Document Server

    Revnivtsev, M; Kniazev, A; Burenin, R; Buckley, D A H; Churazov, E

    2010-01-01

    We study simultaneous X-ray and optical observations of three intermediate polars EX Hya, V1223 Sgr and TV Col with the aim to understand the propagation of matter in their accretion flows. We show that in all cases the power spectra of flux variability of binary systems in X-rays and in optical band are similar to each other and the majority of X-ray and optical fluxes are correlated with time lag <1 sec. These findings support the idea that optical emission of accretion disks, in these binary systems,largely originates as reprocessing of X-ray luminosity of their white dwarfs. In the best obtained dataset of EX Hya we see that the optical lightcurve unambiguously contains some component, which leads the X-ray emission by ~7 sec. We interpret this in the framework of the model of propagating fluctuations and thus deduce the time of travel of the matter from the innermost part of the truncated accretion disk to the white dwarf surface. This value agrees very well with the time expected for matter threaded ...

  8. Ice accretion modeling for wind turbine rotor blades

    Energy Technology Data Exchange (ETDEWEB)

    Chocron, D.; Brahimi, T.; Paraschivoiu, I.; Bombardier, J.A. [Ecole Polytechnique de Montreal (Canada)

    1997-12-31

    The increasing application of wind energy in northern climates implies operation of wind turbines under severe atmospheric icing conditions. Such conditions are well known in the Scandinavian countries, Canada and most of Eastern European countries. An extensive study to develop a procedure for the prediction of ice accretion on wind turbines rotor blades appears to be essential for the safe and economic operation of wind turbines in these cold regions. The objective of the present paper is to develop a computer code capable of simulating the shape and amount of ice which may accumulate on horizontal axis wind turbine blades when operating in icing conditions. The resulting code is capable to predict and simulate the formation of ice in rime and glaze conditions, calculate the flow field and particle trajectories and to perform thermodynamic analysis. It also gives the possibility of studying the effect of different parameters that influence ice formation such as temperature, liquid water content, droplet diameter and accretion time. The analysis has been conducted on different typical airfoils as well as on NASA/DOE Mod-0 wind turbine. Results showed that ice accretion on wind turbines may reduce the power output by more than 20%.

  9. Correlations among Jet, Accretion Disk, and Broad Line Region of Flat Spectrum Radio Quasars

    CERN Document Server

    Zhang, Jin; He, Jian-Jian; Liang, En-Wei; Zhang, Shuang-Nan

    2015-01-01

    The SEDs of 18 GeV FSRQs are collected and compiled from literature, in which both the jet emission and the accretion disk radiation can be observed, in order to investigate the correlations among their jet power (P_jet), accretion disk luminosity (L_disk), and luminosity of broad line region (BLR, L_BLR). On the basis of the SED fits with the jet radiation and accretion disk radiation models, we calculate P_jet and L_disk. No correlation between P_jet with either L_disk or L_BLR is found. With a sub-sample of L_BLR for 13 GeV FSRQs, it is observed that L_BLR is strongly correlated with their L_disk. We also study the BLR covering factors of the GeV FSRQs in our sample, averagely which are smaller than that of the large samples of radio-loud and radio-quiet quasars. P_jet of some GeV FSRQs is higher than L_disk, but P_jet of all the GeV FSRQs is lower than the accretion power of black hole (BH), which is estimated by \\dot{M}c^2=L_disk/0.1, indicating that the total accretion power of BH is sufficient to drive...

  10. Accretion in Saturn's F Ring

    Science.gov (United States)

    Meinke, B. K.; Esposito, L. W.; Stewart, G.

    2012-12-01

    Saturn's F ring is the solar system's principal natural laboratory for direct observation of accretion and disruption processes. The ring resides in the Roche zone, where tidal disruption competes with self-gravity, which allows us to observe the lifecycle of moonlets. Just as nearby moons create structure at the B ring edge (Esposito et al. 2012) and the Keeler gap (Murray 2007), the F ring "shepherding" moons Prometheus and Pandora stir up ring material and create observably changing structures on timescales of days to decades. In fact, Beurle et al (2010) show that Prometheus makes it possible for "distended, yet gravitationally coherent clumps" to form in the F ring, and Barbara and Esposito (2002) predicted a population of ~1 km bodies in the ring. In addition to the observations over the last three decades, the Cassini Ultraviolet Imaging Spectrograph (UVIS) has detected 27 statistically significant features in 101 occultations by Saturn's F ring since July 2004. Seventeen of those 27 features are associated with clumps of ring material. Two features are opaque in occultation, which makes them candidates for solid objects, which we refer to as Moonlets. The 15 other features partially block stellar signal for 22 m to just over 3.7 km along the radial expanse of the occultation. Upon visual inspection of the occultation profile, these features resemble Icicles, thus we will refer to them as such here. The density enhancements responsible for such signal attenuations are likely due to transient clumping of material, evidence that aggregations of material are ubiquitous in the F ring. Our lengthy observing campaign reveals that Icicles are likely transient clumps, while Moonlets are possible solid objects. Optical depth is an indicator of clumping because more-densely aggregated material blocks more light; therefore, it is natural to imagine moonlets as later evolutionary stage of icicle, when looser clumps of material compact to form a feature that appears

  11. Radiation from optically thin accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Tylenda, R. (Polska Akademia Nauk, Torun. Pracownia Astrofizyki)

    1981-01-01

    Accretion discs in cataclysmic variables with low rates of mass transfer, M < or approx. 10/sup 16/g s/sup -1/, have outer regions optically thin in continuum. A simple approach that allows one to calculate the radiation spectra from such discs is presented. A great number of disc models has been obtained in order to study the influence of various parameters (accretion rate, outer radius of the disc, inclination angle, mass of the accreting degenerate dwarf, viscosity parameter) of discs on the outgoing continuous spectra, emission lines and the UBV colours.

  12. Increased Understanding of Accretion in Massive YSOs

    Science.gov (United States)

    De Wit, Willem-Jan; Caratti, A.; Kraus, S.

    2017-06-01

    That massive stars up to 20Msol form by disk accretion is by now reasonably well established. We will present the latest observational results for the formation of single and multiple massive YSOs. By means of optical interferometry using the newly commissioned instrument Gravity at the VLT-I, we show the discovery of a young, embedded, 170AU-wide binary and is the most massive and most compact accreting young binary to date. We will also present the results of a multi-site multi-epoch follow-up campaign of the first well studied accretion outburst in a massive YSO.

  13. A New Paradigm for Gamma Ray Bursts: Long Term Accretion Rate Modulation by an External Accretion Disk

    Science.gov (United States)

    Cannizzo, John; Gehrels, Neil

    2009-01-01

    We present a new way of looking at the very long term evolution of GRBs in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence the efficacy of the process that extracts rotational energy from the black hole and inner accretion disk. The pre-Swift paradigm of achromatic, shallow-to-steep "breaks" in the long term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an "external disk" whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power law decay to the GRB light curves. In this model, the different canonical power law decay segments in the GRB identified by Zhang et al. and Nousek et al. represent different physical states of the accretion disk. We identify a physical disk state with each power law segment.

  14. Diagnosing the Black Hole Accretion Physics of Sgr A*

    Science.gov (United States)

    Fazio, Giovanni; Ashby, Matthew; Baganoff, Frederick; Becklin, Eric; Carey, Sean; Gammie, Charles; Ghez, Andrea; Glaccum, William; Gurwell, Mark; Haggard, Daryl; Hora, Joseph; Ingalls, James; Marrone, Daniel; Meyer, Leo; Morris, Mark; Smith, Howard; Willner, Steven; Witzel, Gunther

    2016-08-01

    The Galactic center offers the closest opportunity for studying accretion onto supermassive black holes. The fluctuating source, Sgr A*, is detected across the electromagnetic spectrum and may originate in the accretion flow or jet. Recent general relativistic magneto-hydrodynamic (GRMHD) models indicate that variability can be produced by a tilted inner disk, gravitational lensing of bright spots in the disk by the hole, or particle acceleration in reconnection events. These models produce different flare characteristics, and in particular better characterization of flares may enable us to distinguish between strong and weakly magnetized disks. Disentangling the power source and emission mechanisms of the flares is a central challenge to our understanding of the Sgr A* accretion flow. Following our successful observations of the variability of Sgr A* with IRAC in 2013 and 2014, we propose simultaneous IRAC (4.5 micron) and Chandra (2-10 keV) observations to (1) probe the accretion physics of Sgr A* on event-horizon scales and (2) detect any effect of the object G2 on Sgr A*. Specifically, we propose six additional epochs of observation, each of 24 uninterrupted hours; four in 2017 July and two in 2018 July. In this proposal we request two 24-hour (86.4 ks) Chandra periods, and are requesting another four through the Chandra TAC to have simultaneous X-ray observations in each of the six Spitzer epochs. Independent of this proposal we will also request NuSTAR (3-79 keV), SMA/ALMA/APEX (0.8 mm), and Keck/Magellan NIR (2.2 micron) observations during the IRAC/Chandra epochs. Only such long-duration, continuous, multi-wavelength observations can achieve a comprehensive view of the dominant emission process(es) and quantify the physical properties near the event horizon. Theoretical models are increasing in physical sophistication, and our study will provide essential constraints for the next generation of models.

  15. Application of a physical continuum model to recent X-ray observations of accreting pulsars

    Science.gov (United States)

    Marcu-Cheatham, Diana Monica; Pottschmidt, Katja; Wolff, Michael Thomas; Becker, Peter A.; Wood, Kent S.; Wilms, Joern; Britton Hemphill, Paul; Gottlieb, Amy; Fuerst, Felix; Schwarm, Fritz-Walter; Ballhausen, Ralf

    2016-04-01

    We present a uniform spectral analysis in the 0.5-50 keV energy range of a sample of accreting pulsars by applying an empirical broad-band continuum cut-off power-law model. We also apply the newly implemented physical continuum model developed by Becker and Wolff (2007, ApJ 654, 435) to a number of high-luminosity sources. The X-ray spectral formation process in this model consists of the Comptonization of bremsstrahlung, cyclotron, and black body photons emitted by the hot, magnetically channeled, accreting plasma near the neutron star surface. This model describes the spectral formation in high-luminosity accreting pulsars, where the dominant deceleration mechanism is via a radiation-dominated radiative shock. The resulting spectra depend on five physical parameters: the mass accretion rate, the radius of the accretion column, the electron temperature and electron scattering cross-sections inside the column, and the magnetic field strength. The empirical model is fitted to Suzaku data of a sample of high-mass X-ray binaries covering a broad luminosity range (0.3-5 x 10 37 erg/s). The physical model is fitted to Suzaku data from luminous sources: LMC X-4, Cen X-3, GX 304-1. We compare the results of the two types of modeling and summarize how they can provide new insight into the process of accretion onto magnetized neutron stars.

  16. Is Episodic Accretion Necessary to Resolve the Luminosity Problem in Low-Mass Protostars?

    Science.gov (United States)

    Sevrinsky, Raymond Andrew; Dunham, Michael

    2017-01-01

    In this contribution, we compare the results of protostellar accretion simulations for scenarios both containing and lacking episodic accretion activity. We determine synthetic observational signatures for collapsing protostars by taking hydrodynamical simulations predicting highly variable episodic accretion events, filtering out the stochastic behavior by applying power law fits to the mass accretion rates onto the disk and central star, and using the filtered rates as inputs to two-dimensional radiative transfer calculations. The spectral energy distributions generated by these calculations are used to calculate standard observational signatures of Lbol and Tbol, and compared directly to a sample of 230 embedded protostars. We explore the degree to which these continually declining accretion models successfully reproduce the observed spread of protostellar luminosities, and examine their consistency with the prior variable models to investigate the degree to which episodic accretion bursts are necessary in protostellar formation theories to match observations of field protostars. The SAO REU program is funded in part by the National Science Foundation REU and Department of Defense ASSURE programs under NSF Grant no. 1262851, and by the Smithsonian Institution.

  17. Accretion-induced variability links young stellar objects, white dwarfs, and black holes.

    Science.gov (United States)

    Scaringi, Simone; Maccarone, Thomas J; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R; Aranzana, Ester; Dhillon, Vikram S; Barros, Susana C C

    2015-10-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies.

  18. Active galactic nuclei at z ~ 1.5: III. Accretion discs and black hole spin

    CERN Document Server

    Capellupo, Daniel M; Lira, Paulina; Trakhtenbrot, Benny; Mejía-Restrepo, Julián

    2016-01-01

    This is the third paper in a series describing the spectroscopic properties of a sample of 39 AGN at $z \\sim 1.5$, selected to cover a large range in black hole mass ($M_{BH}$) and Eddington ratio ($L/L_{Edd}$). In this paper, we continue the analysis of the VLT/X-shooter observations of our sample with the addition of 9 new sources. We use an improved Bayesian procedure, which takes into account intrinsic reddening, and improved $M_{BH}$ estimates, to fit thin accretion disc (AD) models to the observed spectra and constrain the spin parameter ($a_*$) of the central black holes. We can fit 37 out of 39 AGN with the thin AD model, and for those with satisfactory fits, we obtain constraints on the spin parameter of the BHs, with the constraints becoming generally less well defined with decreasing BH mass. Our spin parameter estimates range from $\\sim$$-$0.6 to maximum spin for our sample, and our results are consistent with the "spin-up" scenario of BH spin evolution. We also discuss how the results of our anal...

  19. Active galactic nuclei at z ˜ 1.5 - III. Accretion discs and black hole spin

    Science.gov (United States)

    Capellupo, D. M.; Netzer, H.; Lira, P.; Trakhtenbrot, B.; Mejía-Restrepo, J.

    2016-07-01

    This is the third paper in a series describing the spectroscopic properties of a sample of 39 AGN at z ˜ 1.5, selected to cover a large range in black hole mass (MBH) and Eddington ratio (L/LEdd). In this paper, we continue the analysis of the VLT/X-shooter observations of our sample with the addition of nine new sources. We use an improved Bayesian procedure, which takes into account intrinsic reddening, and improved MBH estimates, to fit thin accretion disc (AD) models to the observed spectra and constrain the spin parameter (a*) of the central black holes. We can fit 37 out of 39 AGN with the thin AD model, and for those with satisfactory fits, we obtain constraints on the spin parameter of the BHs, with the constraints becoming generally less well defined with decreasing BH mass. Our spin parameter estimates range from ˜-0.6 to maximum spin for our sample, and our results are consistent with the `spin-up' scenario of BH spin evolution. We also discuss how the results of our analysis vary with the inclusion of non-simultaneous GALEX photometry in our thin AD fitting. Simultaneous spectra covering the rest-frame optical through far-UV are necessary to definitively test the thin AD theory and obtain the best constraints on the spin parameter.

  20. Spread of Matter over a Neutron-Star Surface During Disk Accretion: Deceleration of Rapid Rotation

    CERN Document Server

    Sunyaev, R A

    2011-01-01

    The problem of disk accretion onto the surface of a neutron star with a weak magnetic field at a luminosity exceeding several percent of Eddington is reduced to the problem of the braking of a hypersonic flow with a velocity that is 0.4-0.5 of the speed of light above the base of the spreading layer -- a dense atmosphere made up of previously fallen matter. We show that turbulent braking in the Prandtl-Karman model with universally accepted coefficients for terrestrial conditions and laboratory experiments and a ladder of interacting gravity waves in a stratified quasi-exponential atmosphere at standard Richardson numbers lead to a spin-up of the massive zone that extends to the ocean made up of a plasma with degenerate electrons. Turbulent braking in the ocean at the boundary with the outer solid crust reduces the rotation velocity to the solid-body rotation velocity of the star. This situation should lead to strong heating of deep atmospheric layers and to the switch-off of the explosive helium burning mech...

  1. The beaming of subhalo accretion

    Science.gov (United States)

    Libeskind, Noam I.

    2016-10-01

    We examine the infall pattern of subhaloes onto hosts in the context of the large-scale structure. We find that the infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (~ 10 Mpc) scale - a scale accessible by current surveys of peculiar velocities - it follows that findings presented here open a new window into the relation between the observed large scale structure unveiled by current surveys of peculiar velocities and the preferential infall direction of the Local Group. This may shed light on the unexpected alignments of dwarf galaxies seen in the Local Group.

  2. The mass profile and accretion history of cold dark matter haloes

    Science.gov (United States)

    Ludlow, Aaron D.; Navarro, Julio F.; Boylan-Kolchin, Michael; Bett, Philip E.; Angulo, Raúl E.; Li, Ming; White, Simon D. M.; Frenk, Carlos; Springel, Volker

    2013-06-01

    We use the Millennium Simulation series to investigate the relation between the accretion history and mass profile of cold dark matter (CDM) haloes. We find that the mean inner density within the scale radius, r-2 (where the halo density profile has isothermal slope), is directly proportional to the critical density of the Universe at the time when the virial mass of the main progenitor equals the mass enclosed within r-2. Scaled to these characteristic values of mass and density, the average mass accretion history, expressed in terms of the critical density of the Universe, M(ρcrit(z)), resembles that of the enclosed density profile, M(), at z = 0. Both follow closely the Navarro, Frenk & White (NFW) profile, which suggests that the similarity of halo mass profiles originates from the mass-independence of halo accretion histories. Support for this interpretation is provided by outlier haloes whose accretion histories deviate from the NFW shape; their mass profiles show correlated deviations from NFW and are better approximated by Einasto profiles. Fitting both M() and M(ρcrit) with either NFW or Einasto profiles yield concentration and shape parameters that are correlated, confirming and extending earlier work that has linked the concentration of a halo with its accretion history. These correlations also confirm that halo structure is insensitive to initial conditions: only haloes whose accretion histories differ greatly from the NFW shape show notable deviations from NFW in their mass profiles. As a result, the NFW profile provides acceptable fits to hot dark matter haloes, which do not form hierarchically, and for fluctuation power spectra other than CDM. Our findings, however, predict a subtle but systematic dependence of mass profile shape on accretion history which, if confirmed, would provide strong support for the link between accretion history and halo structure we propose here.

  3. Accretion disks around a mass with quadrupole

    CERN Document Server

    Abishev, Medeu; Quevedo, Hernando; Toktarbay, Saken

    2015-01-01

    We consider the stability properties of test particles moving along circular orbits around a mass with quadrupole. We show that the quadrupole modifies drastically the properties of an accretion disk made of such test particles.

  4. Foundations of Black Hole Accretion Disk Theory

    National Research Council Canada - National Science Library

    Abramowicz, Marek A; Fragile, P. Chris

    2013-01-01

    This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves...

  5. Radiatively inefficient MHD accretion-ejection structures

    CERN Document Server

    Casse, F; Casse, Fabien; Keppens, Rony

    2004-01-01

    We present magnetohydrodynamic simulations of a resistive accretion disk continuously launching transmagnetosonic, collimated jets. We time-evolve the full set of magnetohydrodynamic equations, but neglect radiative losses in the energetics (radiatively inefficient). Our calculations demonstrate that a jet is self-consistently produced by the interaction of an accretion disk with an open, initially bent large-scale magnetic field. A constant fraction of heated disk material is launched in the inner equipartition disk regions, leading to the formation of a hot corona and a bright collimated, super-fastmagnetosonic jet. We illustrate the complete dynamics of the ``hot'' near steady-state outflow (where thermal pressure $\\simeq$ magnetic pressure) by showing force balance, energy budget and current circuits. The evolution to this near stationary state is analyzed in terms of the temporal variation of energy fluxes controlling the energetics of the accretion disk. We find that unlike advection-dominated accretion...

  6. Accretion disks around a mass with quadrupole

    Science.gov (United States)

    Abishev, M.; Boshkayev, K.; Quevedo, H.; Toktarbay, S.

    We consider the stability properties of circular orbits of test particles moving around a mass with quadrupole. We show that the quadrupole modifies drastically the properties of an accretion disk made of such test particles.

  7. The High-Energy Spectra of Accreting Black Holes Observational Evidence for Bulk-Motion Infall

    CERN Document Server

    Shrader, C R; Shrader, Chris; Titarchuk, Lev

    1998-01-01

    We discuss the emergent spectra from accreting black holes, considering in particular the case where the accretion is characterized by relativistic bulk motion. We suggest that such accretion is likely to occur in a wide variety of black hole enviroments, where the strong gravitational field is expected to dominate the pressure forces, and this likely to lead to a characteristic high-energy spectriscopic signature; an extended power tail. It is in the high (soft) state that matter impinging upon the event horizon can be viewed directly, and intrinsic power-law seen. A test of the model is presented using observational data from the Compton Gamma Ray Observatory and the Rossi X-ray Timing Explorer, covering 2- 200 keV for recent galactic black hole X-ray nova outbursts.

  8. Self-similar Evolution of Self-Gravitating Viscous Accretion Discs

    CERN Document Server

    Illenseer, Tobias F

    2015-01-01

    A new one-dimensional, dynamical model is proposed for geometrically thin, self-gravitating viscous accretion discs. The vertically integrated equations are simplified using the slow accretion limit and the monopole approximation with a time-dependent central point mass to account for self-gravity and accretion. It is shown that the system of partial differential equations can be reduced to a single non-linear advection diffusion equation which describes the time evolution of angular velocity. In order to solve the equation three different turbulent viscosity prescriptions are considered. It is shown that for these parametrizations the differential equation allows for similarity transformations depending only on a single non-dimensional parameter. A detailed analysis of the similarity solutions reveals that this parameter is the initial power law exponent of the angular velocity distribution at large radii. The radial dependence of the self-similar solutions is in most cases given by broken power laws. At sma...

  9. Some Interesting Behaviour of Accreting Particles in the Gap Region of Black Hole Accretion Discs

    Institute of Scientific and Technical Information of China (English)

    WANG Ding-Xiong; XIAO Kan; LEI Wei-Hua

    2001-01-01

    Some interesting behaviour of accreting particles in the gap region between the horizon of the Kerr black hole and the inner edge of the surrounding disc is investigated. The following results are obtained. (i) Spacetime coincidence of the maximum of angular velocity of accreting particles and that of the black hole horizon is extended to the more general disc-accretion. (ii) The possibility is discussed of negative energy of accreting particles in prograde orbit inside the ergosphere of the Kerr black hole, which is surrounded by strong enough magnetic field.

  10. Gas dynamics for accretion disk simulations

    Science.gov (United States)

    Whitehurst, R.

    1994-01-01

    The behavior of accretion disks can largely be understood in terms of the basic physical processes of mass, energy, and momentum conservation. Despite this, detailed modeling of these systems using modern computational techniques is challenging and controversial. Disturbing differences exist between methods used widely in astrophysics, namely Eulerian finite-difference techniques and particle codes such as SPH. Therefore neither technique is fully satisfactory for accretion disk simulations. This paper describes a new fully Lagrangian method designed to resolve these difficulties.

  11. Lyman edges in AGN accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Czerny, B. (Copernicus Astronomical Center, Warsaw (Poland)); Pojmanski, G. (Warsaw Univ. (Poland). Obserwatorium Astronomiczne)

    1990-07-01

    We show that the basic difference in the two principal approaches to predictions of the Lyman edge in an accretion disc lies in the implicit assumption about the density of the radiating gas. Independent from the details, models predict a broad range of the edge sizes, both in absorption and in emission. Observed spectra do not exhibit any strong feature at 912 A but may still be consistent with an accretion disc mechanism if more advanced theory is developed. (author).

  12. Energy-dependent variability from accretion flows

    OpenAIRE

    Zdziarski, Andrzej A.

    2005-01-01

    We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependencies of the level of local variability (as expected from propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As example local emission, we consider blackb...

  13. ASYMMETRIC ACCRETION FLOWS WITHIN A COMMON ENVELOPE

    Energy Technology Data Exchange (ETDEWEB)

    MacLeod, Morgan; Ramirez-Ruiz, Enrico [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)

    2015-04-10

    This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). Flow in the vicinity of the embedded object is gravitationally focused, leading to drag and potentially to gas accretion. This process has been studied numerically and analytically in the context of Hoyle–Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of three-dimensional HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by one to two orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically derived rates of drag and accretion as a function of the density gradient. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.

  14. A Direct Measurement of the Heat Release in the Outer Crust of the Transiently Accreting Neutron Star XTE J1709-267

    NARCIS (Netherlands)

    N. Degenaar; R. Wijnands; J.M. Miller

    2013-01-01

    The heating and cooling of transiently accreting neutron stars provides a powerful probe of the structure and composition of their crust. Observations of superbursts and cooling of accretion-heated neutron stars require more heat release than is accounted for in current models. Obtaining firm constr

  15. Moving mesh cosmology: tracing cosmological gas accretion

    CERN Document Server

    Nelson, Dylan; Genel, Shy; Sijacki, Debora; Keres, Dusan; Springel, Volker; Hernquist, Lars; 10.1093/mnras/sts595

    2013-01-01

    We investigate the nature of gas accretion onto haloes and galaxies at z=2 using cosmological hydrodynamic simulations run with the moving mesh code AREPO. Implementing a Monte Carlo tracer particle scheme to determine the origin and thermodynamic history of accreting gas, we make quantitative comparisons to an otherwise identical simulation run with the smoothed particle hydrodynamics (SPH) code GADGET-3. Contrasting these two numerical approaches, we find significant physical differences in the thermodynamic history of accreted gas in haloes above 10^10.5 solar masses. In agreement with previous work, GADGET simulations show a cold fraction near unity for galaxies forming in massive haloes, implying that only a small percentage of accreted gas heats to an appreciable fraction of the virial temperature during accretion. The same galaxies in AREPO show a much lower cold fraction, <20% in haloes above 10^11 solar masses. This results from a hot gas accretion rate which, at this same halo mass, is an order o...

  16. Magnetospheric accretion and spin-down of the prototypical classical T Tauri star AATau

    CERN Document Server

    Donati, JF; Bouvier, J; Gregory, SG; Grankin, KN; Jardine, MM; Hussain, GAJ; Menard, F; Dougados, C; Unruh, Y; Mohanty, S; Auriere, M; Morin, J; Fares, R

    2010-01-01

    From observations collected with the ESPaDOnS & NARVAL spectropolarimeters at CFHT and TBL, we report the detection of Zeeman signatures on the prototypical classical TTauri star AATau, both in photospheric lines and accretion-powered emission lines. Using time series of unpolarized and circularly polarized spectra, we reconstruct at two epochs maps of the magnetic field, surface brightness and accretion-powered emission of AATau. We find that AATau hosts a 2-3kG magnetic dipole tilted at ~20deg to the rotation axis, and of presumably dynamo origin. We also show that the magnetic poles of AATau host large cool spots at photospheric level and accretion regions at chromospheric level. The logarithmic accretion rate at the surface of AATau at the time of our observations is strongly variable, ranging from -9.6 to -8.5 and equal to -9.2 in average (in Msun/yr); this is an order of magnitude smaller than the disc accretion rate at which the magnetic truncation radius (below which the disc is disrupted by the s...

  17. The inner disc radius in the propeller phase and accretion-propeller transition of neutron stars

    Science.gov (United States)

    Ertan, Ünal

    2017-04-01

    We have investigated the critical conditions required for a steady propeller effect for magnetized neutron stars with optically thick, geometrically thin accretion discs. We have shown through simple analytical calculations that a steady-state propeller mechanism cannot be sustained at an inner disc radius where the viscous and magnetic stresses are balanced. The radius calculated by equating these stresses is usually found to be close to the conventional Alfvén radius for spherical accretion, rA. Our results show that: (1) a steady propeller phase can be established with a maximum inner disc radius that is at least ∼15 times smaller than rA depending on the mass-flow rate of the disc, rotational period and strength of the magnetic dipole field of the star, (2) the critical accretion rate corresponding to the accretion-propeller transition is orders of magnitude lower than the rate estimated by equating rA to the co-rotation radius. Our results are consistent with the properties of the transitional millisecond pulsars that show transitions between the accretion powered X-ray pulsar and the rotational powered radio-pulsar states.

  18. Three Dimensional MHD Simulation of Circumbinary Accretion Disks -2. Net Accretion Rate

    CERN Document Server

    Shi, Ji-Ming

    2015-01-01

    When an accretion disk surrounds a binary rotating in the same sense, the binary exerts strong torques on the gas. Analytic work in the 1D approximation indicated that these torques sharply diminish or even eliminate accretion from the disk onto the binary. However, recent 2D and 3D simulational work has shown at most modest diminution. We present new MHD simulations demonstrating that for binaries with mass ratios of 1 and 0.1 there is essentially no difference between the accretion rate at large radius in the disk and the accretion rate onto the binary. To resolve the discrepancy with earlier analytic estimates, we identify the small subset of gas trajectories traveling from the inner edge of the disk to the binary and show how the full accretion rate is concentrated onto them.

  19. Chaotic cold accretion on to black holes

    Science.gov (United States)

    Gaspari, M.; Ruszkowski, M.; Oh, S. Peng

    2013-07-01

    Bondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when tcool/tff ≲ 10. The clouds decouple from the hot gas, `raining' on to the centre. Subsonic turbulence of just over 100 km s-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (tturb/tcool ≲ 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in

  20. The Final Fates of Accreting Supermassive Stars

    CERN Document Server

    Umeda, Hideyuki; Omukai, Kazuyuki; Yoshida, Naoki

    2016-01-01

    The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have less concentrated structure than fully-convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit $\\gtrsim 10^5~M_\\odot$ derived for the fully-convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with $\\dot M \\lesssim 0.1~M_\\odot~{\\rm yr}^{-1}$. With $\\dot{M} \\simeq 0.3 - 1~M_\\odot~{\\rm yr}^{-1}$, the star becomes GR-unstable during the helium-burning stage ...

  1. Mixing and Accretion in lambda Bootis Stars

    CERN Document Server

    Turcotte, S

    2002-01-01

    Strong evidence for deep mixing has been uncovered for slowly rotating F, and A stars of the main sequence. As the accretion/diffusion model for the formation of lboo stars is heavily dependent on mixing in superficial regions, such deep mixing may have important repercussions on our understanding of these stars. It is shown that deep mixing at a level similar to that of FmAm stars increases the amount of matter that needs to be accreted by the stars with respect with the standard models by some three orders of magnitude. It is also shown that significantly larger accretion rates have to be maintained, as high as $10^{-11}$~M$_\\sun yr^{-1}$, to prevent meridional circulation from canceling the effect of accretion. The existence of old ($\\approx 1$~Gyr) is not a likely outcome of the present models for accretion/diffusion with or without deep mixing. It is argued that lboo stars are potentially very good diagnostics of mixing mechanisms in moderately fast rotators.

  2. The accretion of migrating giant planets

    CERN Document Server

    Dürmann, Christoph

    2016-01-01

    Most studies concerning the growth and evolution of massive planets focus either on their accretion or their migration only. In this work we study both processes concurrently to investigate how they might mutually affect each other. We modeled a 2-dimensional disk with a steady accretion flow onto the central star and embed a Jupiter mass planet at 5.2 au. The disk is locally isothermal and viscosity is modeled using a constant $\\alpha$. The planet is held on a fixed orbit for a few hundred orbits to allow the disk to adapt and carve a gap. After this period, the planet is released and free to move according to the gravitational interaction with the gas disk. The mass accretion onto the planet is modeled by removing a fraction of gas from the inner Hill sphere, and the removed mass and momentum can be added to the planet. Our results show that a fast migrating planet is able to accrete more gas than a slower migrating planet. Utilizing a tracer fluid we analyzed the origin of the accreted gas which comes pred...

  3. Bondi accretion in early-type galaxies

    CERN Document Server

    Korol, V; Pellegrini, S

    2016-01-01

    Accretion onto central massive black holes in galaxies is often modelled with the Bondi solution. In this paper we study a generalization of the classical Bondi accretion theory, considering the additional effects of the gravitational potential of the host galaxy, and of electron scattering in the optically thin limit. We provide a general analysis of the bias in the estimates of the Bondi radius and mass accretion rate, when adopting as fiducial values for the density and temperature at infinity the values of these quantities measured at finite distance from the central black hole. We also give general formulae to compute the correction terms of the critical accretion parameter in relevant asymptotic regimes. A full analytical discussion is presented in the case of an Hernquist galaxy, when the problem reduces to the discussion of a cubic equation, therefore allowing for more than one critical point in the accretion structure. The results are useful for observational works (especially in the case of low-lumi...

  4. Optically-thick accretion discs with advection

    Institute of Scientific and Technical Information of China (English)

    陈林红; 吴枚; 尚仁成

    2002-01-01

    The structures of optically-thick accretion discs with radial advection have been investigated by the iteration and integration algorithms. The advective cooling term changes mostly the inner part of disc solution, and even results in an optically-thick advection-dominated accretion flow (ADAF). Three distinct branches-the outer Shakura-Sunyaev disc (SSD), the inner ADAF and the middle transition layer-are found for a super-Eddington disc. The SSD-ADAF transition radius can be estimated as 18(M/ME)RG where RG is the Schwarzschild radius, M is the mass accretion rate and ME is the Eddington accretion rate. SSD solutions calculated with the iteration and integration methods are identical, while ADAF solutions obtained by these two methods differ greatly. Detailed algorithms and their differences have been analysed. The iteration algorithm is not self-consistent, since it implies that the dimensionless advection factor ξ is invariant, but in the inner ADAF region the variation of ξ is not negligible. The integration algorithm is always effective for the whole region of an optically-thick disc if the accretion rate is no smaller than 10-4ME. For optically-thin discs, the validity of these two algorithms is different. We suggest that the integration method be employed to calculate the global solution of a disc model without assuming ξ to be a constant. We also discuss its application to the emergent continuum spectrum in order to explain observational facts.

  5. Accretion Rates in Herbig Ae stars

    CERN Document Server

    López, R G; Testi, L; Habart, E

    2006-01-01

    Accretion rates from disks around pre-main sequence stars are of importance for our understanding of planetary formation and disk evolution. We provide in this paper estimates of the mass accretion rates in the disks around a large sample of Herbig Ae stars. We obtained medium resolution 2 micron spectra and used the results to compute values of Macc from the measured luminosity of the Br_gamma emission line, using a well established correlation between L(Br_gamma) and the accretion luminosity Lacc. We find that 80% of the stars, all of which have evidence of an associated circumstellar disk, are accreting matter, with rates 3x10^{-9} 10^{-7} Msun/yr. In most HAe stars the accretion rate is sufficiently low that the gas in the inner disk, inside the dust evaporation radius, is optically thin and does not prevent the formation of a puffed-up rim, where dust is directly exposed to the stellar radiation. When compared to the Macc values found for lower-mass stars in the star forming regions Taurus and Ophiuchus,...

  6. Rapid variability as a probe of warped space-time around accreting black holes

    Science.gov (United States)

    Axelsson, Magnus

    2016-07-01

    The geometry of the inner accretion flow of X-ray binaries is complex, with multiple regions contributing to the observed emission. Frequency-resolved spectroscopy is a powerful tool in breaking this spectral degeneracy. We have extracted the spectra of the strong low-frequency quasi-periodic oscillation (QPO) and its harmonic in GX339-4 and XTE J1550-564, and compare these to the time-averaged spectrum and the spectrum of the rapid (plaguing the time-averaged spectrum and revealing the geometry of the accretion flow close to the black hole.

  7. Accretion disks in luminous young stellar objects

    CERN Document Server

    Beltran, M T

    2015-01-01

    An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and therefore predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.

  8. Accretion disks in Algols: progenitors and evolution

    CERN Document Server

    Van Rensbergen, W

    2016-01-01

    There are only a few Algols with measured accretion disk parameters. These measurements provide additional constraints for tracing the origin of individual systems, narrowing down the initial parameter space. We investigate the origin and evolution of 6 Algol systems with accretion disks to find the initial parameters and evolutionary constraints for them. With a modified binary evolution code, series of close binary evolution are calculated to obtain the best match for observed individual systems. Initial parameters for 6 Algol systems with accretion disks were determined matching both the present system parameters and the observed disk characteristics. When RLOF starts during core hydrogen burning of the donor, the disk lifetime was found to be short. The disk luminosity is comparable to the luminosity of the gainer during a large fraction of the disk lifetime.

  9. MHD Turbulence in Accretion Disk Boundary Layers

    CERN Document Server

    Chan, Chi-kwan

    2012-01-01

    The physical modeling of the accretion disk boundary layer, the region where the disk meets the surface of the accreting star, usually relies on the assumption that angular momentum transport is opposite to the radial angular frequency gradient of the disk. The standard model for turbulent shear viscosity, widely adopted in astrophysics, satisfies this assumption by construction. However, this behavior is not supported by numerical simulations of turbulent magnetohydrodynamic (MHD) accretion disks, which show that angular momentum transport driven by the magnetorotational instability is inefficient in this inner disk region. I will discuss the results of a recent study on the generation of hydromagnetic stresses and energy density in the boundary layer around a weakly magnetized star. Our findings suggest that although magnetic energy density can be significantly amplified in this region, angular momentum transport is rather inefficient. This seems consistent with the results obtained in numerical simulations...

  10. Spherical Accretion in Nearby Weakly Active Galaxies

    CERN Document Server

    Moscibrodzka, M A

    2005-01-01

    We consider the sample of weakly active galaxies situated in 'Local Universe' collected in the paper of Pellegrini (2005) with inferred accretion efficiencies from $10^{-2}$ to $10^{-7}$. We apply a model of spherically symmetrical Bondi accretion for given parameters ($M_{BH}$,$T_{\\infty}$,$\\rho_{\\infty}$,) taken from observation. We calculate spectra emitted by the gas accreting onto its central objects using Monte Carlo method including synchrotron and bremsstrahlung photons as seed photons. We compare our results with observed nuclear X-ray luminosities $L_{X,nuc}$ (0.3-10 keV) of the sample. Model is also tested for different external medium parameters ($\\rho_{\\infty}$ and $T_{\\infty}$) and different free parameters of the model. Our model is able to explain most of the observed nuclear luminosities $L_X$ under an assumption that half of the compresion energy is transfered directly to the electrons.

  11. Earth, Moon, Sun, and CV Accretion Disks

    CERN Document Server

    Montgomery, M M

    2009-01-01

    Net tidal torque by the secondary on a misaligned accretion disk, like the net tidal torque by the Moon and the Sun on the equatorial bulge of the spinning and tilted Earth, is suggested by others to be a source to retrograde precession in non-magnetic, accreting Cataclysmic Variable (CV) Dwarf Novae systems that show negative superhumps in their light curves. We investigate this idea in this work. We generate a generic theoretical expression for retrograde precession in spinning disks that are misaligned with the orbital plane. Our generic theoretical expression matches that which describes the retrograde precession of Earths' equinoxes. By making appropriate assumptions, we reduce our generic theoretical expression to those generated by others, or to those used by others, to describe retrograde precession in protostellar, protoplanetary, X-ray binary, non-magnetic CV DN, quasar and black hole systems. We find that differential rotation and effects on the disk by the accretion stream must be addressed. Our a...

  12. Strongly magnetized accretion discs require poloidal flux

    Science.gov (United States)

    Salvesen, Greg; Armitage, Philip J.; Simon, Jacob B.; Begelman, Mitchell C.

    2016-08-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  13. Minidisks in Binary Black Hole Accretion

    CERN Document Server

    Ryan, Geoffrey

    2016-01-01

    Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole ("minidisks"), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using two-dimensional hydrodynamical simulations performed with a new general relativistic version of the moving mesh code Disco. We introduce a co-moving energy variable which enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the ISCO providing a Reynolds stress which causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov-Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling....

  14. Disks, accretion and outflows of brown dwarfs

    CERN Document Server

    Joergens, V; Liu, Y; Pascucci, I; Whelan, E; Alcala, J; Biazzo, K; Costigan, G; Gully-Santiago, M; Henning, Th; Natta, A; Rigliaco, E; Rodriguez-Ledesma, V; Sicilia-Aguilar, A; Tottle, J; Wolf, S

    2012-01-01

    Characterization of the properties of young brown dwarfs are important to constraining the formation of objects at the extreme low-mass end of the IMF. While young brown dwarfs share many properties with solar-mass T Tauri stars, differences may be used as tests of how the physics of accretion/outflow and disk chemistry/dissipation depend on the mass of the central object. This article summarizes the presentations and discussions during the splinter session on 'Disks, accretion and outflows of brown dwarfs' held at the CoolStars17 conference in Barcelona in June 2012. Recent results in the field of brown dwarf disks and outflows include the determination of brown dwarf disk masses and geometries based on Herschel far-IR photometry (70-160 um), accretion properties based on X-Shooter spectra, and new outflow detections in the very low-mass regime.

  15. Strongly magnetized accretion discs require poloidal flux

    CERN Document Server

    Salvesen, Greg; Simon, Jacob B; Begelman, Mitchell C

    2016-01-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  16. Global Models for Embedded, Accreting Protostellar Disks

    CERN Document Server

    Kratter, Kaitlin M; Krumholz, Mark R

    2007-01-01

    Most analytic work to date on protostellar disks has focused on disks in isolation from their environments. However, observations are now beginning to probe the earliest, most embedded phases of star formation, during which disks are rapidly accreting from their parent cores and cannot be modeled in isolation. We present a simple, one-zone model of protostellar accretion disks with high mass infall rates. Our model combines a self-consistent calculation of disk temperatures with an approximate treatment of angular momentum transport via several mechanisms. We use this model to survey the properties of protostellar disks across a wide range of stellar masses and evolutionary times, and make predictions for disks' masses, sizes, spiral structure, and fragmentation that will be directly testable by future large-scale surveys of deeply embedded disks. We define a dimensionless accretion-rotation parameter which, in conjunction with the disk's temperature, controls the disk evolution. We track the dominant mode of...

  17. Reverberation Mapping of AGN Accretion Disks

    Science.gov (United States)

    Fausnaugh, Michael; AGN STORM Collaboration

    2017-01-01

    I will discuss new reverberation mapping results that allow us to investigate the temperature structure of AGN accretion disks. By measuring time-delays between broad-band continuum light curves, we can determine the size of the disk as a function of wavelength. I will discuss the detection of continuum lags in NGC 5548 reported by the AGN STORM project and implications for the accretion disk. I will also present evidence for continuum lags in two other AGN for which we recently measured black hole masses from continuum-Hbeta reverberations. The mass measurements allow us to compare the continuum lags to predictions from standard thin disk theory, and our results indicate that the accretion disks are larger than the simplest expectations.

  18. Simulating a Thin Accretion Disk Using PLUTO

    Science.gov (United States)

    Phillipson, Rebecca; Vogeley, Michael S.; Boyd, Patricia T.

    2017-08-01

    Accreting black hole systems such as X-ray binaries and active galactic nuclei exhibit variability in their luminosity on many timescales ranging from milliseconds to tens of days, and even hundreds of days. The mechanism(s) driving this variability and the relationship between short- and long-term variability is poorly understood. Current studies on accretion disks seek to determine how the changes in black hole mass, the rate at which mass accretes onto the central black hole, and the external environment affect the variability on scales ranging from stellar-mass black holes to supermassive black holes. Traditionally, the fluid mechanics equations governing accretion disks have been simplified by considering only the kinematics of the disk, and perhaps magnetic fields, in order for their phenomenological behavior to be predicted analytically. We seek to employ numerical techniques to study accretion disks including more complicated physics traditionally ignored in order to more accurately understand their behavior over time. We present a proof-of-concept three dimensional, global simulation using the astrophysical hydrodynamic code PLUTO of a simplified thin disk model about a central black hole which will serve as the basis for development of more complicated models including external effects such as radiation and magnetic fields. We also develop a tool to generate a synthetic light curve that displays the variability in luminosity of the simulation over time. The preliminary simulation and accompanying synthetic light curve demonstrate that PLUTO is a reliable code to perform sophisticated simulations of accretion disk systems which can then be compared to observational results.

  19. Generalized Similarity for Accretion/Decretion Disks

    Science.gov (United States)

    Rafikov, Roman R.

    2016-10-01

    Decretion (or external) disks are gas disks freely expanding to large radii due to their internal stresses. They are expected to naturally arise in tidal disruption events, around Be stars, in mass-losing post-main-sequence binaries, as a result of supernova fallback, etc. Their evolution is theoretically understood in two regimes: when the central object does not exert torque on the disk (a standard assumption for conventional accretion disks) or when no mass inflow (or outflow) occurs at the disk center. However, many astrophysical objects—circumbinary disks, Be stars, neutron stars accreting in a propeller regime, etc.—feature non-zero torque simultaneously with the non-zero accretion (or ejection of mass) at the disk center. We provide a general description for the evolution of such disks (both linear and nonlinear) in the self-similar regime, to which the disk should asymptotically converge with time. We identify a similarity parameter λ, which is uniquely related to the degree, to which the central mass accretion is suppressed by the non-zero central torque. The known decretion disk solutions correspond to the two discrete values of λ, while our new solutions cover a continuum of its physically allowed values, corresponding to either accretion or mass ejection by the central object. A direct relationship between λ and central \\dot{M} and torque is also established. We describe the time evolution of the various disk characteristics for different λ, and show that the observable properties (spectrum and luminosity evolution) of the decretion disks, in general, are different from the standard accretion disks with no central torque.

  20. A truncated accretion disk in the galactic black hole candidate source H1743-322

    Institute of Scientific and Technical Information of China (English)

    Kandulapati Sriram; Vivek Kumar Agrawal; Arikkala Raghurama Rao

    2009-01-01

    To investigate the geometry of the accretion disk in the source H1743-322, we have carded out a detailed X-ray temporal and spectral study using RXTE pointed observations. We have selected all data pertaining to the Steep Power Law (SPL) state during the 2003 outburst of this source. We find anti-correlated hard X-ray lags in three of the observations and the changes in the spectral and timing parameters (like the QPO fre-quency) confirm the idea of a truncated accretion disk in this source. Compiling data from similar observations of other sources, we find a correlation between the fractional change in the QPO frequency and the observed delay. We suggest that these observations indicate a definite size scale in the inner accretion disk (the radius of the truncated disk) and we explain the observed correlation using various disk parameters like Compton cooling time scale, viscous time scale etc..

  1. A truncated accretion disk in the galactic black hole source H1743-322

    CERN Document Server

    Sriram, K; Rao, A R

    2009-01-01

    To investigate the geometry of the accretion disk in the source H1743-322, we have carried out a detailed X-ray temporal and spectral study using RXTE pointed observations. We have selected all data pertaining to the Steep Power Law (SPL) state during the 2003 outburst of this source. We find anti-correlated hard X-ray lags in three of the observations and the changes in the spectral and timing parameters (like the QPO frequency) confirm the idea of a truncated accretion disk in this source. Compiling data from similar observations from other sources, we find a correlation between the fractional change in the QPO frequency and the observed delay. We suggest that these observations indicate a definite size scale in the inner accretion disk (the radius of the truncated disk) and we explain the observed correlation using various disk parameters like Compton cooling time scale, viscous time scale etc..

  2. Spiral shocks in the accretion disc of IP Peg during outburst maximum

    Science.gov (United States)

    Harlaftis, E. T.; Steeghs, D.; Horne, K.; Martín, E.; Magazzú, A.

    1999-06-01

    In response to our recent discovery of spiral arms in the accretion disc of IP Peg during rise to outburst, we have obtained time-resolved spectrophotometry of IP Peg during outburst maximum. In particular, indirect imaging of He II 4686, using Doppler tomography, shows a two-arm spiral pattern on the disc image, which confirms repeatability over different outbursts. The jump in He II intensity (a factor of more than 2) and in velocity (~ 200-300 km s^-1) clarifies the shock nature of the spiral structure. The He II shocks show an azimuthal extent of ~ 90 deg, a shallow power-law emissivity ~ V^-1, an upper limit of 30 deg in opening angle, and a flux contribution of 15 per cent of the total disc emission. We discuss the results in view of recent simulations of accretion discs which show that spiral shocks can be raised in the accretion disc by the secondary star.

  3. Alignment of magnetized accretion disks and relativistic jets with spinning black holes.

    Science.gov (United States)

    McKinney, Jonathan C; Tchekhovskoy, Alexander; Blandford, Roger D

    2013-01-04

    Accreting black holes (BHs) produce intense radiation and powerful relativistic jets, which are affected by the BH's spin magnitude and direction. Although thin disks might align with the BH spin axis via the Bardeen-Petterson effect, this does not apply to jet systems with thick disks. We used fully three-dimensional general relativistic magnetohydrodynamical simulations to study accreting BHs with various spin vectors and disk thicknesses and with magnetic flux reaching saturation. Our simulations reveal a "magneto-spin alignment" mechanism that causes magnetized disks and jets to align with the BH spin near BHs and to reorient with the outer disk farther away. This mechanism has implications for the evolution of BH mass and spin, BH feedback on host galaxies, and resolved BH images for the accreting BHs in SgrA* and M87.

  4. Accreting Scalar-Field Models of Dark Energy Onto Morris-Thorne Wormhole

    Science.gov (United States)

    Chattopadhyay, Surajit; Pasqua, Antonio; Radinschi, Irina

    2016-10-01

    The present paper reports a study on accreting tachyon, Dirac-Born-Infeld essence and h-essence scalar field models of dark energy onto Morris-Thorne wormhole. Using three different parameterisation schemes and taking H = {H_0} + {{{H_1}} over t}, we have derived the mass of the wormhole for all of the three parameterisation schemes that are able to get hold of both quintessence and phantom behaviour. With suitable choice of parameters, we observed that accreting scalar field dark energy models are increasing the mass of the wormhole in the phantom phase and the mass is decreasing in the quintessence phase. Finally, we have considered accretion with power law form of scale factor and without any parameterisation scheme for the equation of state parameter and observed the fact that phantom-type dark energy supports the existence of wormholes.

  5. Jittering-jets explosion triggered by the standing accretion shock instability

    CERN Document Server

    Papish, Oded; Soker, Noam

    2015-01-01

    We show that the standing accretion shock instability (SASI) that has been used to ease the shock revival in core collapse supernovae (CCSNe) neutrino-driven explosion models, might play a much more decisive role in supplying the stochastic angular momentum required to trigger an explosion with jittering jets. To play a minor role in neutrino-based explosion models, the kinetic energy of the gas inside the stalled shock associated with the transverse (non-radial) motion should be about more than ten percent of the energy of the accreted gas. We find that this implies a stochastic angular momentum that can reach about five percent of the Keplerian specific angular momentum around the newly born neutron star. Such an accretion flow leaves an open conical region along the poles with an average opening angle of about 5 degrees. The outflow from the open polar region powers an explosion according to the jittering-jets model.

  6. Constraint on the black hole spin of M87 from the accretion-jet model

    Science.gov (United States)

    Feng, Jianchao; Wu, Qingwen

    2017-09-01

    The millimetre bump, as found in high-resolution multiwaveband observations of M87 by Prieto et al., most possibly comes from the synchrotron emission of thermal electrons in advection-dominated accretion flow (ADAF). It is possible to constrain the accretion rate near the horizon if both the nuclear millimetre emission and its polarization are produced by the hot plasma in the accretion flow. The jet power of M87 has been extensively explored, which is around 8_-3^{+7}× 10^{42} erg s-1 based on the analysis of the X-ray cavity. The black hole (BH) spin can be estimated if the jet power and the accretion rate near the horizon are known. We model the multiwavelength spectral energy distribution (SED) of M87 with a coupled ADAF-jet model surrounding a Kerr BH, where the full set of relativistic hydrodynamical equations of the ADAF are solved. The hybrid jet formation model, as a variant of the Blandford-Znajek model, is used to model the jet power. We find that the SMBH should be fast rotating with a dimensionless spin parameter a_{*}˜eq 0.98_-0.02^{+0.012}.

  7. Energy, momentum and mass outflows and feedback from thick accretion discs around rotating black holes

    CERN Document Server

    Sadowski, A; Penna, R; Zhu, Y

    2013-01-01

    A set of long-duration general relativistic magnetohydrodynamic simulations of radiatively inefficient accretion discs around rotating black holes are presented, and are used to estimate the energy, mass and momentum outflow rates from such systems. Outflows occur via two fairly distinct modes: a relativistic jet and a sub-relativistic wind. The jet power depends strongly on the black hole spin and on the magnetic flux at the horizon. Unless these are very small, the energy output in the jet dominates over that in the wind. In the limit of a rapidly spinning black hole accreting in the magnetically arrested limit, when the magnetic flux at the black hole is maximum, the jet power exceeds the total rate of accretion of rest mass energy. However, because of strong collimation, the jet probably does not have a significant effect on its surrounding. In the case of an accreting supermassive black hole, external feedback via a jet is likely important only on the largest galaxy cluster scales. The power in the wind ...

  8. Planetesimal accretion in binary star systems

    CERN Document Server

    Marzari, Francesco; Scholl, Hans

    2007-01-01

    Numerical simulations of planetesimal accretion in circumprimary and circumbinary orbits are described. The secular perturbations by the com- panion star and gas drag are included in our models. We derive limits on the parameters of the binary system for which accretion and then planetary forma- tion are possible. In the circumbinary case we also outline the radial distance from the baricenter of the stars beyond which accumulation always occurs. Hy- drodynamical simulations are also presented to validate our N-body approach based on the axisymmetric approximation for the gas of the disk.

  9. Accretion disc flows around FU Orionis stars

    Science.gov (United States)

    Clarke, C. J.; Lin, D. N. C.; Papaloizou, J. C. B.

    1989-01-01

    The accretion disk model of FU Orionis systems in outburst is investigated by examining the time-dependent behavior of a disk around a low-mass protostar that accretes at 0.00001-0.0001 solar masses/yr. It is found that the disk may be stabilized against the thermal ionization instability by the effect of advective heat transport and that it may therefore exist in the quasi-steady-state observed in post-outburst FU Orionis systems. The disk models are used to discuss the cosmochemical consequences of possible FU Ori events during the evolution of the primordial solar nebula.

  10. Massive Star Formation: Accreting from Companion

    Indian Academy of Sciences (India)

    X. Chen; J. S. Zhang

    2014-09-01

    We report the possible accretion from companion in the massive star forming region (G350.69–0.49). This region seems to be a binary system composed of a diffuse object (possible nebulae or UC HII region) and a Massive Young Stellar Object (MYSO) seen in Spitzer IRAC image. The diffuse object and MYSO are connected by the shock-excited 4.5 m emission, suggesting that the massive star may form through accreting material from the companion in this system.

  11. Accretion and nuclear activity of quiescent supermassive black holes. II: optical study and interpretation

    CERN Document Server

    Soria, R; Fabbiano, G; Baldi, A; Elvis, M; Jerjen, H; Pellegrini, S; Siemiginowska, A; Soria, Roberto; Graham, Alister W.; Fabbiano, Giuseppina; Baldi, Alessandro; Elvis, Martin; Jerjen, Helmut; Pellegrini, Silvia; Siemiginowska, Aneta

    2006-01-01

    Our X-ray study of the nuclear activity in a new sample of six quiescent early-type galaxies, and in a larger sample from the literature, confirmed (Soria et al., Paper I) that the Bondi accretion rate of diffuse hot gas is not a good indicator of the supermassive black hole (SMBH) X-ray luminosity. Here we suggest that a more reliable estimate of the accretion rate must include the gas released by the stellar population inside the sphere of influence of the SMBH, in addition to the Bondi inflow of hot gas across that surface. We use optical surface-brightness profiles to estimate the mass-loss rate from stars in the nuclear region: we show that for our sample of galaxies it is an order of magnitude higher (~ 10^{-4} - 10^{-3} M_sun/yr) than the Bondi inflow rate of hot gas, as estimated from Chandra (Paper I). Only by taking into account both sources of fuel can we constrain the true accretion rate, the accretion efficiency, and the power budget. Radiatively efficient accretion is ruled out, for quiescent SM...

  12. A two-fluid model for black-hole accretion flows: particle acceleration and disc structure

    Science.gov (United States)

    Lee, Jason P.; Becker, Peter A.

    2017-02-01

    Hot, tenuous advection-dominated accretion flows around black holes are ideal sites for the Fermi acceleration of relativistic particles at standing shock waves in the accretion disc. Previous work has demonstrated that the shock-acceleration process can be efficient enough to power the observed, strong outflows in radio-loud active galaxies such as M87. However, the dynamical effect (back-reaction) on the flow, exerted by the pressure of the relativistic particles, has not been previously considered, and this effect can have a significant influence on the disc structure. We reexamine the problem by developing a new, two-fluid model for the structure of the accretion disc that includes the dynamical effect of the relativistic particle pressure, combined with the pressure of the background (thermal) gas. The new model is analogous to the two-fluid model of cosmic ray acceleration in supernova-driven shock waves. As part of the model, we also develop a new set of shock jump conditions, which are solved along with the hydrodynamic conservation equations to determine the structure of the accretion disc. The solutions include the formation of a mildly relativistic outflow (jet) at the shock radius, driven by the relativistic particles accelerated in the disc. One of our main conclusions is that in the context of the new two-fluid accretion model, global smooth (shock-free) solutions do not exist, and the disc must always contain a standing shock wave, at least in the inviscid case considered here.

  13. Workshop on Physics of Accretion Disks Around Compact and Young Stars

    Science.gov (United States)

    Liang, E (Editor); Stepinski, T. F. (Editor)

    1995-01-01

    The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster presentations, and a list of workshop participants. Accretion disks are believed to surround many stars. Some of these disks form around compact stars, such as white dwarfs, neutron stars, or black holes that are members of binary systems and reveal themselves as a power source, especially in the x-ray and gamma regions of the spectrum. On the other hand, protostellar disks are believed to be accretion disks associated with young, pre-main-sequence stars and manifest themselves mostly in infrared and radio observations. These disks are considered to be a natural outcome of the star formation process. The focus of this workshop included theory and observations relevant to accretion disks around compact objects and newly forming stars, with the primary purpose of bringing the two communities together for intellectual cross-fertilization. The nature of the workshop was exploratory, to see how much interaction is possible between distinct communities and to better realize the local potential in this subject. A critical workshop activity was identification and documentation of key issues that are of mutual interest to both communities.

  14. The S2 star as a probe of the accretion disk of Sgr A*

    CERN Document Server

    Giannios, Dimitrios

    2013-01-01

    How accretion proceeds around the massive black hole in the Galactic center and other highly sub-Eddington accretors remains poorly understood. The orbit of the S2 star in the Galactic center passes through the accretion disk of the massive black hole and any observational signature from such interaction may be used as an accretion probe. Because of its early stellar type, S2 is expected to possess a fairly powerful wind. We show here that the ram pressure of the accretion disk shocks the stellar wind fairly close to the star. The shocked fluid reaches a temperature of ~ 1 keV and cools efficiently through optically thin, thermal bremsstrahlung emission. The radiation from the shocked wind peaks around the epoch of the pericenter passage of the star at a luminosity potentially comparable to the quiescent emission detected from Sgr A*. Detection of shocked wind radiation can constrain the density of the accretion disk at a distance of several thousands of gravitational radii from the black hole.

  15. Observational appearance of inefficient accretion flows and jets in 3D GRMHD simulations: Application to Sgr~A*

    CERN Document Server

    Moscibrodzka, Monika; Shiokawa, Hotaka; Gammie, Charles F

    2014-01-01

    Radiatively inefficient accretion flows (RIAFs) are believed to power supermassive black holes (SMBH) in the underluminous cores of galaxies. Such black holes are typically accompanied by flat-spectrum radio cores indicating the presence of moderately relativistic jets. One of the best constrained RIAFs is associated with the SMBH in the Galactic center, Sgr A*. Since the plasma in RIAFs is only weakly collisional, the dynamics and the radiative properties of these systems are very uncertain. Here we want to study the impact of varying electron temperature on the appearance of accretion flows and jets. Using 3-D GRMHD accretion flow simulations, we use ray tracing methods to predict spectra and radio images of RIAFs allowing for different electron heating mechanisms in the in- and outflowing parts of the simulations. We find that small changes in the electron temperature can result in dramatic differences in the relative dominance of jets and accretion flows. Application to Sgr A* shows that radio spectrum an...

  16. Bondi accretion in trumpet geometries

    Science.gov (United States)

    Miller, August J.; Baumgarte, Thomas W.

    2017-02-01

    The Bondi solution, which describes the radial inflow of a gas onto a non-rotating black hole, provides a powerful test for numerical relativistic codes. However, the Bondi solution is usually derived in Schwarzschild coordinates, which are not well suited for dynamical spacetime evolutions. Instead, many current numerical relativistic codes adopt moving-puncture coordinates, which render black holes in trumpet geometries. Here we transform the Bondi solution into trumpet coordinates, which result in regular expressions for the fluid flow extending into the black-hole interior. We also evolve these solutions numerically and demonstrate their usefulness for testing and calibrating numerical codes.

  17. On the resonant behavior of longitudinally vibrating accreting rods

    CSIR Research Space (South Africa)

    Shatalov, M

    2012-09-01

    Full Text Available The theory of accreting structures is a new and fast developing branch of analytical mechanics basing on the theory of partial differential and integral equations. In the present paper the authors analyze qualitative properties of accreting rods...

  18. Bondi-Hoyle-Lyttleton Accretion onto a Protoplanetary Disk

    CERN Document Server

    Moeckel, Nickolas

    2009-01-01

    Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this `tail-end' accretion for planet formation was recently highlighted by Throop & Bally (2008). While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such ...

  19. Spin equilibrium in strongly-magnetized accreting stars

    CERN Document Server

    D'Angelo, Caroline

    2016-01-01

    The spin rate of a strongly-magnetized accreting star is regulated by the interaction between the star's magnetic field and the accreting gas. These systems are often hypothesized to be in `spin equilibrium' with their surrounding accretion flows such that the net spin change of the star as a result of accretion is very small. This condition requires that the accretion rate changes more slowly than it takes the star to reach spin equilibrium. However, this is not true for most magnetically accreting stars, which have strongly variable accretion outbursts (by one to many orders of magnitude) on timescales much shorter than the time it would take to reach spin equilibrium. This paper examines how accretion outbursts affect the time a star takes to reach spin equilibrium and its final equilibrium spin period. I consider several different models for angular momentum loss -- where angular momentum is carried away in an outflow (the standard `propeller', centrifugally-launched outflow), where most angular momentum ...

  20. Stability of black hole accretion disks

    Directory of Open Access Journals (Sweden)

    Czerny B.

    2012-12-01

    Full Text Available We discuss the issues of stability of accretion disks that may undergo the limit-cycle oscillations due to the two main types of thermal-viscous instabilities. These are induced either by the domination of radiation pressure in the innermost regions close to the central black hole, or by the partial ionization of hydrogen in the zone of appropriate temperatures. These physical processes may lead to the intermittent activity in AGN on timescales between hundreds and millions of years. We list a number of observational facts that support the idea of the cyclic activity in high accretion rate sources. We conclude however that the observed features of quasars may provide only indirect signatures of the underlying instabilities. Also, the support from the sources with stellar mass black holes, whose variability timescales are observationally feasible, is limited to a few cases of the microquasars. Therefore we consider a number of plausible mechanisms of stabilization of the limit cycle oscillations in high accretion rate accretion disks. The newly found is the stabilizing effect of the stochastic viscosity fluctuations.

  1. Accretion Models for Young Neutron Stars

    OpenAIRE

    Alpar, M. Ali

    2003-01-01

    Interaction with possible fallback material, along with the magnetic fields and rotation rates at birth should determine the fates and categories of young neutron stars. This paper addresses some issues related to pure or hybrid accretion models for explaining the properties of young neutron stars.

  2. Realizability of stationary spherically symmetric transonic accretion

    CERN Document Server

    Ray, A K; Ray, Arnab K.

    2002-01-01

    The spherically symmetric stationary transonic (Bondi) flow is considered a classic example of an accretion flow. This flow, however, is along a separatrix, which is usually not physically realizable. We demonstrate, using a pedagogical example, that it is the dynamics which selects the transonic flow.

  3. Viscosity in accretion discs. [for binary stars

    Science.gov (United States)

    Katz, J. I.

    1980-01-01

    Both HerX-1 and SS433 may contain accretion disks slaved to a precessing companion star. If so, it is possible to bound the effective viscosity in these disks. The results, in terms of the disk parameter alpha, are lower bounds of 0.01 for HerX-1 and of 0.1 for SS433.

  4. Radiation of accretion discs: the eclipses

    Energy Technology Data Exchange (ETDEWEB)

    Schwarzenberg-Czerny, A.

    1984-05-01

    Light curves have been calculated for eclipses of the accretion disc in a cataclysmic binary. The Roche geometry of the cool component was taken into account and the stellar atmospheres were interpolated to provide the local spectrum of the radiation from the disc. The dependence of the light curve on the parameters of the disc is discussed.

  5. Jets at lowest mass accretion rates

    NARCIS (Netherlands)

    Maitra, D.; Cantrell, A.; Markoff, S.; Falcke, H.; Miller, J.; Bailyn, C.

    2011-01-01

    We present results of recent observations and theoretical modeling of data from black holes accreting at very low luminosities (L/LEdd ≲ 10−8). We discuss our newly developed time-dependent model for episodic ejection of relativistic plasma within a jet framework, and a successful application of thi

  6. Turbulent Comptonization in Relativistic Accretion Disks

    CERN Document Server

    Socrates, A; Blaes, Omer M; Socrates, Aristotle; Davis, Shane W.; Blaes, Omer

    2006-01-01

    Turbulent Comptonization, a potentially important damping and radiation mechanism in relativistic accretion flows, is discussed. Particular emphasis is placed on the physical basis, relative importance, and thermodynamics of turbulent Comptonization. The effects of metal-absorption opacity on the spectral component resulting from turbulent Comptonization is considered as well.

  7. Two-dimensional vortices and accretion disks

    NARCIS (Netherlands)

    Nauta, Michiel Doede

    2001-01-01

    Observations show that there are disks around certain stars that slowly rain down on the central (compact) object: accretion disks. The rate of depletion of the disk might be slow but is still larger than was expected on theoretical grounds. That is why it has been suggested that the disks are tu

  8. Oligarchic planetesimal accretion and giant planet formation

    CERN Document Server

    Fortier, A; Brunini, A

    2007-01-01

    Aims. In the context of the core instability model, we present calculations of in situ giant planet formation. The oligarchic growth regime of solid protoplanets is the model adopted for the growth of the core. Methods. The full differential equations of giant planet formation were numerically solved with an adaptation of a Henyey-type code. The planetesimals accretion rate was coupled in a self-consistent way to the envelope's evolution. Results. We performed several simulations for the formation of a Jupiter-like object by assuming various surface densities for the protoplanetary disc and two different sizes for the accreted planetesimals. We find that the atmospheric gas drag gives rise to a major enhancement on the effective capture radius of the protoplanet, thus leading to an average timescale reduction of 30% -- 55% and ultimately to an increase by a factor of 2 of the final mass of solids accreted as compared to the situation in which drag effects are neglected. With regard to the size of accreted pla...

  9. Probing thermonuclear burning on accreting neutron stars

    NARCIS (Netherlands)

    Keek, L.

    2008-01-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes

  10. Cooling of Accretion-Heated Neutron Stars

    Indian Academy of Sciences (India)

    Rudy Wijnands; Nathalie Degenaar; Dany Page

    2017-09-01

    We present a brief, observational review about the study of the cooling behaviour of accretion-heated neutron stars and the inferences about the neutron-star crust and core that have been obtained from these studies. Accretion of matter during outbursts can heat the crust out of thermal equilibrium with the core and after the accretion episodes are over, the crust will cool down until crust-core equilibrium is restored. We discuss the observed properties of the crust cooling sources and what has been learned about the physics of neutron-star crusts. We also briefly discuss those systems that have been observed long after their outbursts were over, i.e, during times when the crust and core are expected to be in thermal equilibrium. The surface temperature is then a direct probe for the core temperature. By comparing the expected temperatures based on estimates of the accretion history of the targets with the observed ones, the physics of neutron-star cores can be investigated. Finally, we discuss similar studies performed for strongly magnetized neutron stars in which the magnetic field might play an important role in the heating and cooling of the neutron stars.

  11. Millisecond phenomena in mass accreting neutron stars

    NARCIS (Netherlands)

    van der Klis, M.; Cohen, L.

    2007-01-01

    The past twelve years have seen the discovery, with NASA's Rossi X-ray Timing Explorer (RXTE), of several long-predicted phenomena associated with the accretion of matter onto a neutron star in a binary (double) star system. These phenomena are observed in the strong X-ray emission produced by these

  12. The annual mean sketches and climatological variability of the volume and heat transports through the inter-basin passages:A study based on 1 400-year spin up of MOM4p1

    Institute of Scientific and Technical Information of China (English)

    ZHU Yaohua; WEI Zexun; WANG Yonggang; GUAN Yuping; WANG Xinyi

    2014-01-01

    The annual mean volume and heat transport sketches through the inter-basin passages and transoceanic sections have been constructed based on 1 400-year spin up results of the MOM4p1. The spin up starts from a state of rest, driven by the monthly climatological mean force from the NOAA World Ocean Atlas (1994). The volume transport sketch reveals the northward transport throughout the Pacific and southward trans-port at all latitudes in the Atlantic. The annual mean strength of the Pacific-Arctic-Atlantic through flow is 0.63×106 m3/s in the Bering Strait. The majority of the northward volume transport in the southern Pacific turns into the Indonesian through flow (ITF) and joins the Indian Ocean equatorial current, which subse-quently flows out southward from the Mozambique Channel, with its majority superimposed on the Ant-arctic Circumpolar Current (ACC). This anti-cyclonic circulation around Australia has a strength of 11×106 m3/s according to the model-produced result. The atmospheric fresh water transport, known as P-E+R (pre-cipitation minus evaporation plus runoff ), constructs a complement to the horizontal volume transport of the ocean. The annual mean heat transport sketch exhibits a northward heat transport in the Atlantic and poleward heat transport in the global ocean. The surface heat flux acts as a complement to the horizontal heat transport of the ocean. The climatological volume transports describe the most important features through the inter-basin passages and in the associated basins, including:the positive P-E+R in the Arctic substantially strengthening the East Greenland Current in summer;semiannual variability of the volume transport in the Drake Passage and the southern Atlantic-Indian Ocean passage;and annual transport vari-ability of the ITF intensifying in the boreal summer. The climatological heat transports show heat storage in July and heat deficit in January in the Arctic;heat storage in January and heat deficit in July in the

  13. Gravitational Waves from Fallback Accretion onto Neutron Stars

    CERN Document Server

    Piro, Anthony L

    2012-01-01

    Massive stars generally end their lives as neutron stars (NSs) or black holes (BHs), with NS formation typically occurring at the low mass end and collapse to a BH more likely at the high mass end. In an intermediate regime, with a mass range that depends on the uncertain details of rotation and mass loss during the star's life, a NS is initially formed which then experiences fallback accretion and collapse to a BH. The electromagnetic consequence of such an event is not clear. Depending on the progenitor's structure, possibilities range from a long gamma-ray burst to a Type II supernova (that may or may not be jet-powered) to a collapse with a weak electromagnetic signature. Gravitational waves (GWs) provide the exciting opportunity to peer through the envelope of a dying massive star and directly probe what is occurring inside. We explore whether fallback onto young NSs can be detected by ground-based interferometers. When the incoming material has sufficient angular momentum to form a disk, the accretion s...

  14. Evolving Nonthermal Electrons in Simulations of Black Hole Accretion

    Science.gov (United States)

    Chael, Andrew; Narayan, Ramesh; Sadowski, Aleksander

    2017-06-01

    Current simulations of hot accretion flows around black holes assume either a single-temperature gas or, at best, a two-temperature gas with thermal ions and electrons. However, processes like magnetic reconnection and shocks can accelerate electrons into a nonthermal distribution, which will not quickly thermalise at the very low densities found in many systems. Such nonthermal electrons have been invoked to explain the infrared and X-ray spectra and strong variability of Sagittarius A* (Sgr A*), the black hole at the Galactic Center. We present a method for self-consistent evolution of a nonthermal electron population in the GRMHD code KORAL. The electron distribution is tracked across Lorentz factor space and is evolved in space and time, in parallel with thermal electrons, thermal ions, and radiation. At present, for simplicity, energy injection into the nonthermal distribution is taken as a fixed fraction of the local electron viscous heating rate. Numerical results are presented for a model with a low mass accretion rate similar to Sgr A*. We find that the presence of a nonthermal population of electrons has negligible effect on the overall dynamics of the system. Relative to a purely thermal simulation, the radiative power in the nonthermal simulation is enhanced at large radii and at high frequencies. The energy distribution of the nonthermal electrons shows a synchrotron cooling break, with the break Lorentz factor varying with location and time, reflecting the complex interplay between the local viscous heating rate, magnetic field strength, and fluid velocity.

  15. Variabilities of gamma-ray bursts from black hole hyper-accretion discs

    Science.gov (United States)

    Lin, Da-Bin; Lu, Zu-Jia; Mu, Hui-Jun; Liu, Tong; Hou, Shu-Jin; Lü, Jing; Gu, Wei-Min; Liang, En-Wei

    2016-11-01

    The emission from black hole binaries (BHBs) and active galactic nuclei (AGNs) display significant aperiodic variabilities. The most promising explanation for these variabilities is the propagating fluctuations in the accretion flow. It is natural to expect that the mechanism driving variabilities in BHBs and AGNs may operate in a black hole hyper-accretion disc, which is believed to power gamma-ray bursts (GRBs). We study the variabilities of jet power in GRBs based on the model of propagating fluctuations. It is found that the variabilities of jet power and the temporal profile of erratic spikes in this scenario are similar to those in observed light curves of prompt gamma-ray emission of GRBs. Our results show that the mechanism driving X-ray variabilities in BHBs and AGNs may operate in the central engine to drive the variabilities of GRBs.

  16. Variabilities of Gamma-ray Bursts from Black Hole Hyper-accretion Disks

    CERN Document Server

    Lin, Da-Bin; Mu, Hui-Jun; Liu, Tong; Hou, Shu-Jin; Lv, Jing; Gu, Wei-Min; Liang, En-Wei

    2016-01-01

    The emission from black hole binaries (BHBs) and active galactic nuclei (AGNs) displays significant aperiodic variabilities. The most promising explanation for these variabilities is the propagating fluctuations in the accretion flow. It is natural to expect that the mechanism driving variabilities in BHBs and AGNs may operate in a black hole hyper-accretion disk, which is believed to power gamma-ray bursts (GRBs). We study the variabilities of jet power in GRBs based on the model of propagating fluctuations. It is found that the variabilities of jet power and the temporal profile of erratic spikes in this scenario are similar to those in observed light curves of prompt gamma-ray emission of GRBs. Our results show that the mechanism driving X-ray variabilities in BHBs and AGNs may operate in the central engine to drive the variabilities of GRBs.

  17. Spin-Up in a Rectangular Cylinder

    Science.gov (United States)

    1993-12-01

    cylinder by scaling as follows: I I IElt , and p = E’,X, 3.22 where we have scaled the radial and vertical flow to be higher order in Ekman number than the...two flow visualization systems, and the rectangular tank with prepared water. Fig- ure 4.1 is a schematic of this system, which we describe below.I I

  18. Accretion from debris disks onto white dwarfs : Fingering (thermohaline) instability and derived accretion rates

    CERN Document Server

    Deal, M; Vauclair, G; Vauclair, S; Wachlin, F C

    2013-01-01

    Recent observations of a large number of DA and DB white dwarfs show evidence of debris disks, which are the remnants of old planetary systems. The infrared excess detected with \\emph{Spitzer} and the lines of heavy elements observed in their atmospheres with high-resolution spectroscopy converge on the idea that planetary material accretes onto these stars. Accretion rates have been derived by several authors with the assumption of a steady state between accretion and gravitational settling. The results are unrealistically different for DA and DB white dwarfs. When heavy matter is accreted onto stars, it induces an inverse $\\mu$-gradient that leads to fingering (thermohaline) convection. The aim of this letter is to study the impact of this specific process on the derived accretion rates in white dwarfs and on the difference between DA and DB. We solve the diffusion equation for the accreted heavy elements with a time-dependent method. The models we use have been obtained both with the IRAP code, which compu...

  19. The relation between accretion rates and the initial mass function in hydrodynamical simulations of star formation

    CERN Document Server

    Maschberger, Th; Clarke, C J; Moraux, E

    2013-01-01

    We analyse a hydrodynamical simulation of star formation. Sink particles in the simulations which represent stars show episodic growth, which is presumably accretion from a core that can be regularly replenished in response to the fluctuating conditions in the local environment. The accretion rates follow $\\dot{m} \\propto m^{2/3}$, as expected from accretion in a gas-dominated potential, but with substantial variations over-laid on this. The growth times follow an exponential distribution which is tapered at long times due to the finite length of the simulation. The initial collapse masses have an approximately lognormal distribution with already an onset of a power-law at large masses. The sink particle mass function can be reproduced with a non-linear stochastic process, with fluctuating accretion rates $\\propto m^{2/3}$, a distribution of seed masses and a distribution of growth times. All three factors contribute equally to the form of the final sink mass function. We find that the upper power law tail of...

  20. Stellar and Quasar Feedback in Concert: Effects on AGN Accretion, Obscuration, and Outflows

    CERN Document Server

    Hopkins, Philip F; Faucher-Giguere, Claude-Andre; Quataert, Eliot; Murray, Norman

    2015-01-01

    We use hydrodynamic simulations to study the interaction of realistic active galactic nucleus (AGN) feedback mechanisms (accretion-disk winds & Compton heating) with a multi-phase interstellar medium (ISM). Our ISM model includes radiative cooling and explicit stellar feedback from multiple processes. We simulate radii ~0.1-100 pc around an isolated (non-merging) black hole. These are the scales where the accretion rate onto the black hole is determined and where AGN-powered winds and radiation couple to the ISM. Our primary results include: (1) The black hole accretion rate on these scales is determined by exchange of angular momentum between gas and stars in gravitational instabilities. This produces accretion rates of ~0.03-1 Msun/yr, sufficient to power a luminous AGN. (2) The gas disk in the galactic nucleus undergoes an initial burst of star formation followed by several Myrs where stellar feedback suppresses the star formation rate per dynamical time. (3) AGN winds injected at small radii with mome...

  1. New Insights on the Accretion Disk-Winds Connection in Radio-Loud AGNs from Suzaku

    Science.gov (United States)

    Tombesi, F.; Sambruna, R. M.; Reeves, J. N.; Braito, V.; Cappi, M.; Reynolds, S.; Mushotzky, R. F.

    2011-01-01

    From the spectral analysis of long Suzaku observations of five radio-loud AGNs we have been able to discover the presence of ultra-fast outflows with velocities ,,approx.0.1 c in three of them, namely 3C III, 3C 120 and 3C 390.3. They are consistent with being accretion disk winds/outflows. We also performed a follow-up on 3C III to monitor its outflow on approx.7 days time-scales and detected an anti-correlated variability of a possible relativistic emission line with respect to blue-shifted Fe K features, following a flux increase. This provides the first direct evidence for an accretion disc-wind connection in an AGN. The mass outflow rate of these outflows can be comparable to the accretion rate and their mechanical power can correspond to a significant fraction of the bolometric luminosity and is comparable to their typical jet power. Therefore, they can possibly play a significant role in the expected feedback from AGNs and can give us further clues on the relation between the accretion disk and the formation of winds/jets.

  2. New insights on the accretion disk-winds connection in radio-loud AGNs from Suzaku

    CERN Document Server

    Tombesi, F; Reeves, J N; Braito, V; Cappi, M; Reynolds, C S; Mushotzky, R F

    2011-01-01

    From the spectral analysis of long Suzaku observations of five radio-loud AGNs we have been able to discover the presence of ultra-fast outflows with velocities ~0.1c in three of them, namely 3C 111, 3C 120 and 3C 390.3. They are consistent with being accretion disk winds/outflows. We also performed a follow-up on 3C 111 to monitor its outflow on ~7days time-scales and detected an anti-correlated variability of a possible relativistic emission line with respect to blue-shifted Fe K features, following a flux increase. This provides the first direct evidence for an accretion disk-wind connection in an AGN. The mass outflow rate of these outflows can be comparable to the accretion rate and their mechanical power can correspond to a significant fraction of the bolometric luminosity and is comparable to their typical jet power. Therefore, they can possibly play a significant role in the expected feedback from AGNs and can give us further clues on the relation between the accretion disk and the formation of winds/...

  3. Radiative efficiency, variability and Bondi accretion onto massive black holes: from mechanical to quasar feedback in brightest cluster galaxies

    CERN Document Server

    Russell, H R; Edge, A C; Hogan, M T; Main, R A; Vantyghem, A N

    2012-01-01

    We examine unresolved nuclear X-ray sources in 57 brightest cluster galaxies to study the relationship between nuclear X-ray emission and accretion onto supermassive black holes (SMBHs). The majority of the clusters in our sample have prominent X-ray cavities embedded in the surrounding hot atmospheres, which we use to estimate mean jet power and average accretion rate onto the SMBHs over the past several hundred Myr. We find that ~50% of the sample have detectable nuclear X-ray emission. The nuclear X-ray luminosity is correlated with average accretion rate determined using X-ray cavities, which is consistent with the hypothesis that nuclear X-ray emission traces ongoing accretion. The results imply that jets in systems that have experienced recent AGN outbursts, in the last ~10^7yr, are `on' at least half of the time. Nuclear X-ray sources become more luminous with respect to the mechanical jet power as the mean accretion rate rises. We show that nuclear radiation exceeds the jet power when the mean accreti...

  4. Accretion and Orbital Inspiral in Gas-assisted Supermassive Black Hole Binary Mergers

    Science.gov (United States)

    Rafikov, Roman R.

    2016-08-01

    Many galaxies are expected to harbor binary supermassive black holes (SMBHs) in their centers. Their interaction with the surrounding gas results in the accretion and exchange of angular momentum via tidal torques, facilitating binary inspiral. Here, we explore the non-trivial coupling between these two processes and analyze how the global properties of externally supplied circumbinary disks depend on the binary accretion rate. By formulating our results in terms of the angular momentum flux driven by internal stresses, we come up with a very simple classification of the possible global disk structures, which differ from the standard constant \\dot{M} accretion disk solution. The suppression of accretion by the binary tides, leading to a significant mass accumulation in the inner disk, accelerates binary inspiral. We show that once the disk region strongly perturbed by the viscously transmitted tidal torque exceeds the binary semimajor axis, the binary can merge in less than its mass-doubling time due to accretion. Thus, unlike the inspirals driven by stellar scattering, the gas-assisted merger can occur even if the binary is embedded in a relatively low-mass disk (lower than its own mass). This is important for resolving the “last parsec” problem for SMBH binaries and understanding powerful gravitational wave sources in the universe. We argue that the enhancement of accretion by the binary found in some recent simulations cannot persist for a long time and should not affect the long-term orbital inspiral. We also review existing simulations of SMBH binary-disk coupling and propose a numerical setup which is particularly well suited to verifying our theoretical predictions.

  5. Clumpy wind accretion in supergiant neutron star high mass X-ray binaries

    Science.gov (United States)

    Bozzo, E.; Oskinova, L.; Feldmeier, A.; Falanga, M.

    2016-05-01

    The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.

  6. In-flight photogrammetric measurement of wing ice accretions

    Science.gov (United States)

    Mcknight, R. C.; Palko, R. L.; Humes, R. L.

    1986-01-01

    A photographic instrumentation system was developed for the Lewis icing research aircraft to measure wind ice accretions during flight. The system generates stereo photographs of the accretions which are then photogrammetrically measured by the Air Force Arnold Engineering and Development Center. The measurements yield a survey of spatial coordinates of an accretion's surface to an accuracy of at least + or - 0.08 cm. The accretions can then be matched to corresponding icing cloud and aerodynamic measurements. The system is being used to measure rime, mixed, and clear natural ice accretions.

  7. Theory of disk accretion onto supermassive black holes

    CERN Document Server

    Armitage, P J

    2004-01-01

    Accretion onto supermassive black holes produces both the dramatic phenomena associated with active galactic nuclei and the underwhelming displays seen in the Galactic Center and most other nearby galaxies. I review selected aspects of the current theoretical understanding of black hole accretion, emphasizing the role of magnetohydrodynamic turbulence and gravitational instabilities in driving the actual accretion and the importance of the efficacy of cooling in determining the structure and observational appearance of the accretion flow. Ongoing investigations into the dynamics of the plunging region, the origin of variability in the accretion process, and the evolution of warped, twisted, or eccentric disks are summarized.

  8. Accretion disc onto a static non-baryonic compact object

    CERN Document Server

    Torres, D F

    2002-01-01

    We study the emissivity properties of a geometrically thin, optically thick, steady accretion disc about a static boson star. Starting from a numerical computation of the metric potentials and the rotational velocities of the particles in the vicinity of the compact object, we obtain the power per unit area, the temperature of the disc, and the spectrum of the emitted radiation. In order to see if different central objects could be actually distinguished, all these results are compared with the case of a central Schwarzschild black hole of equal mass. We considered different situations both for the boson star, assumed with and without self-interactions, and the disc, whose internal commencement can be closer to the center than in the black hole case. We finally make some considerations about the Eddington luminosity, which becomes radially dependent for a transparent object. We found that, particularly at high energies, differences in the emitted spectrum are notorious. Reasons for that are discussed.

  9. The mass accretion rate of galaxy clusters: a measurable quantity

    CERN Document Server

    De Boni, Cristiano; Diaferio, Antonaldo; Giocoli, Carlo; Baldi, Marco

    2015-01-01

    We explore the possibility of measuring the mass accretion rate of galaxy clusters by using dense galaxy redshift surveys of their outer regions. By approximating the accretion with the infall of a spherical shell, the mass accretion rate only depends on the mass profile of the cluster in a thin shell at radii larger than $R_{200}$. This approximation is rather crude in hierarchical clustering scenarios, where both smooth accretion and aggregation of smaller dark matter haloes contribute to the mass accretion of clusters. Nevertheless, in the redshift range $z=[0,1]$, our prescription returns an average mass accretion rate within $20 \\%$ of the average rate derived with the more realistic merger trees of dark matter haloes extracted from $N$-body simulations. The mass accretion rate of galaxy clusters has been the topic of numerous detailed numerical and theoretical investigations, but so far it has remained inaccessible to measurements in the real Universe. Our result suggests that measuring the mass accreti...

  10. Thermally Driven Winds from Radiatively Inefficient Accretion Flows

    CERN Document Server

    Kawabata, Ryoji

    2009-01-01

    Radiatively inefficient accretion flows (RIAFs) are common feature of low-luminosity accretion flows, including quiescent states of X-ray binaries and low-lunimosity active galactic nuclei. Thermally driven winds are expected from such hot accretion flows. By assuming that the flow has self-similarity structure in the radial direction, we solve the vertical structure of the wind and accretion flows simultaneously and evaluate the mass loss rates by wind. We find that the ratio of the outflow rate to the accretion rate is approximately unity for a viscosity parameter, alpha lesssim 0.1, despite some uncertainties in the angular momentum and temperature distributions. That is, the accretion rate in the RIAFs is roughly proportional to the radius. Moreover, we elucidate the effect of cooling by wind on the underneath accretion flow, finding that this effect could be important for calculating energy spectrum of the RIAF. Observational implications are briefly discussed in the context of Sgr A*.

  11. Effects of ice accretion on the aerodynamics of bridge cables

    DEFF Research Database (Denmark)

    Demartino, C.; Koss, Holger; Georgakis, Christos T.;

    2015-01-01

    Undesirable wind induced vibrations of bridge cables can occur when atmospheric conditions are such to generate ice accretion. This paper contains the results of an extensive investigation of the effects of ice accretion due to in-cloud icing, on the aerodynamic characteristics of bridge hangers...... and stay cables. The aim of this paper is twofold; first, it was investigated the ice accretion process and the final shape of the ice accreted; then the aerodynamics of the ice accreted bridge cables was characterized, and related to the ice shape. Different climatic conditions, i.e. combinations...... of the ice accretions is given in the paper. Only for the bridge hanger case, a short description of the evolution of the ice accretions is given. The aerodynamic force coefficients were then measured with varying yaw angle, angle of attack and wind speed, and are presented and discussed in the paper...

  12. AS ABOVE, SO BELOW: EXPLOITING MASS SCALING IN BLACK HOLE ACCRETION TO BREAK DEGENERACIES IN SPECTRAL INTERPRETATION

    Energy Technology Data Exchange (ETDEWEB)

    Markoff, Sera; Silva, Catia V. [Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1098 XH Amsterdam (Netherlands); Nowak, Michael A. [Massachusetts Institute of Technology, Kavli Institute for Astrophysics, Cambridge, MA 02139 (United States); Gallo, Elena; Plotkin, Richard M. [Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042 (United States); Hynes, Robert [Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803-4001 (United States); Wilms, Jörn [Dr. Karl Remeis-Sternwarte and ECAP, Universität Erlangen-Nürnberg, D-96049 Bamberg (Germany); Maitra, Dipankar [Department of Physics and Astronomy, Wheaton College, Norton, MA 02766 (United States); Drappeau, Samia, E-mail: S.B.Markoff@uva.nl, E-mail: C.V.DeJesusSilva@uva.nl, E-mail: mnowak@space.mit.edu, E-mail: egallo@umich.edu, E-mail: rih@redstick.phys.lsu.edu, E-mail: joern.wilms@sternwarte.uni-erlangen.de, E-mail: maitra_dipankar@wheatoncollege.edu, E-mail: samia.drappeau@irap.omp.eu [CNRS, IRAP, BP 44346, F-31028 Toulouse cedex 4 (France)

    2015-10-20

    Over the past decade, evidence has mounted that several aspects of black hole (BH) accretion physics proceed in a mass-invariant way. One of the best examples of this scaling is the empirical “fundamental plane of BH accretion” relation linking mass, radio, and X-ray luminosity over eight orders of magnitude in BH mass. The currently favored theoretical interpretation of this relation is that the physics governing power output in weakly accreting BHs depends more on relative accretion rate than on mass. In order to test this theory, we explore whether a mass-invariant approach can simultaneously explain the broadband spectral energy distributions from two BHs at opposite ends of the mass scale but that are at similar Eddington accretion fractions. We find that the same model, with the same value of several fitted physical parameters expressed in mass-scaling units to enforce self-similarity, can provide a good description of two data sets from V404 Cyg and M81*, a stellar and supermassive BH, respectively. Furthermore, only one of several potential emission scenarios for the X-ray band is successful, suggesting it is the dominant process driving the fundamental plane relation at this accretion rate. This approach thus holds promise for breaking current degeneracies in the interpretation of BH high-energy spectra and for constructing better prescriptions of BH accretion for use in various local and cosmological feedback applications.

  13. Short-Term Variability of X-rays from Accreting Neutron Star Vela X-1: II. Monte-Carlo Modeling

    CERN Document Server

    Odaka, Hirokazu; Tanaka, Yasuyuki T; Watanabe, Shin; Takahashi, Tadayuki; Makishima, Kazuo

    2013-01-01

    We develop a Monte Carlo Comptonization model for the X-ray spectrum of accretion-powered pulsars. Simple, spherical, thermal Comptonization models give harder spectra for higher optical depth, while the observational data from Vela X-1 show that the spectra are harder at higher luminosity. This suggests a physical interpretation where the optical depth of the accreting plasma increases with mass accretion rate. We develop a detailed Monte-Carlo model of the accretion flow, including the effects of the strong magnetic field ($\\sim 10^{12}$ G) both in geometrically constraining the flow into an accretion column, and in reducing the cross section. We treat bulk-motion Comptonization of the infalling material as well as thermal Comptonization. These model spectra can match the observed broad-band {\\it Suzaku} data from Vela X-1 over a wide range of mass accretion rates. The model can also explain the so-called "low state", in which the uminosity decreases by an order of magnitude. Here, thermal Comptonization sh...

  14. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni

    CERN Document Server

    Muñoz-Darias, T; Sánchez, D Mata; Fender, R P; Padilla, M Armas; Linares, M; Ponti, G; Charles, P A; Mooley, K P; Rodriguez, J

    2016-01-01

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black hole transients show outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disc encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient black hole transient V404 Cyg, and interpreted as disrupted mass flow into the inner regions of its large accretion disc. Here, we report on the discovery of a sustained outer accretion disc wind in V404 Cyg, which is unlike any seen previously. We find that the outflowing wind is neutral, has a large covering factor, expands at 1% of the speed of light and triggers a nebular phase once accretion sharply drops and the ejecta become optically thin. The large expelled mass (> 10^-8 Msun) indicates that the outburst was prematurely ended when a sizeable fracti...

  15. Accretion onto black holes and relativistic jets

    CERN Document Server

    Belloni, Tomaso

    2007-01-01

    Relativistic jets from Active Galactic Nuclei are known since decades, but the study of the connection between accretion and ejection in these systems is hampered by the long time scales associated to these events. The past decade has seen a rapid advancement due to the observation of similar radio jets in galactic X-ray binaries, where the time scales are much shorter. A clear connection between accretion and ejection has been found for these systems, together with a solid characterization of the phenomenological properties of their outbursts. This wealth of new results has led to a detailed comparison between X-ray binaries and AGN, from which a number of correlations and scaling laws has been established. Here I briefly review the current observational status.

  16. Magnetised accretion discs in Kerr spacetimes

    CERN Document Server

    Ranea-Sandoval, Ignacio F

    2014-01-01

    We study the effect caused by external magnetic fields on the observed thermal spectra and iron line profiles of thin accretion discs formed around Kerr black holes and naked singularities. We aim to provide a tool that can be used to estimate the presence of magnetic fields in the neighbourhood of a compact object and to probe the cosmic censorship conjecture in these particular astrophysical environments. We developed a numerical scheme able to calculate thermal spectra of magnetised Page-Thorne accretion discs formed around rotating black holes and naked singularities as seen by an arbitrary distant observer. We incorporated two different magnetic field configurations: uniform and dipolar, using a perturbative scheme in the coupling constant between matter and magnetic field strength. Under the same assumptions, we obtained observed synthetic line profiles of the 6.4 keV fluorescent iron line. We show that an external magnetic field produces potentially observable modifications on the thermal energy spectr...

  17. Quasistars: Accreting black holes inside massive envelopes

    CERN Document Server

    Begelman, Mitchell C; Armitage, Philip J

    2007-01-01

    We study the structure and evolution of "quasistars," accreting black holes embedded within massive hydrostatic gaseous envelopes. These configurations may model the early growth of supermassive black hole seeds. The accretion rate onto the black hole adjusts so that the luminosity carried by the convective envelope equals the Eddington limit for the total mass. This greatly exceeds the Eddington limit for the black hole mass alone, leading to rapid growth of the black hole. We use analytic models and numerical stellar structure calculations to study the structure and evolution of quasistars. We derive analytically the scaling of the photospheric temperature with the black hole mass and envelope mass, and show that it decreases with time as the black hole mass increases. Once the photospheric temperature becomes lower than 10000 K, the photospheric opacity drops precipitously and the photospheric temperature hits a limiting value, analogous to the Hayashi track for red giants and protostars, below which no hy...

  18. Ice Accretion on Wind Turbine Blades

    DEFF Research Database (Denmark)

    Hudecz, Adriána; Koss, Holger; Hansen, Martin Otto Laver

    2013-01-01

    In this paper, both experimental and numerical simulations of the effects of ice accretion on a NACA 64-618 airfoil section with 7° angle of attack are presented. The wind tunnel tests were conducted in a closed-circuit climatic wind tunnel at Force Technology in Denmark. The changes of aerodynamic...... forces were monitored as ice was building up on the airfoil for glaze, rime and mixed ice. In the first part of the numerical analysis, the resulted ice profiles of the wind tunnel tests were compared to profiles estimated by using the 2D ice accretion code TURBICE. In the second part, Ansys Fluent...... of the rime iced ice profile follows the streamlines quite well, disturbing the flow the least. The TURBICE analysis agrees fairly with the profiles produced during the wind tunnel testing....

  19. Volatile accretion history of the Earth.

    Science.gov (United States)

    Wood, B J; Halliday, A N; Rehkämper, M

    2010-10-28

    It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albarède paints a different picture, proposing that the Earth first formed as a dry planet which, like the Moon, was devoid of volatile constituents. He suggests that the Earth's complement of volatile elements was only established later, by the addition of a small veneer of volatile-rich material at ∼100 Myr (here and elsewhere, ages are relative to the origin of the Solar System). Here we argue that the Earth's mass balance of moderately volatile elements is inconsistent with Albarède's hypothesis but is well explained by the standard model of accretion from partially volatile-depleted material, accompanied by core formation.

  20. The properties of external accretion discs

    Energy Technology Data Exchange (ETDEWEB)

    Pringle, J.E. (Space Telescope Science Inst., Baltimore, MD (USA))

    1991-02-15

    The properties of external accretion discs (discs with a central source of angular momentum) are explored both analytically and numerically. An illustrative example of the effect of a disc of material around a binary star on the stellar separation is considered. We consider a Greens-function-type solution in which an initial ring of matter is put in orbit around the central binary. We find that the solution splits temporally into three parts. First, the disc evolves as a standard accretion disc unaware of the inner boundary condition. Secondly, the disc interacts with the inner boundary and changes its character to become, thirdly, an outflowing disc which is propelled outwards by the source of angular momentum at the centre. (author).

  1. Eclipse Mapping: Astrotomography of Accretion Discs

    CERN Document Server

    Baptista, Raymundo

    2015-01-01

    The Eclipse Mapping Method is an indirect imaging technique that transforms the shape of the eclipse light curve into a map of the surface brightness distribution of the occulted regions. Three decades of application of this technique to the investigation of the structure, the spectrum and the time evolution of accretion discs around white dwarfs in cataclysmic variables have enriched our understanding of these accretion devices with a wealth of details such as (but not limited to) moving heating/cooling waves during outbursts in dwarf novae, tidally-induced spiral shocks of emitting gas with sub-Keplerian velocities, elliptical precessing discs associated to superhumps, and measurements of the radial run of the disc viscosity through the mapping of the disc flickering sources. This chapter reviews the principles of the method, discusses its performance, limitations, useful error propagation procedures, as well as highlights a selection of applications aimed at showing the possible scientific problems that ha...

  2. Energy-dependent variability from accretion flows

    CERN Document Server

    Zdziarski, A A

    2005-01-01

    We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependencies of the level of local variability (as expected from propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As example local emission, we consider blackbody, Wien and thermal Comptonization spectra. In addition to numerical results, we present a number of analytical formulae for the resulting rms. We also find an analytical formula for the disc Wien spectrum, which we find to be a very good approximation to the disc blackbody. We compare our results to the rms spectrum observed in an ultrasoft state of GRS 1915+105.

  3. Satellites in discs regulating the accretion luminosity

    CERN Document Server

    Syer, D; Syer, Dave; Clarke, Cathie

    1995-01-01

    We demonstrate, using a simple analytic model, that the presence of a massive satellite can globally modify the structure and emission properties of an accretion disc to which it is tidally coupled. We show, using two levels of numerical approximation, that the analytic model gives reasonable results. The results are applicable to two astrophysical situations. In the case of an active galactic nucleus, we consider the case of a \\sim 10^3\\Msun compact companion to the central black-hole and show that it could modulate the emitted spectrum on a timescale of \\sim10^5 years. In the case of a T Tauri accretion disc, a satellite such as a sub-dwarf or giant planet could modify the disc spectral energy distribution over a substantial fraction of the T Tauri star lifetime.

  4. Turbulent Comptonization in Black Hole Accretion Disks

    CERN Document Server

    Socrates, A; Blaes, Omer M; Socrates, Aristotle; Davis, Shane W.; Blaes, Omer

    2004-01-01

    In the inner-most regions of radiation pressure supported accretion disks, the turbulent magnetic pressure may greatly exceed that of the gas. If this is the case, it is possible for bulk Alfvenic motions driven by the magnetorotational instability (MRI) to surpass the electron thermal velocity. Bulk rather than thermal Comptonization may then be the dominant radiative process which mediates gravitational energy release. For sufficiently large turbulent stresses, we show that turbulent Comptonization produces a significant contribution to the far-UV and X-ray emission of black hole accretion disks. The existence of this spectral component provides a means of obtaining direct observational constraints on the nature of the turbulence itself. We describe how this component may affect the spectral energy distributions and variability properties of X-ray binaries and active galactic nuclei.

  5. Compositional evolution during rocky protoplanet accretion

    CERN Document Server

    Carter, Philip J; Elliott, Tim; Walter, Michael J; Stewart, Sarah T

    2015-01-01

    The Earth appears non-chondritic in its abundances of refractory lithophile elements, posing a significant problem for our understanding of its formation and evolution. It has been suggested that this non-chondritic composition may be explained by collisional erosion of differentiated planetesimals of originally chondritic composition. In this work, we present N-body simulations of terrestrial planet formation that track the growth of planetary embryos from planetesimals. We simulate evolution through the runaway and oligarchic growth phases under the Grand Tack model and in the absence of giant planets. These simulations include a state-of-the-art collision model which allows multiple collision outcomes, such as accretion, erosion, and bouncing events, that enables tracking of the evolving core mass fraction of accreting planetesimals. We show that the embryos grown during this intermediate stage of planet formation exhibit a range of core mass fractions, and that with significant dynamical excitation, enoug...

  6. On the Stability of Cubic Galileon Accretion

    CERN Document Server

    Bergliaffa, Santiago P E

    2016-01-01

    We examine the stability of steady-state galileon accretion for the case of a Schwarzshild black hole. Considering the galileon action up to the cubic term in a static and spherically symmetric background we obtain the general solution for the equation of motion which is divided in two branches. By perturbing this solution we define an effective metric which determines the propagation of fluctuations. In this general picture we establish the position of the sonic horizon together with the matching condition of the two branches on it. Restricting to the case of a Schwarzschild background, we show, via the analysis of the energy of the perturbations and its time derivative, that the accreting field is linearly stable.

  7. Variable protostellar accretion with episodic bursts

    CERN Document Server

    Vorobyov, Eduard I

    2015-01-01

    We present the latest development of the disk gravitational instability and fragmentation model, originally introduced by us to explain episodic accretion bursts in the early stages of star formation. Using our numerical hydrodynamics model with improved disk thermal balance and star-disk interaction, we computed the evolution of protostellar disks formed from the gravitational collapse of prestellar cores. In agreement with our previous studies, we find that cores of higher initial mass and angular momentum produce disks that are more favorable to gravitational instability and fragmentation, while a higher background irradiation and magnetic fields moderate the disk tendency to fragment. The protostellar accretion in our models is time-variable, thanks to the nonlinear interaction between different spiral modes in the gravitationally unstable disk, and can undergo episodic bursts when fragments migrate onto the star owing to the gravitational interaction with other fragments or spiral arms. Most bursts occur...

  8. Early dust evolution in protostellar accretion disks

    OpenAIRE

    2000-01-01

    We investigate dust dynamics and evolution during the formation of a protostellar accretion disk around intermediate mass stars via 2D numerical simulations. Using three different detailed dust models, compact spherical particles, fractal BPCA grains, and BCCA grains, we find that even during the early collapse and the first 10,000 yr of dynamical disk evolution, the initial dust size distribution is strongly modified. Close to the disk's midplane coagulation produces dust particles of sizes ...

  9. Massive thin accretion discs. Pt. 2; Polarization

    Energy Technology Data Exchange (ETDEWEB)

    Laor, A.; Netzer, H. (Tel Aviv Univ. (Israel)); Piran, T. (Hebrew Univ., Jerusalem (Israel). Racah Inst. of Physics)

    1990-02-15

    Thin accretion discs around massive black holes are believed to produce much of the observed optical-UV emission from AGN. Classical calculations predict that this radiation is highly polarized at large inclination angles, in contrast to observations of quasars and Seyfert galaxies. We have calculated the spectrum and polarization of such discs using an improved radiative transfer method with all the relevant opacity sources, and a full general relativistic treatment of the radiation propagation. (author).

  10. Broken discs: warp propagation in accretion discs

    OpenAIRE

    Nixon, Chris; King, Andrew

    2012-01-01

    We simulate the viscous evolution of an accretion disc around a spinning black hole. In general any such disc is misaligned, and warped by the Lense-Thirring effect. Unlike previous studies we use effective viscosities constrained to be consistent with the internal fluid dynamics of the disc. We find that nonlinear fluid effects, which reduce the effective viscosities in warped regions, can promote the breaking of the disc into two distinct planes. This occurs when the Shakura & Sunyaev dimen...

  11. Satellites in Discs: Regulating the Accretion Luminosity

    OpenAIRE

    Syer, Dave; Clarke, Cathie

    1995-01-01

    We demonstrate, using a simple analytic model, that the presence of a massive satellite can globally modify the structure and emission properties of an accretion disc to which it is tidally coupled. We show, using two levels of numerical approximation, that the analytic model gives reasonable results. The results are applicable to two astrophysical situations. In the case of an active galactic nucleus, we consider the case of a $\\sim 10^3\\Msun$ compact companion to the central black-hole and ...

  12. Probing thermonuclear burning on accreting neutron stars

    OpenAIRE

    Keek, L.

    2008-01-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes thermonuclear burning in the neutron star envelope, creating carbon and heavier elements. The fusion process may proceed in an unstable manner, resulting in a thermonuclear runaway. Within one seco...

  13. Interaction of Accretion Shocks with Winds

    Indian Academy of Sciences (India)

    Kinsuk Acharya; Sandip K. Chakrabarti; D. Molteni

    2002-03-01

    Accretion shocks are known to oscillate in presence of cooling processes in the disk. This oscillation may also cause quasi-periodic oscillations of black holes. In the presence of strong winds, these shocks have oscillations in vertical direction as well.We show examples of shock oscillations under the influence of both the effects. When the shocks are absent and the flow is cooler, the wind becomes weaker and the vertical oscillation becomes negligible.

  14. Numerical simulations of dissipationless disk accretion

    Science.gov (United States)

    Bogovalov, S. V.; Tronin, I. V.

    2017-09-01

    Our goal is to study the regime of disk accretion in which almost all of the angular momentum and energy is carried away by the wind outflowing from the disk in numerical experiments. For this type of accretion the kinetic energy flux in the outflowing wind can exceed considerably the bolometric luminosity of the accretion disk, what is observed in the plasma flow from galactic nuclei in a number of cases. In this paper we consider the nonrelativistic case of an outflow from a cold Keplerian disk. All of the conclusions derived previously for such a system in the self-similar approximation are shown to be correct. The numerical results agree well with the analytical predictions. The inclination angle of the magnetic field lines in the disk is less than 60°, which ensures a free wind outflow from the disk, while the energy flux per wind particle is greater than the particle rotation energy in its Keplerian orbit by several orders of magnitude, provided that the ratio r A/ r ≫ 1, where r A is the Alfvénic radius and r is the radius of the Keplerian orbit. In this case, the particle kinetic energy reaches half the maximum possible energy in the simulation region. The magnetic field collimates the outflowing wind near the rotation axis and decollimates appreciably the wind outflowing from the outer disk periphery.

  15. Magnetic Field Transport in Accretion Disks

    Science.gov (United States)

    Jafari, Amir; Vishniac, Ethan

    2017-06-01

    The most plausible theories for launching astrophysical jets rely on strong magnetic fields at the inner parts of some accretion disks. An internal dynamo can in principle generate small scale magnetic fields in situ but generating a large scale field in a disk seems a difficult task in the dynamo theories. In fact, as far as numerous numerical experiments indicate, a dynamo-generated field in general would not be coherent enough over the large length scales of order the disk's radius. Instead, a large scale poloidal field dragged in from the environment, and compressed by the accretion, provides a more promising possibility. The difficulty in the latter picture, however, arises from the reconnection of the radial field component across the mid-plane which annihilates the field faster than it is dragged inward by the accretion. We suggest that a combination of different effects, including magnetic buoyancy and turbulent pumping, is responsible for the vertical transport of the field lines toward the surface of the disk. The radial component of the poloidal field vanishes at the mid-plane, which efficiently impedes reconnection, and grows exponentially toward the surface where it can become much larger than the vertical field component. This allows the poloidal field to be efficiently advected to small radii until the allowed bending angle drops to of order unity, and the field can drive a strong outflow.

  16. Minidisks in Binary Black Hole Accretion

    Science.gov (United States)

    Ryan, Geoffrey; MacFadyen, Andrew

    2017-02-01

    Newtonian simulations have demonstrated that accretion onto binary black holes produces accretion disks around each black hole (“minidisks”), fed by gas streams flowing through the circumbinary cavity from the surrounding circumbinary disk. We study the dynamics and radiation of an individual black hole minidisk using 2D hydrodynamical simulations performed with a new general relativistic version of the moving-mesh code Disco. We introduce a comoving energy variable that enables highly accurate integration of these high Mach number flows. Tidally induced spiral shock waves are excited in the disk and propagate through the innermost stable circular orbit, providing a Reynolds stress that causes efficient accretion by purely hydrodynamic means and producing a radiative signature brighter in hard X-rays than the Novikov-Thorne model. Disk cooling is provided by a local blackbody prescription that allows the disk to evolve self-consistently to a temperature profile where hydrodynamic heating is balanced by radiative cooling. We find that the spiral shock structure is in agreement with the relativistic dispersion relation for tightly wound linear waves. We measure the shock-induced dissipation and find outward angular momentum transport corresponding to an effective alpha parameter of order 0.01. We perform ray-tracing image calculations from the simulations to produce theoretical minidisk spectra and viewing-angle-dependent images for comparison with observations.

  17. Water Masers in AGN Accretion Disks

    Science.gov (United States)

    Braatz, J. A.; Reid, M. J.; Greenhill, L. J.; Kuo, C.-Y.; Condon, J. J.; Lo, K.-Y.; Henkel, C.

    2009-08-01

    Water vapor masers at 22 GHz have been detected in over 100 galaxies, most of them AGNs. High resolution VLBI observations of these masers provide the only opportunity for direct imaging of sub-parsec structure in AGN accretion disks. The key science goals associated with such observations are concentrated in two areas. First, observations of nearby, bright sources, exemplified by NGC 4258, enable unique investigations of accretion disk geometry, substructure, thickness, and rotation properties. Second, when combined with spectral line monitoring, VLBI imaging and subsequent disk modeling enables the estimation of a distance to the host galaxy independent of standard candle arguments. In this contribution we present VLBI observations of two maser disk systems in galaxies well into the Hubble flow, UGC 3789 and NGC 6323. A long term goal in these studies is to measure the Hubble constant with high precision and, as a complement to CMB observations, constrain several key cosmological parameters, including the equation of state for dark energy. Observations with VSOP-2 at 22 GHz will have the resolution critical for mapping substructure in these accretion disks and will contribute to reducing systematic errors in the measurement of distances to galaxies.

  18. Magnetically driven accretion in protoplanetary discs

    CERN Document Server

    Simon, Jacob B; Kunz, Matthew W; Armitage, Philip J

    2015-01-01

    We characterize magnetically driven accretion at radii between 1 au and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary di...

  19. Modeling the Accretion Structure of AU Mon

    CERN Document Server

    Atwood-Stone, Corwin; Richards, Mercedes T; Budaj, Jan; Peters, Geraldine J

    2012-01-01

    AU Mon is a long-period (11.113 d) Algol-type binary system with a persistent accretion disk that is apparent as double-peaked H-alpha emission. We present previously unpublished optical spectra of AU Mon which were obtained over several years with dense orbital phase coverage. We utilize these data, along with archival UV spectra, to model the temperature and structure of the accretion disk and the gas stream. Synthetic spectral profiles for lines including H-alpha, H-beta, and the Al III and Si IV doublets were computed with the Shellspec program. The best match between the model spectra and the observations is obtained for an accretion disk of inner/outer radius 5.1/23 R_sun, thickness of 5.2 R_sun, density of 1.0e-13 g/cm^3, and maximum temperature of 14000 K, along with a gas stream at a temperature of ~8000 K transferring ~2.4e-9 M_sun/yr. We show H-alpha Doppler tomograms of the velocity structure of the gas, constructed from difference profiles calculated through sequentially subtracting contributions...

  20. Accretion on to Magnetic White Dwarfs

    Directory of Open Access Journals (Sweden)

    Wickramasinghe Dayal

    2014-01-01

    The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is “blobby”. This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence for blobby accretion in the generally lower field and disced IPs.

  1. Generalized Similarity for Accretion/Decretion Disks

    CERN Document Server

    Rafikov, Roman R

    2016-01-01

    Decretion (or external) disks are gas disks freely expanding to large radii due to their internal stresses. They are expected to naturally arise in tidal disruption events, around Be stars, in mass-losing post main sequence binaries, as a result of supernova fallback, etc. Their evolution is theoretically understood in two regimes: when the central object does not exert torque on the disk (a standard assumption for conventional accretion disks) or when no mass inflow (or outflow) occurs at the disk center. However, many astrophysical objects - circumbinary disks, Be stars, neutron stars accreting in a propeller regime, etc. - feature non-zero torque simultaneously with the non-zero accretion (or ejection of mass) at the disk center. We provide a general description for the evolution of such disks (both linear and non-linear) in the self-similar regime, to which the disk should asymptotically converge with time. We identify a similarity parameter $\\lambda$, which is uniquely related to the degree, to which the...

  2. Dynamical Masses of Accreting White Dwarfs

    Science.gov (United States)

    Pala, A. F.; Gänsckie, B. T.

    2017-03-01

    The mass retention efficiency is a key question in both the theoretical and observational study of accreting white dwarfs in interacting binaries, with important implications for their potential as progenitors for type Ia supernovae (SNe Ia). Canonical wisdom is that classical nova eruptions erode the white dwarf mass, and consequently, cataclysmic variables (CVs) have been excluded from the SN Ia progenitor discussion. However the average mass of white dwarfs in CVs is substantially higher (≃ 0.83 M⊙) than that of single white dwarfs (≃ 0.64 M ⊙), in stark contrast to expectations based on current classical nova models. This finding is based on a sample of ≃ 30 CV white dwarfs with accurate mass measurements, most of them in eclipsing systems. Given the fundamental importance of the mass evolution of accreting white dwarfs, it is necessary to enlarge this sample and to diversify the methods used for measuring masses. We have begun a systematic study of 27 CVs to almost double the number of CV white dwarfs with an accurate mass measurement. Using VLT/X-shooter phase-resolved observations, we can measure the white dwarf masses to a few percent, and will be able to answer the question whether accreting CV white dwarfs grow in mass.

  3. Sub-Keplerian accretion onto circumstellar disks

    CERN Document Server

    Visser, R

    2010-01-01

    Models of the formation, evolution and photoevaporation of circumstellar disks are an essential ingredient in many theories of the formation of planetary systems. The ratio of disk mass over stellar mass in the circumstellar phase of a disk is largely determined by the angular momentum of the original cloud core from which the system was formed. While full 3D or 2D axisymmetric hydrodynamical models of accretion onto the disk automatically treat all aspects of angular momentum, this is not so trivial for 1D and semi-2D viscous disk models. Since 1D and semi-2D disk models are still very useful for long-term evolutionary modelling of disks with relatively little numerical effort, we investigate how the 2D nature of accretion affects the formation and evolution of the disk in such models. A proper treatment of this problem requires a correction for the sub-Keplerian velocity at which accretion takes place. We develop an update of our semi-2D time-dependent disk evolution model to properly treat the effects of s...

  4. Terrane accretion: Insights from numerical modelling

    Science.gov (United States)

    Vogt, Katharina; Gerya, Taras

    2016-04-01

    The oceanic crust is not homogenous, but contains significantly thicker crust than norm, i.e. extinct arcs, spreading ridges, detached continental fragments, volcanic piles or oceanic swells. These (crustal) fragments may collide with continental crust and form accretionary complexes, contributing to its growth. We analyse this process using a thermo-mechanical computer model (i2vis) of an ocean-continent subduction zone. In this model the oceanic plate can bend spontaneously under the control of visco-plastic rheologies. It moreover incorporates effects such as mineralogical phase changes, fluid release and consumption, partial melting and melt extraction. Based on our 2-D experiments we suggest that the lithospheric buoyancy of the downgoing slab and the rheological strength of crustal material may result in a variety of accretionary processes. In addition to terrane subduction, we are able to identify three distinct modes of terrane accretion: frontal accretion, basal accretion and underplating plateaus. We show that crustal fragments may dock onto continental crust and cease subduction, be scrapped off the downgoing plate, or subduct to greater depth prior to slab break off and subsequent exhumation. Direct consequences of these processes include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes, partial melting and crustal growth.

  5. Effect of Gas Accretion Disc Profile on Orbital Parameters of the Accreted Stars

    Science.gov (United States)

    Shukirgaliyev, Bekdaulet T.; Panamarev, Taras P.; Naurzbaeva, Aisha Zh.; Kalambay, Mukhagali T.; Makukov, Maxim A.; Vilkoviskij, Emmanuil Y.; Omarov, Chingis T.; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-10-01

    The results of studies of the effect of the gas disk and its profile on the dynamics of active galactic nuclei are presented. The study was conducted with a numerical model of galactic nucleus based on phiGRAPE+GPU comprising three subsystems - a central supermassive black hole, gaseous accretion disc, and compact stellar cluster. The evolution of the compact stellar cluster is modeled with direct integration (N-body simulation), while the black hole and gaseous disc are represented phenomenologically: the black hole is introduced as an external potential (fixed in space but variable in time due to black hole mass growth), and the gaseous disc is introduced as spatial time-independent density distribution. We examined and compared with each other orbital parameters of accreting stars for model of the galactic nucleus with gas disc of constant and variable thickness, as well as without gas. It was found that in the presence of a gaseous disk almost half of the accreted particles interact strongly with the gas and are captured by the disc before accretion, while more than 85% of particles are affected to some extent by the disc prior to accretion. This suggests that interaction of the stellar cluster with the gas disk in the galactic nucleus might lead to the formation of stellar disk in the central part of the nucleus.

  6. Inner Accretion Disk Regions of Black Hole X-ray Binaries

    Science.gov (United States)

    Salvesen, Greg

    2015-01-01

    The innermost regions of accretion disks in black hole X-ray binaries dominate the observed X-ray emission, which is the main diagnostic that one uses to gain insights into the physics of black holes and accretion. The standard spectrum predicted from a geometrically thin, optically thick disk experiences non-trivial modification due to conspiring physical effects operating within the vertical disk structure such as Comptonization, free-free emission/absorption, bound-free opacities, and energy dissipation by magnetic processes. The complicated interplay of these effects cause the seed accretion disk spectrum to become hardened and it is this hardened emergent spectrum that we observe. To zeroth order, this hardening can be described by a phenomenological parameter called the spectral hardening factor.In practice, the adopted degree of spectral hardening is confined to lie within a rather restrictive range. I will discuss the following consequences of relaxing this criterion, while still requiring the spectral hardening factor to take on physically plausible values. Examining multiple state transitions of the black hole X-ray binary GX 339-4 with archival data from the Rossi X-ray Timing Explorer, I will show that appealing to a spectral hardening factor that varies during state transitions provides a viable alternative to a truncated disk model for the evolution of the inner accretion disk. Having demonstrated that moderate degrees of accretion disk spectral hardening cannot be ruled out by observations, I will explore this possibility from a theoretical standpoint. Extending previous work on radiative transfer modeling coupled to the vertical disk structure, I present the impacts on the emergent accretion disk spectrum caused by disk inclination and by allowing accretion power to be dissipated in the corona. Using magnetohydrodynamic simulations of a localized patch of the accretion disk (i.e., shearing box) performed with the Athena code, I will present the

  7. Extracting Energy Magnetically from Plunging Region of Black-Hole Accretion Disk

    Institute of Scientific and Technical Information of China (English)

    LIU Dong-Mei; YE Yong-Chun; WANG Ding-Xiong

    2007-01-01

    An analytical expression for the jet power extracted from the plunging region between a black hole (BH)horizon and the inner edge of the disk (hereafter the PL power) is derived based on an improved equivalent circuit in BH magnetosphere with a mapping relation between the radial coordinate of the plunging region and that of the remote astrophysical load.It is shown that the PL power is of great importance in explaining jet power and dominates over the BZ and DL powers for a wide value range of the BH spin.In addition,we show that the PL power derived in our model can be fitted with the strong jet powers of several 3CR FR I radio galaxies,which cannot be explained by virtue of the BZ mechanism.Furthermore,the condition for negative energy of the accreting particles in the plunging region is discussed with the validity of the second law of BH thermodynamics.

  8. Core Forensics: Earth's Accretion and Differentiation

    Science.gov (United States)

    Badro, J.; Brodholt, J. P.; Siebert, J.; Piet, H.; Ryerson, F. J.

    2013-12-01

    Earth's accretion and its primitive differentiation are intimately interlinked processes. One way to constrain accretionary processes is by looking at the major differentiation event that took place during accretion: core formation. Understanding core formation and core composition can certainly shed a new light on early and late accretionary processes. On the other hand, testing certain accretionary models and hypothesis (fluxes, chemistries, timing) allows -short of validating them- at the very least to unambiguously refute them, through the 'filter'' of core formation and composition. Earth's core formed during accretion as a result of melting, phase-separation, and segregation of accretionary building blocks (from meteorites to planetesimals). The bulk composition of the core and mantle depends on the evolution (pressure, temperature, composition) of core extraction during accretion. The entire process left a compositional imprint on both reservoirs: (1) in the silicate Earth, in terms of siderophile trace-element (Ni, Co, V, Cr, among others) concentrations and isotopic fractionation (Si, Cu, among others), a record that is observed in present-day mantle rocks; and (2) on the core, in terms of major element composition and light elements dissolved in the metal, a record that is observed by seismology through the core density-deficit. This imprint constitutes actually a fairly impressive set of evidence (siderophile element concentration and fractionation, volatile and siderophile element isotopic fractionation), can be used today to trace back the primordial processes that occurred 4.5 billion years ago. We are seeking to provide an overhaul of the standard core formation/composition models, by using a new rationale that bridges geophysics and geochemistry. The new ingredients are (1) new laser-heated diamond anvil cell partitioning data, dramatically extending the previous P-T conditions for experimental work, (2) ab initio molecular dynamics calculations to

  9. Accretion in the Early Kuiper Belt; 2, Fragmentation

    CERN Document Server

    Kenyon, S J; Kenyon, Scott J.; Luu, Jane X.

    1999-01-01

    We describe new planetesimal accretion calculations in the Kuiper Belt that include fragmentation and velocity evolution. All models produce two power law cumulative size distributions, N_C propto r^{-q}, with q = 2.5 for radii less than 0.3-3 km and q = 3 for radii exceeding 1-3 km. The power law indices are nearly independent of the initial mass in the annulus, the initial eccentricity of the planetesimal swarm, and the initial size distribution of the planetesimal swarm. The transition between the two power laws moves to larger radii as the initial eccentricity increases. The maximum size of objects depends on their intrinsic tensile strength; Pluto formation requires a strength exceeding 300 erg per gram. Our models yield formation timescales for Pluto-sized objects of 30-40 Myr for a minimum mass solar nebula. The production of several `Plutos' and more than 10^5 50 km radius Kuiper Belt objects leaves most of the initial mass in 0.1-10 km radius objects that can be collisionally depleted over the age of...

  10. Wind interactions above accretion discs: a model for broad-line regions and collimated outflow

    Energy Technology Data Exchange (ETDEWEB)

    Smith, M.D.; Raine, D.J. (Leicester Univ. (UK). Dept. of Astronomy)

    1985-01-15

    The interaction of a wind from an active galactic nucleus with a Compton-heating-induced wind from an accretion disc is studied. The nuclear wind is taken as initially supersonic and spherically symmetric. The disc wind arises when the disc surface is exposed to a hard and powerful X-ray source. Three classes of interaction are identified in terms of the relation between the pressure on the disc surface and the corresponding thermal and ram pressures in the nuclear wind.

  11. Hyper-accreting black hole as GRB central engine. I: Baryon loading in GRB jets

    CERN Document Server

    Lei, Wei-Hua; Liang, En-Wei

    2012-01-01

    A hyper-accreting stellar-mass black hole has been long speculated as the best candidate of central engine of gamma-ray bursts (GRBs). Recent rich observations of GRBs by space missions such as Swift and Fermi pose new constraints on GRB central engine models. In this paper, we study the baryon loading processes of a GRB jet launched from a black hole central engine. We consider a relativistic jet powered by $\

  12. Phantom Accretion onto the Schwarzschild de-Sitter Black Hole

    Institute of Scientific and Technical Information of China (English)

    M Sharif; G Abbas

    2011-01-01

    We deal with phantom energy accretion onto the Schwarzschild de-Sitter black hole. The energy flux conservation, relativistic Bernoulli equation and mass Bux conservation equation are formulated to discuss the phantom accretion. We discuss the conditions for critical accretion. It is found that the mass of the black hole decreases due to phantom accretion. There exist two critical points which lie in the exterior of horizons (black hole and cosmological horizons). The results for the phantom energy accretion onto the Schwarzschild black hole can be recovered by taking A → 0.%@@ We deal with phantom energy accretion onto the Schwarzschild de-Sitter black hole.The energy flux conserva-tion,relativistic Bernoulli equation and mass flux conservation equation are formulated to discuss the phantom accretion.We discuss the conditions for critical accretion.It is found that the mass of the black hole decreases due to phantom accretion.There exist two critical points which lie in the exterior of horizons(black hole and cosmological horizons).The results for the phantom energy accretion onto the Schwarzschild black hole can be recovered by taking ∧→0.

  13. Magnetospheric accretion on the fully-convective classical T Tauri star DN Tau

    CERN Document Server

    Donati, JF; Alencar, SHP; Hussain, G; Bouvier, J; Jardine, MM; Menard, F; Dougados, C; Romanova, MM

    2013-01-01

    We report here results of spectropolarimetric observations of the classical T Tauri star DN Tau carried out (at 2 epochs) with ESPaDOnS at the Canada-France-Hawaii Telescope within the `Magnetic Protostars and Planets' programme. We infer that DN Tau, with a photospheric temperature of 3,950+-50 K, a luminosity of 0.8+-0.2 Lsun and a rotation period of 6.32 d, is a ~2Myr-old fully-convective 0.65+-0.05 Msun star with a radius of 1.9+-0.2 Dsun, viewed at an inclination of 35+-10degr. Clear circularly-polarized Zeeman signatures are detected in both photospheric and accretion-powered emission lines, probing longitudinal fields of up to 1.8 kG (in the He1 D3 accretion proxy). Rotational modulation of Zeeman signatures, detected both in photospheric and accretion lines, is different between our 2 runs, providing further evidence that fields of cTTSs are generated by non-stationary dynamos. Using tomographic imaging, we reconstruct maps of the large-scale field, of the photospheric brightness and of the accretion-...

  14. Diagnosing the Black Hole Accretion Physics of Sgr A*: Spitzer/Chandra Observations

    Science.gov (United States)

    Hora, Joseph L.; Fazio, Giovanni G.; Willner, Steven P.; Gurwell, Mark A.; Smith, Howard Alan; Ashby, Matthew; Baganoff, Frederick K.; Witzel, Gunther; Morris, Mark; Ghez, Andrea M.; Meyer, Leo; Becklin, Eric E.; Ingalls, James G.; Glaccum, William J.; Carey, Sean J.; Haggard, Daryl; Marrone, Daniel P.; Gammie, Charles F.

    2017-01-01

    The Galactic center offers the closest opportunity for studying accretion onto a supermassive black hole. The fluctuating source, Sgr A*, is detected across the electromagnetic spectrum and its flux may originate in either the accretion flow or a jet, or both. Disentangling the power source and emission mechanisms of the flares is a central challenge to our understanding of the Sgr A* accretion flow. Recent general relativistic magneto-hydrodynamic (GRMHD) models indicate that variability can be produced by a tilted inner disk, gravitational lensing of bright spots in the disk by the hole, or particle acceleration in reconnection events. These models produce different flare characteristics, and better characterization of flares may enable us to distinguish between strong and weakly magnetized disks. Following our successful Spitzer observations of the variability of Sgr A* in 2013 and 2014, we have undertaken a program of simultaneous IRAC (4.5 micron) and Chandra (2-10 keV) observations to (1) probe the accretion physics of Sgr A* on event-horizon scales and (2) detect any effect of the object G2 on Sgr A*. In addition, several ground-based observatories participated in the campaigns, at wavelengths including radio, sub-mm, and the near-infrared. We will present initial Spitzer/Chandra results from the two 24-hour epochs in 2016 July. Only such long-duration, continuous, multi-wavelength observations can achieve a comprehensive view of the dominant emission process(es) and quantify the physical properties near the event horizon.

  15. Looking into the Theory of Pulsar Accretion: Cen X-3 and XTE J1946+274

    CERN Document Server

    Marcu, Diana M; Gottlieb, Amy M; Wolff, Michael T; Becker, Peter A; Wilms, Joern; Ferrigno, Carlo; Wood, Kent S

    2015-01-01

    This is an overview of pulsar accretion modeling. The physics of pulsar accretion, i.e., the process of plasma flow onto the neutron star surface, can be constrained from the spectral properties of the X-ray source. We discuss a new implementation of the physical continuum model developed by Becker and Wolff (2007, ApJ 654, 435). The model incorporates Comptonized blackbody, bremsstrahlung, and cyclotron emission. We discuss preliminary results of applying the new tool to the test cases of Suzaku data of Cen X-3 and XTE J1946+274. Cen X-3 is a persistent accreting pulsar with an O-star companion observed during a bright period. XTE J1946+274 is a transient accreting pulsar with a Be companion observed during a dim period. Both sources show spectra that are well described with an empirical Fermi Dirac cutoff power law model. We extend the spectral analysis by making the first steps towards a physical description of Cen X-3 and XTE J1946+274.

  16. The self-regulated AGN feedback loop: the role of chaotic cold accretion

    CERN Document Server

    Gaspari, M

    2015-01-01

    Supermassive black hole accretion and feedback play central role in the evolution of galaxies, groups, and clusters. I review how AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion (CCA). In a turbulent and heated atmosphere, cold clouds and kpc-scale filaments condense out of the plasma via thermal instability and rain toward the black hole. In the nucleus, the recurrent chaotic collisions between the cold clouds, filaments, and central torus promote angular momentum cancellation or mixing, boosting the accretion rate up to 100 times the Bondi rate. The rapid variability triggers powerful AGN outflows, which quench the cooling flow and star formation without destroying the cool core. The AGN heating stifles the formation of multiphase gas and accretion, the feedback subsides and the hot halo is allowed to cool again, restarting a new cycle. Ultimately, CCA creates a symbiotic link between the black hole and the whole host via a tight self-regulate...

  17. Resonance oscillation of radiative shock waves in accretion disks around compact objects

    CERN Document Server

    Molteni, D; Chakrabarti, S K; Molteni, Diego M; Chakrabarti, Sandip K

    1995-01-01

    We extend our previous numerical simulation of accretion disks with shock waves when cooling effects are also included. We consider bremsstrahlung and other power law processes: \\Lambda \\propto T^{\\alpha} \\rho^2 to mimic cooling in our simulation. We employ {\\it Smoothed Particle Hydrodynamics} technique as in the past. We observe that for a given angular momentum of the flow, the shock wave undergoes a steady, radial oscillation with the period is roughly equal to the cooling time. Oscillations seem to take place when the disk and cooling parameters (i.e., accretion rate, cooling process) are such that the infall time from shock is of the same order as the post-shock cooling time. The amplitude of oscillation could be up to ten percent of the distance of the shock wave from the black hole when the black hole is accreting. When the accretion is impossible due to the centrifugal barrier, the amplitude variation could be much larger. Due to the oscillation, the energy output from the disk is also seen to vary q...

  18. Crossing the Eddington limit: examining disk spectra at high accretion rates

    CERN Document Server

    Sutton, Andrew D; Roberts, Timothy P; Middleton, Matthew J; Soria, Roberto; Done, Chris

    2016-01-01

    The faintest ultraluminous X-ray sources (ULXs), those with 0.3-10 keV luminosities 1 < L_X/10^39 < 3 erg s^-1, tend to have X-ray spectra that are disk-like but broader than expected for thin accretion disks. These `broadened disk' spectra are thought to indicate near- or mildly super-Eddington accretion onto stellar remnant black holes. Here we report that a sample of bright thermal-dominant black hole binaries, which have Eddington ratios constrained to moderate values, also show broadened disk spectra in the 0.3-10 keV band at an order of magnitude lower luminosities. This broadening would be missed in studies that only look above ~2 keV. While this may suggest that broadened disk ULXs could be powered by accretion onto massive stellar remnant black holes with close to maximal spin, we argue in favor of a scenario where they are at close to the Eddington luminosity, such that radiation pressure would be expected to result in geometrically slim, advective accretion disks. However, this implies that a...

  19. A Monte Carlo study of the spectra from inhomogeneous accretion flow

    Science.gov (United States)

    Yu, Xiao-Di; Li, Ya-Ping; Xie, Fu-Guo; Ma, Ren-Yi

    2017-08-01

    A model of an inhomogeneous accretion flow, in which cold clumps are surrounded by hot gas or corona, has been proposed to explain the spectral features of black hole X-ray binaries. In this work, we try to find possible observational features in the continuum that can indicate the existence of clumps. The spectra of an inhomogeneous accretion flow are calculated via the Monte Carlo method. Since the corresponding accretion flow is unsteady and complex, the accretion flow is described by a set of free parameters, the ranges of which can include the real cases. The influences of the parameters are investigated. It is found that the thermal component of the spectra deviates from multi-color black body spectra in the middle power-law part. On one hand, a warp appears due to the gaps between the clumps and the outer cold disk, and on the other hand, the slope of the line connecting the thermal peaks deviates from 4/3. The warp feature, as well as the correlation between the thermal peak at higher frequency and the spectral index, possibly indicate the existence of clumps, and are worthy of further investigation with more self-consistent models.

  20. Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole

    CERN Document Server

    Tchekhovskoy, Alexander; McKinney, Jonathan C

    2011-01-01

    We describe global, 3D, time-dependent, non-radiative, general-relativistic, magnetohydrodynamic simulations of accreting black holes (BHs). The simulations are designed to transport a large amount of magnetic flux to the center, more than the BH can swallow. The excess magnetic flux remains outside the BH, impedes accretion, and leads to a magnetically arrested disc. We find powerful outflows. For a BH with spin parameter a = 0.5, the efficiency with which the accretion system generates outflowing energy in jets and winds is eta ~ 30%. For a = 0.99, we find eta ~ 140%, which means that more energy flows out of the BH than flows in. Thus, the gravitational mass of the BH decreases with time. This simulation represents an unambiguous demonstration, within an astrophysically plausible scenario, of the extraction of net energy from a spinning BH via the Penrose-Blandford-Znajek mechanism. We suggest that magnetically arrested accretion might explain observations of AGN with apparent eta ~ few x 100%.

  1. The role of Compton heating on radiation-regulated accretion on to black holes

    CERN Document Server

    Park, KwangHo; Di Matteo, Tiziana; Reynolds, Christopher S

    2014-01-01

    We investigate the role of Compton heating in radiation-regulated accretion on to black holes from a neutral dense medium using 1D radiation-hydrodynamic simulations. We focus on the relative effects of Compton-heating and photo-heating as a function of the spectral slope {\\alpha}, assuming a power-law spectrum in the energy range of 13.6 eV--100 keV. While Compton heating is dominant only close to the black hole, it can reduce the accretion rate to 0.1 % ($l \\propto \\dot{m}^2$ model)--0.01 % ($l \\propto \\dot{m}$ model) of the Bondi accretion rate when the BH radiation is hard ({\\alpha} ~ 1), where $l$ and $\\dot{m}$ are the luminosity and accretion rate normalised by Eddington rates, respectively. The oscillatory behaviour otherwise typically seen in simulations with {\\alpha} > 1, become suppressed when {\\alpha} ~ 1 only for the $l \\propto \\dot{m}$ model. The relative importance of the Compton heating over photo-heating decreases and the oscillatory behaviour becomes stronger as the spectrum softens. When the...

  2. Coronal outflow dominated accretion discs a new possibility for low luminosity black holes?

    CERN Document Server

    Merloni, A

    2002-01-01

    The spectral energy distributions of galactic black holes in the low/hard state and of low-luminosity AGN possess many common features, the most prominent being: compact, flat (or inverted) spectrum radio cores with high brightness temperatures; excess red and infrared emission, often correlated with the radio flux; an extremely weak (or absent) quasi-thermal hump and a hard X-ray power-law with high energy cut-off. These sources are thought to be accreting at low rates and advection (or convection) dominated accretion flows are usually considered the best candidates to explain them. Here we present an alternative possibility, involving strong, unbound, magnetic coronae generated by geometrically thin, optically thick accretion discs at low accretion rates. First we show that, if angular momentum transport in the disc is due to magnetic turbulent stresses, the magnetic energy density and effective viscous stresses inside the disc are proportional to the geometric mean of the total (gas plus radiation) and gas...

  3. The nuclear accretion in the FR I radio galaxy IC4296 from CHANDRA and VLBA observations

    CERN Document Server

    Pellegrini, S; Comastri, A; Fabbiano, G; Fiore, F; Vignali, C; Morganti, R; Trinchieri, G

    2003-01-01

    A high angular resolution study of the nucleus of the FR I galaxy IC4296 using Chandra ACIS-S and VLBA observations is presented, with the aim of studying the nature of the accretion process. Pointlike and hard X-ray emission is found, well described by a moderately absorbed power law of Gamma=1.48^{+0.42}_{-0.34}; no iron fluorescence line from cold material is detected. The 0.3-10 keV luminosity is 2.4\\times 10^{41} erg/s, that is \\sim 400 times lower than the accretion luminosity resulting from the estimated Bondi mass accretion rate and a radiative efficiency of 10%. On the parsec scale a jet and a counter-jet extend out from a central unresolved ``core'' in the 8.4 GHz image. Their orientation is in good agreement with that of the large scale jets and their bulk speed is relativistic. The parsec scale spectrum is convex over 2-22 GHz. The observed nuclear luminosity is not likely to be reconciled with the accretion luminosity by assuming that Compton thick material surrounds the nucleus. Low radiative ef...

  4. Hydromagnetic flows from accretion discs and the production of radio jets

    Energy Technology Data Exchange (ETDEWEB)

    Blandford, R.D.; Payne, D.G. (California Inst. of Tech., Pasadena (USA). Theoretical Astrophysics)

    1982-06-01

    We examine the possibility that energy and angular momentum are removed magnetically from accretion discs, by field lines that leave the disc surface and extend to large distances. We illustrate this mechanism by solving the equations of magnetohydrodynamics, assuming infinite conductivity, for axially symmetric, self-similar, cold magnetospheric flow from a Keplerian accretion disc. We show that a centrifugally driven outflow of matter from the disc is possible, if the poloidal component of the magnetic field makes an angle of less than 60/sup 0/ with the disc surface. At large distances from the disc, the toroidal component of the magnetic field becomes important and collimates the outflow into a pair of anti-parallel jets moving perpendicular to the disc. Close to the disc, the flow is probably driven by gas pressure in a hot magnetically dominated corona. In this way, magnetic stresses can extract the angular momentum from a thin accretion disc and thus enable matter to be accreted, independently of the presence of viscosity. These jet solutions have the property that most of the power is concentrated within a central core, while most of the angular momentum and magnetic flux is carried near the jet walls.

  5. Liberation of specific angular momentum through radiation and scattering in relativistic black hole accretion discs

    CERN Document Server

    Stevens, Adam R H

    2015-01-01

    A key component of explaining the array of galaxies observed in the Universe is the feedback of active galactic nuclei, each powered by a massive black hole's accretion disc. For accretion to occur, angular momentum must be lost by that which is accreted. Electromagnetic radiation must offer some respite in this regard, the contribution for which is quantified in this paper using solely general relativity under the thin-disc regime. Herein, I calculate extremised situations where photons are entirely responsible for energy removal in the disc and then extend and relate this to the standard relativistic accretion disc outlined by Novikov & Thorne that includes the effect of viscosity. While there is potential for the contribution of angular-momentum removal from photons to be >~1% out to ~10^4 Schwarzschild radii, especially if the disc is irradiated and is liberated of angular momentum through scattering, it is more likely of order 10^2 Schwarzschild radii if thermal emission from the disc itself is stron...

  6. Clumpy wind accretion in supergiant neutron star high mass X-ray binaries

    CERN Document Server

    Bozzo, E; Feldmeier, A; Falanga, M

    2016-01-01

    The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the non-stationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total time scale of several hours), the transition of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the non-stationary wind. Th...

  7. Active galactic nuclei. III - Accretion flow in an externally supplied cluster of black holes

    Science.gov (United States)

    Pacholczyk, A. G.; Stoeger, W. R.; Stepinski, T. F.

    1989-01-01

    This third paper in the series modeling QSOs and AGNs as clusters of accreting black holes studies the accretion flow within an externally supplied cluster. Significant radiation will be emitted by the cluster core, but the black holes in the outer halo, where the flow is considered spherically symmetric, will not contribute much to the overall luminosity of the source because of their large velocities relative to the infalling gas and therefore their small accretion radii. As a result, the scenario discussed in Paper I will refer to the cluster cores, rather than to entire clusters. This will steepen the high-frequency region of the spectrum unless inverse Compton scattering is effective. In many cases accretion flow in the central part of the cluster will be optically thick to electron scattering, resulting in a spectrum featuring optically thick radiative component in addition to power-law regimes. The fitting of these spectra to QSO and AGN observations is discussed, and application to 3C 273 is worked out as an example.

  8. Active galactic nuclei. III. Accretion flow in an externally supplied cluster of black holes

    Energy Technology Data Exchange (ETDEWEB)

    Pacholczyk, A.G.; Stoeger, W.R.; Stepinski, T.F. (Steward Observatory, Tucson, AZ (USA); Arizona Univ., Tucson (USA))

    1989-08-01

    This third paper in the series modeling QSOs and AGNs as clusters of accreting black holes studies the accretion flow within an externally supplied cluster. Significant radiation will be emitted by the cluster core, but the black holes in the outer halo, where the flow is considered spherically symmetric, will not contribute much to the overall luminosity of the source because of their large velocities relative to the infalling gas and therefore their small accretion radii. As a result, the scenario discussed in Paper I will refer to the cluster cores, rather than to entire clusters. This will steepen the high-frequency region of the spectrum unless inverse Compton scattering is effective. In many cases accretion flow in the central part of the cluster will be optically thick to electron scattering, resulting in a spectrum featuring optically thick radiative component in addition to power-law regimes. The fitting of these spectra to QSO and AGN observations is discussed, and application to 3C 273 is worked out as an example. 14 refs.

  9. Accretion disk winds as the jet suppression mechanism in the microquasar GRS 1915+105.

    Science.gov (United States)

    Neilsen, Joseph; Lee, Julia C

    2009-03-26

    Stellar-mass black holes with relativistic jets, also known as microquasars, mimic the behaviour of quasars and active galactic nuclei. Because timescales around stellar-mass black holes are orders of magnitude smaller than those around more distant supermassive black holes, microquasars are ideal nearby 'laboratories' for studying the evolution of accretion disks and jet formation in black-hole systems. Whereas studies of black holes have revealed a complex array of accretion activity, the mechanisms that trigger and suppress jet formation remain a mystery. Here we report the presence of a broad emission line in the faint, hard states and narrow absorption lines in the bright, soft states of the microquasar GRS 1915+105. ('Hard' and 'soft' denote the character of the emitted X-rays.) Because the hard states exhibit prominent radio jets, we argue that the broad emission line arises when the jet illuminates the inner accretion disk. The jet is weak or absent during the soft states, and we show that the absorption lines originate when the powerful radiation field around the black hole drives a hot wind off the accretion disk. Our analysis shows that this wind carries enough mass away from the disk to halt the flow of matter into the radio jet.

  10. A model for the waveform behavior of accreting millisecond pulsars: Nearly aligned magnetic fields and wandering emission regions

    CERN Document Server

    Lamb, Frederick K; Van Wassenhove, Sandor; Chamberlain, Robert T; Lo, Ka Ho; Clare, Alexander; Yu, Wenfei; Miller, M Coleman

    2008-01-01

    Ten accretion-powered millisecond pulsars are now known. We show that many properties of their X-ray oscillations can be understood if the X-ray emitting regions of most are near their spin axes but wander. This is to be expected if their magnetic poles are close to their spin axes, so that accreting gas is channeled there. As the accretion rate and structure of the inner disk vary, gas will be channeled to different locations on the stellar surface, causing the X-ray emitting regions to move with respect to the magnetic field. This model can explain the small amplitudes and nearly sinusoidal waveforms of most of these pulsars and the large, rapid phase variations of several. It may also explain why accretion-powered millisecond pulsars are difficult to detect, why all found so far are transients, and why the oscillations of a few are intermittent. The model can be tested by comparing with observations the correlated waveform changes that it predicts, including changes with accretion rate.

  11. Light Curves from an MHD Simulation of a Black Hole Accretion Disk

    Science.gov (United States)

    Schnittman, Jeremy D.; Krolik, Julian H.; Hawley, John F.

    2006-11-01

    We use a relativistic ray-tracing code to calculate the light curves observed from a global, general relativistic, magnetohydrodynamic simulation of an accretion flow onto a Schwarzschild black hole. We apply three basic emission models to sample different properties of the time-dependent accretion disk. With one of these models, which assumes thermal blackbody emission and free-free absorption, we can predict qualitative features of the high-frequency power spectrum from stellar-mass black holes in the ``thermal dominant'' state. The simulated power spectrum is characterized by a power law of index Γ~3 and total rms fractional variance of ~1% near the orbital frequency at the innermost stable orbit. Initial results indicate the existence of transient QPO peaks with frequency ratios of nearly 2:3 at a 99.9% confidence limit, but they are not generic features, because at any given time they are seen only from certain observer directions. In addition, we present detailed analysis of the azimuthal structure of the accretion disk and the evolution of density perturbations in the inner disk. These ``hot-spot'' structures appear to be roughly self-similar over a range of disk radii, with a single characteristic size δφ=25deg and δr/r=0.3, and typical lifetimes Tl~0.3Torb.

  12. Wind from black hole accretion disk as the driver of a molecular outflow in a galaxy

    CERN Document Server

    Tombesi, F; Veilleux, S; Reeves, J N; Gonzalez-Alfonso, E; Reynolds, C S

    2015-01-01

    Powerful winds driven by active galactic nuclei (AGN) are often invoked to play a fundamental role in the evolution of both supermassive black holes (SMBHs) and their host galaxies, quenching star formation and explaining the tight SMBH-galaxy relations. Recent observations of large-scale molecular outflows in ultra-luminous infrared galaxies (ULIRGs) have provided the evidence to support these studies, as they directly trace the gas out of which stars form. Theoretical models suggest an origin of these outflows as energy-conserving flows driven by fast AGN accretion disk winds. Previous claims of a connection between large-scale molecular outflows and AGN activity in ULIRGs were incomplete because they were lacking the detection of the putative inner wind. Conversely, studies of powerful AGN accretion disk winds to date have focused only on X-ray observations of local Seyferts and a few higher redshift quasars. Here we show the clear detection of a powerful AGN accretion disk wind with a mildly relativistic ...

  13. Riverbank erosion induced by gravel bar accretion

    Science.gov (United States)

    Klösch, Mario; Habersack, Helmut

    2010-05-01

    Riverbank erosion is known to be strongly fluvially controlled and determination of shear stresses at the bank surface and at the bank toe is a crucial point in bank erosion modeling. In many modeling attempts hydraulics are simulated separately in a hydrodynamic-numerical model and the simulated shear stresses are further applied onto the bank surface in a bank erosion model. Hydrodynamics are usually simulated at a constant geometry. However, in some cases bed geometry may vary strongly during the event, changing the conditions for hydrodynamics along the bank. This research seeks to investigate the effect of gravel bar accretion during high discharges on final bank retreat. At a restored section of the Drava River bed widenings have been implemented to counter bed degradation. There, in an initiated side-arm, self-dynamic widening strongly affects bed development and long-term connectivity to the main channel. Understanding the riverbank erosion processes there would help to improve planning of future restoration measures. At one riverbank section in the side-arm large bank retreat was measured repeatedly after several flow events. This section is situated between two groins with a distance of 60 m, which act as lateral boundaries to the self-widening channel. In front of this bank section a gravel bar developed. During low flow condition most discharge of the side-arm flows beside the gravel bar along the bank, but shear stresses are too low for triggering bank erosion. For higher discharges results from a two-dimensional hydrodynamic-numerical model suggested shear stresses there to be generally low during the entire events. At some discharges the modeled flow velocities even showed to be recirculating along the bank. These results didn't explain the observed bank retreat. Based on the modeled shear stresses, bank erosion models would have greatly underestimated the bank retreat induced by the investigated events. Repeated surveys after events applying

  14. Volatile loss from accreting icy protoplanets

    Science.gov (United States)

    Stevenson, D. J.

    1993-03-01

    A large self-gravitating body does not easily lose significant mass because the escape velocity is much larger than the sound speed of atmosphere-forming species under ambient thermal conditions. The most significant exceptions to this are giant impacts or impact jetting by fast-moving projectiles. A very small object (e.g. a comet) also does not easily lose significant volatile mass upon formation because the energy release associated with its accretion is so small. (It can however lose a great deal of mass if it is subsequently moved closer to the Sun.) I argue that there is an intermediate mass range (corresponding to bodies with radii of approximately 300-800 km) for which the ambient steady-state mass loss is a maximum. By ambient, I mean those conditions pertaining to the formation region of the body. By steady state, I mean to exclude infrequent traumas (giant impacts). The existence of a preferred intermediate mass arises through the competition of growing gravitational containment and growing energy release by accretion; it corresponds typically to GM/(Rcs2) approximately equals 2 to 4, where M is the protoplanet mass of radius R, and cs is the sound speed. Several factors determine the amount of volatile loss is this vulnerable zone during accretion but in general the loss is a substantial fraction of the volatiles, sometimes approaching 100 percent. The principal implication is that bodies larger than a few hundred kilometers in radius will not have a 'primitive' (i.e. cometary) composition. This is relevant for understanding Triton, Pluto, Charon, and perhaps Chiron.

  15. Accretion-disc model spectra for dwarf-nova stars

    OpenAIRE

    Idan, Irit; Lasota, Jean-Pierre; Hameury, Jean-Marie; Shaviv, Giora

    2008-01-01

    Radiation from accretion discs in cataclysmic variable stars (CVs) provides fundamental information about the properties of these close binary systems and about the physics of accretion in general. The detailed diagnostics of accretion disc structure can be achieved by including in its description all the relevant heating and cooling physical mechanism, in particular the convective energy transport that, although dominant at temperatures less than about 10 000 K, is usually not taken into acc...

  16. Smearing of mass accretion rate variation by viscous processes in accretion disks in compact binary systems

    Science.gov (United States)

    Ghosh, A.; Chakrabarti, Sandip K.

    2016-09-01

    Variation of mass supply rate from the companion can be smeared out by viscous processes inside an accretion disk. Hence, by the time the flow reaches the inner edge, the variation in X-rays need not reflect the true variation of the mass supply rate at the outer edge. However, if the viscosity fluctuates around a mean value, one would expect the viscous time scale t_{{visc}} also to spread around a mean value. In high mass X-ray binaries, which are thought to be primarily wind-fed, the size of the viscous Keplerian disk is smaller and thus such a spread could be lower as compared to the low mass X-ray binaries which are primarily fed by Roche lobe overflow. If there is an increasing or decreasing trend in viscosity, the interval between enhanced emission would be modified systematically. In the absence of a detailed knowledge about the variation of mass supply rates at the outer edge, we study ideal circumstances where modulation must take place exactly in orbital time scales, such as when there is an ellipticity in the orbit. We study a few compact binaries using long term All Sky monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) and all sky survey data (15-50 keV) of Swift satellites by different methods to look for such smearing effects and to infer what these results can tell us about the viscous processes inside the respective disks. We employ three different methods to seek imprints of periodicity on the X-ray variation and found that in all the cases, the location of the peak in the power density spectra is consistent with the orbital frequencies. Interestingly, in high mass X-ray binaries the peaks are sharp with high rms values, consistent with a small Keplerian disk in a wind fed system. However, in low mass X-ray binaries with larger Keplerian disk component, the peaks are spreaded out with much lower rms values. X-ray reflections, or superhump phenomena which may also cause such X-ray modulations would not be affected by the size of

  17. Accretion and evaporation of modified Hayward black hole

    Energy Technology Data Exchange (ETDEWEB)

    Debnath, Ujjal [Indian Institute of Engineering Science and Technology, Department of Mathematics, Howrah (India)

    2015-03-01

    We assume the most general static spherically symmetric black hole metric. The accretion of any general kind of fluid flow around the black hole is investigated. The accretion of the fluid flow around the modified Hayward black hole is analyzed, and we then calculate the critical point, the fluid's four-velocity, and the velocity of sound during the accretion process. Also the nature of the dynamical mass of the black hole during accretion of the fluid flow, taking into consideration Hawking radiation from the black hole, i.e., evaporation of the black hole, is analyzed. (orig.)

  18. Waves and Instabilities in Accretion Disks MHD Spectroscopic Analysis

    CERN Document Server

    Keppens, R; Goedbloed, J P

    2002-01-01

    A complete analytical and numerical treatment of all magnetohydrodynamic waves and instabilities for radially stratified, magnetized accretion disks is presented. The instabilities are a possible source of anomalous transport. While recovering results on known hydrodynamicand both weak- and strong-field magnetohydrodynamic perturbations, the full magnetohydrodynamic spectra for a realistic accretion disk model demonstrates a much richer variety of instabilities accessible to the plasma than previously realized. We show that both weakly and strongly magnetized accretion disks are prone to strong non-axisymmetric instabilities.The ability to characterize all waves arising in accretion disks holds great promise for magnetohydrodynamic spectroscopic analysis.

  19. Do observations reveal accretion discs in intermediate polars

    Energy Technology Data Exchange (ETDEWEB)

    Hellier, C. (University Coll., London (UK). Mullard Space Science Lab.)

    1991-08-15

    It has been proposed that intermediate polars do not accrete through discs and that they may not possess discs. Observations of eclipses and emission lines provide strong evidence that discs are present in many intermediate polars, although it is less clear whether the accretion flows through these discs. An analysis of the EXOSAT database shows that many systems have orbital and beat period modulations which are small compared to the spin-pulses, suggesting disc accretion. There are, though, exceptions, notably TX Col where dominant orbital and beat period modulations indicate discless accretion. (author).

  20. Convection-Dominated Accretion Flows with Radiative Cooling

    Institute of Scientific and Technical Information of China (English)

    LI Shuang-Liang; XUE Li; LU Ju-Fu

    2007-01-01

    @@ By numerically solving the set of basic equations describing black hole accretion flows with low accretion rates,we show that although the dynamical structure of these flows is essentially unaffected by radiative processes in comparison with the case in which the radiation is not considered, the radiative cooling can be more important than the advective cooling in the flow's convection-dominated zone, and this result may have implications to distinguish observationally convection-dominated accretion flows from advection-dominated accretion flows.

  1. Accretion Onto a Charged Higher-Dimensional Black Hole

    CERN Document Server

    Sharif, M

    2016-01-01

    This paper deals with the steady-state polytropic fluid accretion onto a higher-dimensional Reissner-Nordstr$\\ddot{o}$m black hole. We formulate the generalized mass flux conservation equation, energy flux conservation and relativistic Bernoulli equation to discuss the accretion process. The critical accretion is investigated by finding critical radius, critical sound velocity and critical flow velocity. We also explore gas compression and temperature profiles to analyze the asymptotic behavior. It is found that the results for Schwarzschild black hole are recovered when $q=0$ in four dimensions. We conclude that accretion process in higher dimensions becomes slower in the presence of charge.

  2. Deceleration Effect of Magnetic Field on Black Hole Accretion Disks

    Institute of Scientific and Technical Information of China (English)

    WANG Ding-Xiong

    2000-01-01

    The deceleration effect of magnetic field near the horizon of a spinning black hole (BH) of accretion disk is investigated in the Blandford-Znajek (BZ) process. It is shown that rates of change with respect to time for both the angular velocities of BH horizon and accreting particles at the inner edge of an accretion disk are reduced in the BZ process, behaving with non-monotonous evolution characteristics. This result implies that the magnetic field near the BH horizon has & deceleration effect not only on the spinning BH but also on the surrounding accretion disk.

  3. Brown dwarf accretion: Nonconventional star formation over very long timescales

    Directory of Open Access Journals (Sweden)

    Ćirković Milan M.

    2005-01-01

    Full Text Available We investigate the process of accretion of interstellar gas by the Galactic population of brown dwarfs over very long timescales typical for physical eschatology. In particular, we use the classical Hoyle-Lyttleton-Bondi accretion model to investigate the rate at which brown dwarfs collect enough additional mass to become red dwarfs, accretion-induced changes in the mass function of the low- mass objects, and the corresponding accretion heating of brown dwarfs. In addition, we show how we can make the definition of the final mass function for stellar objects more precise.

  4. ACCRETION DISKS IN TWO-DIMENSIONAL HOYLE-LYTTLETON FLOW

    Energy Technology Data Exchange (ETDEWEB)

    Blondin, John M., E-mail: John_Blondin@ncsu.edu [Department of Physics, North Carolina State University, Raleigh, NC 27695-8202 (United States)

    2013-04-20

    We investigate the flip-flop instability observed in two-dimensional planar hydrodynamic simulations of Hoyle-Lyttleton accretion in the case of an accreting object with a radius much smaller than the nominal accretion radius, as one would expect in astrophysically relevant situations. Contrary to previous results with larger accretors, accretion from a homogenous medium onto a small accretor is characterized by a robust, quasi-Keplerian accretion disk. For gas with a ratio of specific heats of 5/3, such a disk remains locked in one direction for a uniform ambient medium. The accretion flow is more variable for gas with a ratio of specific heats of 4/3, with more dynamical interaction of the disk flow with the bow shock leading to occasional flips in the direction of rotation of the accretion disk. In both cases the accretion of angular momentum is determined by the flow pattern behind the accretion shock rather than by the parameters of the upstream flow.

  5. Centrifugally driven winds from protostellar accretion discs - I. Formulation and initial results

    Science.gov (United States)

    Nolan, C. A.; Salmeron, R.; Federrath, C.; Bicknell, G. V.; Sutherland, R. S.

    2017-10-01

    Protostellar discs play an important role in star formation, acting as the primary mass reservoir for accretion on to young stars and regulating the extent to which angular momentum and gas is released back into stellar nurseries through the launching of powerful disc winds. In this study, we explore how disc structure relates to the properties of the wind-launching region, mapping out the regions of protostellar discs where wind launching could be viable. We combine a series of 1.5D semi-analytic, steady-state, vertical disc-wind solutions into a radially extended 1+1.5D model, incorporating all three diffusion mechanisms (Ohm, Hall and ambipolar). We observe that the majority of mass outflow via disc winds occurs over a radial width of a fraction of an astronomical unit, with outflow rates attenuating rapidly on either side. We also find that the mass accretion rate, magnetic field strength and surface density profile each have significant effects on both the location of the wind-launching region and the ejection/accretion ratio \\dot{M}_out/\\dot{M}_in. Increasing either the accretion rate or the magnetic field strength corresponds to a shift of the wind-launching region to smaller radii and a decrease in \\dot{M}_out/\\dot{M}_in, while increasing the surface density corresponds to launching regions at larger radii with increased \\dot{M}_out/\\dot{M}_in. Finally, we discover a class of disc winds containing an ineffective launching configuration at intermediate radii, leading to two radially separated regions of wind launching and diminished \\dot{M}_out/\\dot{M}_in. We find that the wind locations and ejection/accretion ratio are consistent with current observational and theoretical estimates.

  6. Testing Models of Circum-Binary-AGN Accretion for PSO J334.2028+01.4075

    Science.gov (United States)

    Foord, Adi; Gultekin, Kayhan; Reynolds, Mark

    2017-08-01

    We present analysis of new Chandra data of PSO J334.2028+01.4075 (PSO J334 hereafter), a strong binary AGN candidate discovered by Liu et al. (2015) based on periodic variation of the optical flux. Recent radio coverage presented in Mooley et al. (2017) further supports that PSO J334 is a binary black hole system, as the quasar was found to be lobe-dominated with a twisted radio structure, possibly due to a precessing jet. With no prior X-ray coverage for PSO J334, our new 50 ksec Chandra observation allows for the unique opportunity to differentiate between a single or binary-AGN system, and if a binary, can characterize the mode of accretion. The two most basic sets of predictions via simulations of circum-binary accretion model are a “cavity”, where the inner region of the accretion disk is mostly empty and emission is truncated blueward of the wavelength associated with the temperature of the innermost ring, or “minidisks”, where there is substantial accretion onto one or both of the members of the binary, each with their own shock-heated thin-disk accretion system. We find the X-ray emission to be well-fit with a heavily absorbed power-law, incompatible with the cavity scenario. Further, we construct an SED of PSO J334 by combining radio through X-ray observations and compare it to standard QSO SEDs. We discuss the implications of the comparison between the SED of PSO J334 and that of a single AGN, and assess the likelihood of the binary model for PSO J334.

  7. Diffusive Particle Acceleration in Shocked, Viscous Accretion Disks: Green's Function Energy Distribution

    Science.gov (United States)

    Becker, Peter A.; Das, Santabrata; Le, Truong

    2011-12-01

    The acceleration of relativistic particles in a viscous accretion disk containing a standing shock is investigated as a possible explanation for the energetic outflows observed around radio-loud black holes. The energy/space distribution of the accelerated particles is computed by solving a transport equation that includes the effects of first-order Fermi acceleration, bulk advection, spatial diffusion, and particle escape. The velocity profile of the accreting gas is described using a model for shocked viscous disks recently developed by the authors, and the corresponding Green's function distribution for the accelerated particles in the disk and the outflow is obtained using a classical method based on eigenfunction analysis. The accretion-driven, diffusive shock acceleration scenario explored here is conceptually similar to the standard model for the acceleration of cosmic rays at supernova-driven shocks. However, in the disk application, the distribution of the accelerated particles is much harder than would be expected for a plane-parallel shock with the same compression ratio. Hence the disk environment plays a key role in enhancing the efficiency of the shock acceleration process. The presence of the shock helps to stabilize the disk by reducing the Bernoulli parameter, while channeling the excess binding energy into the escaping relativistic particles. In applications to M87 and Sgr A*, we find that the kinetic power in the jet is {\\sim}0.01\\,\\dot{M} c^2, and the outflowing relativistic particles have a mean energy ~300 times larger than that of the thermal gas in the disk at the shock radius. Our results suggest that a standing shock may be an essential ingredient in accretion onto underfed black holes, helping to resolve the long-standing problem of the stability of advection-dominated accretion disks.

  8. Vertical angular momentum transfer from accretion discs and the formation of large-scale collimated jets

    Energy Technology Data Exchange (ETDEWEB)

    Casse, F [AstroParticule et Cosmologie (APC), Universite Paris Diderot, 10, rue A. Domon et L. Duquet 75205, Paris Cedex 13 (France)], E-mail: fcasse@apc.univ-paris7.fr

    2008-12-15

    In this paper I present an overview of the favoured scenario explaining the presence of twin cylindrical astrophysical jets in the vicinity of accretion discs. These jets are made of plasma and host large-scale magnetic fields. The twin jets flow away from the accreting system in opposite directions, perpendicular to the plane of the accretion disc. In the scenario presented in this paper, the accretion disc interacts with the magnetic field in such a way that the disc angular momentum is removed from the disc and transported away along the magnetic field lines. Such a transport is the source of the jet phenomenon as the angular momentum is given back to a tiny amount of material extracted from the disc. This outflow is then powered by the disc rotation as the disc is able to enter an accretion motion where matter releases its gravitational energy. The angular momentum carried by the jet is actually present through the existence of an electric current. In the jet cylindrical geometry, the presence of this current is able to provide a collimating mechanism where the magnetic field pinches the plasma column. This mechanism is very close to the one acting in tokamak reactors. Apart from explaining how the plasma outflow is able to be self-confined by the magnetic field present in the flow, this scenario is also able to explain how jet mass can be accelerated thanks to the magnetohydrodynamics Poynting flux escaping from the disc. In this presentation I finally present the constraints arising from the scenario, in particular upon the turbulent transport coefficient required to get a steady structure.

  9. On the effect of injection of gas in the numerical simulation of accretion flows

    Institute of Scientific and Technical Information of China (English)

    2010-01-01

    We investigate the effects of various ways of injection of gas at the outer boundary in the numerical simulations of non-viscous accretion flows.We study three models.In Model A,we inject material around the equatorial plane.In Models B and C,fullrange θ injection is used(we employ spherical coordinates).In all three models,the injected material has the same density distribution with polar angle θ.From the equatorial region to the polar regions,angular momentum of the injected material of Model B decreases faster than that in Model C.For all of the models,after a transient episode of infall at the beginning of the simulations,the gas piles up in the equatorial regions outside the black hole and forms a thick torus bounded by a centrifugal barrier.We find that the accretion rates of Models B and C are more than ten times higher than that in Model A.In Model A,there is weak accretion only in the torus and outflows are found on the surface of the torus.In Model B,we find strong inflows on the surface of its torus,and the accretion in the torus is weak.In Model C,strong inflows also occur on the surface of its torus,but the accretion regions are narrower and there are strong outflows in its torus.In all of our models,the time-averaged density,pressure and angular momentum in the equatorial region can be described by a radial power law,with P ∝r-3/2,P ∝r-2 and l∝r0.

  10. Accretion Disk Signatures in Type I X-Ray Bursts: Prospects for Future Missions

    Science.gov (United States)

    Keek, L.; Wolf, Z.; Ballantyne, D. R.

    2016-07-01

    Type I X-ray bursts and superbursts from accreting neutron stars illuminate the accretion disk and produce a reflection signal that evolves as the burst fades. Examining the evolution of reflection features in the spectra will provide insight into the burst-disk interaction, a potentially powerful probe of accretion disk physics. At present, reflection has been observed during only two bursts of exceptional duration. We investigate the detectability of reflection signatures with four of the latest well-studied X-ray observatory concepts: Hitomi, Neutron Star Interior Composition Explorer (NICER), Athena, and Large Observatory For X-ray Timing (LOFT). Burst spectra are modeled for different values for the flux, temperature, and the disk ionization parameter, which are representative for most known bursts and sources. The effective area and throughput of a Hitomi-like telescope are insufficient for characterizing burst reflection features. NICER and Athena will detect reflection signatures in Type I bursts with peak fluxes ≳10-7.5 erg cm-2 s-1 and also effectively constrain the reflection parameters for bright bursts with fluxes of ˜10-7 erg cm-2 s-1 in exposures of several seconds. Thus, these observatories will provide crucial new insight into the interaction of accretion flows and X-ray bursts. For sources with low line-of-sight absorption, the wide bandpass of these instruments allows for the detection of soft X-ray reflection features, which are sensitive to the disk metallicity and density. The large collecting area that is part of the LOFT design would revolutionize the field by tracing the evolution of the accretion geometry in detail throughout short bursts.

  11. On the Accretion Rates and Radiative Efficiencies of the Highest-redshift Quasars

    Science.gov (United States)

    Trakhtenbrot, Benny; Volonteri, Marta; Natarajan, Priyamvada

    2017-02-01

    We estimate the accretion rates onto the supermassive black holes that power 20 of the highest-redshift quasars, at z≳ 5.8, including the quasar with the highest redshift known to date—ULAS J1120 at z = 7.09. The analysis is based on the observed (rest-frame) optical luminosities and reliable “virial” estimates of the BH masses of the quasars, and utilizes scaling relations derived from thin accretion disk theory. The mass accretion rates through the postulated disks cover a wide range, {\\dot{M}}{disk}≃ 4{--}190 {M}ȯ {{yr}}-1, with most of the objects (80%) having {\\dot{M}}{disk}≃ 10{--}65 {M}ȯ {{yr}}-1, confirming the Eddington-limited nature of the accretion flows. By combining our estimates of {\\dot{M}}{disk} with conservative, lower limits on the bolometric luminosities of the quasars, we investigate which alternative values of η best account for all the available data. We find that the vast majority of quasars (∼85%) can be explained with radiative efficiencies in the range η ≃ 0.03{--}0.3, with a median value close to the commonly assumed η = 0.1. Within this range, we obtain conservative estimates of η ≳ 0.14 for ULAS J1120 and SDSS J0100 (at z = 6.3), and of ≳ 0.19 for SDSS J1148 (at z=6.41; assuming their BH masses are accurate). The implied accretion timescales are generally in the range {t}{acc}\\equiv {M}{BH}/{\\dot{M}}{BH}≃ 0.1{--}1 {Gyr}, suggesting that most quasars could have had ∼ 1{--}10 mass e-foldings since BH seed formation. Our analysis therefore demonstrates that the available luminosities and masses for the highest-redshift quasars can be explained self-consistently within the thin, radiatively efficient accretion disk paradigm. Episodes of radiatively inefficient, “super-critical” accretion may have occurred at significantly earlier epochs (i.e., z≳ 10).

  12. The Chaotic Light Curves of Accreting Black Holes

    Science.gov (United States)

    Kazanas, Demosthenes

    2007-01-01

    We present model light curves for accreting Black Hole Candidates (BHC) based on a recently developed model of these sources. According to this model, the observed light curves and aperiodic variability of BHC are due to a series of soft photon injections at random (Poisson) intervals and the stochastic nature of the Comptonization process in converting these soft photons to the observed high energy radiation. The additional assumption of our model is that the Comptonization process takes place in an extended but non-uniform hot plasma corona surrounding the compact object. We compute the corresponding Power Spectral Densities (PSD), autocorrelation functions, time skewness of the light curves and time lags between the light curves of the sources at different photon energies and compare our results to observation. Our model reproduces the observed light curves well, in that it provides good fits to their overall morphology (as manifest by the autocorrelation and time skewness) and also to their PSDs and time lags, by producing most of the variability power at time scales 2 a few seconds, while at the same time allowing for shots of a few msec in duration, in accordance with observation. We suggest that refinement of this type of model along with spectral and phase lag information can be used to probe the structure of this class of high energy sources.

  13. Seismology of Rapidly Rotating Accreting White Dwarfs

    CERN Document Server

    Townsley, Dean M; Bildsten, Lars

    2016-01-01

    A number of White Dwarfs (WDs) in cataclysmic binaries have shown brightness variations consistent with non-radial oscillations as observed in isolated WDs. A few objects have been well-characterized with photometric campaigns in the hopes of gleaning information about the mass, spin, and possibly internal structural characteristics. The novel aspect of this work is the possiblity to measure or constrain the interior structure and spin rate of WDs which have spent gigayears accreting material from their companion, undergoing thousands of nova outbursts in the process. In addition, variations in the surface temperature affect the site of mode driving, and provide unique and challenging tests for mode driving theories previously applied to isolated WD's. Having undergone long-term accretion, these WDs are expected to have been spun up. Spin periods in the range 60-100 seconds have been measured by other means for two objects, GW Lib and V455 And. Compared to typical mode frequencies, the spin frequency may be s...

  14. Formation of redbacks via accretion induced collapse

    CERN Document Server

    Smedley, Sarah L; Ferrario, Lilia; Wickramasinghe, Dayal T

    2014-01-01

    We examine the growing class of binary millisecond pulsars known as redbacks. In these systems the pulsar's companion has a mass between 0.1 and about 0.5 solar masses in an orbital period of less than 1.5 days. All show extended radio eclipses associated with circumbinary material. They do not lie on the period-companion mass relation expected from the canonical intermediate-mass X-ray binary evolution in which the companion filled its Roche lobe as a red giant and has now lost its envelope and cooled as a white dwarf. The redbacks lie closer to, but usually at higher period than, the period-companion mass relation followed by cataclysmic variables and low-mass X-ray binaries. In order to turn on as a pulsar mass accretion on to a neutron star must be sufficiently weak, considerably weaker than expected in systems with low-mass main-sequence companions driven together by magnetic braking or gravitational radiation. If a neutron star is formed by accretion induced collapse of a white dwarf as it approaches th...

  15. Magnetic field evolution of accreting neutron stars

    CERN Document Server

    Istomin, Ya N

    2016-01-01

    The flow of a matter, accreting onto a magnetized neutron star, is accompanied by an electric current. The closing of the electric current occurs in the crust of a neutron stars in the polar region across the magnetic field. But the conductivity of the crust along the magnetic field greatly exceeds the conductivity across the field, so the current penetrates deep into the crust down up to the super conducting core. The magnetic field, generated by the accretion current, increases greatly with the depth of penetration due to the Hall conductivity of the crust is also much larger than the transverse conductivity. As a result, the current begins to flow mainly in the toroidal direction, creating a strong longitudinal magnetic field, far exceeding an initial dipole field. This field exists only in the narrow polar tube of $r$ width, narrowing with the depth, i.e. with increasing of the crust density $\\rho$, $r\\propto \\rho^{-1/4}$. Accordingly, the magnetic field $B$ in the tube increases with the depth, $B\\propto...

  16. NUMERICAL SIMULATION OF ICE ACCRETION ON AIRFOIL

    Directory of Open Access Journals (Sweden)

    Nicusor ALEXANDRESCU

    2009-09-01

    Full Text Available This work consists in the simulation of the ice accretion in the leading edge of aerodynamic profiles and our proposed model encompasses: geometry generation, calculation of the potential flow around the body, boundary layer thickness computation, water droplet trajectory computation, heat and mass balances and the consequent modification of the geometry by the ice growth. The flow calculation is realized with panel methods, using only segments defined over the body contour. The viscous effects are considered using the Karman-Pohlhausen method for the laminar boundary layer. The local heat transfer coefficient is obtained by applying the Smith-Spalding method for the thermal boundary layer. The ice accretion limits and the collection efficiency are determined by computing water droplet trajectories impinging the surface. The heat transfer process is analyzed with an energy and a mass balance in each segment defining the body. Finally, the geometry is modified by the addition of the computed ice thickness to the respective panel. The process by repeating all the steps. The model validation is done using a selection of problems with experimental solution, CIRA (the CESAR project. Hereinafter, results are obtained for different aerodynamic profiles, angles of attack and meteorological parameters

  17. Magnetic fields in primordial accretion disks

    Science.gov (United States)

    Latif, M. A.; Schleicher, D. R. G.

    2016-01-01

    Magnetic fields are considered a vital ingredient of contemporary star formation and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations and are subsequently amplified by the small-scale dynamo, leading to a strong, tangled magnetic field. We explore how the magnetic field provided by the small-scale dynamo is further amplified via the α-Ω dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop. III star with 10M⊙ and an accretion rate of 10-3M⊙ yr-1, and a supermassive star with 105M⊙ and an accretion rate of 10-1M⊙ yr-1. For the 10M⊙ Pop. III star, we find that coherent magnetic fields can be produced on scales of at least 100 AU, which are sufficient to drive a jet with a luminosity of 100L⊙ and a mass outflow rate of 10-3.7M⊙ yr-1. For the supermassive star, the dynamical timescales in its environment are even shorter, implying smaller orbital timescales and an efficient magnetization out to at least 1000 AU. The jet luminosity corresponds to ~106.0L⊙ and a mass outflow rate of 10-2.1M⊙ yr-1. We expect that the feedback from the supermassive star can have a relevant impact on its host galaxy.

  18. Magnetic fields in primordial accretion disks

    CERN Document Server

    Latif, Muhammad A

    2016-01-01

    Magnetic fields are considered as a vital ingredient of contemporary star formation, and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations, and are subsequently amplified by the small-scale dynamo, leading to a strong tangled magnetic field. Here we explore how the magnetic field provided by the small-scale dynamo is further amplified via the $\\alpha-\\Omega$ dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop.~III star with $10$~M$_\\odot$ and an accretion rate of $10^{-3}$~M$_\\odot$~yr$^{-1}$, and a supermassive star with $10^5$~M$_\\odot$ and an accretion rate of $10^{-1}$~M$_\\odot$~yr$^{-1}$. For the $10$~M$_\\odot$ Pop.~III star, we find that coherent magnetic fields can be produced on scales of at least $100$~AU, which are sufficient to drive a jet with a luminosity of $100$~L$_\\odot$ and a mass outflow ra...

  19. Spiral waves in accretion discs - observations

    CERN Document Server

    Steeghs, D

    2000-01-01

    I review the observational evidence for spiral structure in the accretion discs of cataclysmic variables (CVs). Doppler tomography is ideally suited to resolve and map such co-rotating patterns and allows a straightforward comparison with theory. The dwarf nova IP Pegasi presents the best studied case, carrying two spiral arms in a wide range of emission lines throughout its outbursts. Both arms appear at the locations where tidally driven spiral waves are expected, with the arm closest to the gas stream weaker in the lines compared to the arm closest to the companion. Eclipse data indicates sub-Keplerian velocities in the outer disc. The dramatic disc structure changes in dwarf novae on timescales of days to weeks, provide unique opportunities for our understanding of angular momentum transport and the role of density waves on the structure of accretion discs. I present an extension to the Doppler tomography technique that relaxes one of the basic assumptions of tomography, and is able to map modulated emiss...

  20. A Wind Accretion Model for HLX-1

    CERN Document Server

    Miller, M Coleman; Maccarone, Thomas J

    2014-01-01

    The brightest ultraluminous X-ray source currently known, HLX-1, has been observed to undergo five outburst cycles. The periodicity of these outbursts, and their high inferred maximum accretion rates of $\\sim{\\rm few}\\times 10^{-4} M_\\odot {\\rm yr}^{-1}$, naturally suggest Roche lobe overflow at the pericenter of an eccentric orbit. It is, however, difficult for the Roche lobe overflow model to explain the apparent trend of decreasing decay times over the different outbursts while the integrated luminosity also drops. Thus if the trend is real rather than simply being a reflection of the complex physics of accretion disks, a different scenario may be necessary. We present a speculative model in which, within the last decade, a high-mass giant star had most of its envelope tidally stripped by the $\\sim 10^{4-5} M_\\odot$ black hole in HLX-1, and the remaining core plus low-mass hydrogen envelope now feeds the hole with a strong wind. This model can explain the short decay time of the disk, and could explain the...

  1. Accretion, Outflows, and Winds of Magnetized Stars

    CERN Document Server

    Romanova, M M

    2016-01-01

    Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars...

  2. Quantifying Rapid Variability in Accreting Compact Objects

    CERN Document Server

    Van der Klis, M

    1997-01-01

    I discuss some practical aspects of the analysis of millisecond time variability X-ray data obtained from accreting neutron stars and black holes. First I give an account of the statistical methods that are at present commonly applied in this field. These are mostly based on Fourier techniques. To a large extent these methods work well: they give astronomers the answers they need. Then I discuss a number of statistical questions that astronomers don't really know how to solve properly and that statisticians may have ideas about. These questions have to do with the highest and the lowest frequency ranges accessible in the Fourier analysis: how do you determine the shortest time scale present in the variability, how do you measure steep low-frequency noise. The point is stressed that in order for any method that resolves these issues to become popular, it is necessary to retain the capabilities the current methods already have in quantifying the complex, concurrent variability processes characteristic of accret...

  3. Eclipse Mapping: Astrotomography of Accretion Discs

    Science.gov (United States)

    Baptista, Raymundo

    The Eclipse Mapping Method is an indirect imaging technique that transforms the shape of the eclipse light curve into a map of the surface brightness distribution of the occulted regions. Three decades of application of this technique to the investigation of the structure, the spectrum and the time evolution of accretion discs around white dwarfs in cataclysmic variables have enriched our understanding of these accretion devices with a wealth of details such as (but not limited to) moving heating/cooling waves during outbursts in dwarf novae, tidally-induced spiral shocks of emitting gas with sub-Keplerian velocities, elliptical precessing discs associated to superhumps, and measurements of the radial run of the disc viscosity through the mapping of the disc flickering sources. This chapter reviews the principles of the method, discusses its performance, limitations, useful error propagation procedures, as well as highlights a selection of applications aimed at showing the possible scientific problems that have been and may be addresses with it.

  4. Probing thermonuclear burning on accreting neutron stars

    Science.gov (United States)

    Keek, L.

    2008-12-01

    Neutron stars are the most compact stars that can be directly observed, which makes them ideal laboratories to study physics at extreme densities. Neutron stars in low-mass X-ray binaries accrete hydrogen and helium from a lower-mass companion star through Roche lobe overflow. This matter undergoes thermonuclear burning in the neutron star envelope, creating carbon and heavier elements. The fusion process may proceed in an unstable manner, resulting in a thermonuclear runaway. Within one second the entire surface is burned, which is observable as a sharp rise in the emitted X-ray flux: a type I X-ray burst. Afterwards the neutron star surface cools down on a timescale of ten to one hundred seconds. During these bursts the surface of an accreting neutron star can be observed directly, which makes them instrumental for studying this type of stars. We have studied rare kinds of X-ray bursts. One such rare burst is the superburst, which lasts a thousand times longer than an ordinary burst. Superbursts are thought to result from the explosive burning of a thick carbon layer, which lies deeper inside the neutron star, close to a layer known as the crust. A prerequisite for the occurrence of a superburst is a high enough temperature, which is set by the temperature of the crust and the heat conductivity of the envelope. The latter is lowered by the presence of heavy elements that are produced during normal X-ray bursts. Using a large set of observations from the Wide Field Camera's onboard the BeppoSAX satellite, we find that, at high accretion rate, sources which do not exhibit normal bursts likely have a longer superburst recurrence time, than the observed superburst recurrence time of one burster. We analyze in detail the first superburst from a transient source, which went into outburst only 55 days before the superburst. Recent models of the neutron star crust predict that this is too small a time to heat the crust sufficiently for superburst ignition, indicating

  5. Testing black hole neutrino-dominated accretion discs for long-duration gamma-ray bursts

    CERN Document Server

    Song, Cui-Ying; Gu, Wei-Min; Tian, Jian-Xiang

    2016-01-01

    Long-duration gamma-ray bursts (LGRBs) are generally considered to originate from the massive collapsars. It is believed that the central engine of gamma-ray bursts (GRBs) is a neutrino-dominated accretion flow (NDAF) around a rotating stellar-mass black hole (BH). The neutrino annihilation above the NDAF is a feasible mechanism to power GRB. In this work, we analyse the distributions of the isotropic gamma-ray radiated energy and jet kinetic energy of 48 LGRBs. According to the NDAF and fireball models, we estimate the mean accreted masses of LGRBs in our sample to investigate whether the NDAFs can power LGRBs with the reasonable BH parameters and conversion efficiency of neutrino annihilation. The results indicate that most of the values of the accreted masses are less than $5~M_\\odot$ for the extreme Kerr BHs and high conversion efficiency. It suggests that the NDAFs may be suitable for most of LGRBs except for some extremely high energy sources.

  6. Testing black hole neutrino-dominated accretion discs for long-duration gamma-ray bursts

    Science.gov (United States)

    Song, Cui-Ying; Liu, Tong; Gu, Wei-Min; Tian, Jian-Xiang

    2016-05-01

    Long-duration gamma-ray bursts (LGRBs) are generally considered to originate from the massive collapsars. It is believed that the central engine of gamma-ray bursts (GRBs) is a neutrino-dominated accretion flow (NDAF) around a rotating stellar-mass black hole (BH). The neutrino annihilation above the NDAF is a feasible mechanism to power GRB. In this work, we analyse the distributions of the isotropic gamma-ray-radiated energy and jet kinetic energy of 48 LGRBs. According to the NDAF and fireball models, we estimate the mean accreted masses of LGRBs in our sample to investigate whether the NDAFs can power LGRBs with the reasonable BH parameters and conversion efficiency of neutrino annihilation. The results indicate that most of the values of the accreted masses are less than 5 M⊙ for the extreme Kerr BHs and high conversion efficiency. It suggests that the NDAFs may be suitable for most of LGRBs except for some extremely high energy sources.

  7. Non-stationary dynamo & magnetospheric accretion processes of the classical T Tauri star V2129 Oph

    CERN Document Server

    Donati, JF; Walter, FM; Gregory, SG; Skelly, MB; Hussain, GAJ; Flaccomio, E; Argiroffi, C; Grankin, KN; Jardine, MM; Menard, F; Dougados, C; Romanova, MM

    2010-01-01

    We report here the first results of a multi-wavelength campaign focussing on magnetospheric accretion processes of the classical TTauri star (cTTS) V2129Oph. In this paper, we present spectropolarimetric observations collected in 2009 July with ESPaDOnS at the Canada-France-Hawaii Telescope (CFHT). Circularly polarised Zeeman signatures are clearly detected, both in photospheric absorption and accretion-powered emission lines, from time-series of which we reconstruct new maps of the magnetic field, photospheric brightness and accretion-powered emission at the surface of V2129Oph using our newest tomographic imaging tool -- to be compared with those derived from our old 2005 June data set, reanalyzed in the exact same way. We find that in 2009 July, V2129Oph hosts octupolar & dipolar field components of about 2.1 & 0.9kG respectively, both tilted by about 20deg with respect to the rotation axis; we conclude that the large-scale magnetic topology changed significantly since 2005 June (when the octupole ...

  8. X-Ray Spectra from MHD Simulations of Accreting Black Holes

    Science.gov (United States)

    Schnittman, Jeremy D.; Krolik, Julian H.; Noble, Scott C.

    2012-01-01

    We present the results of a new global radiation transport code coupled to a general relativistic magneto-hydrodynamic simulation of an accreting, nonrotating black hole. For the first time, we are able to explain from first principles in a self-consistent way the X-ray spectra observed from stellar-mass black holes, including a thermal peak, Compton reflection hump, power-law tail, and broad iron line. Varying only the mass accretion rate, we are able to reproduce the low/hard, steep power-law, and thermal-dominant states seen in most galactic black hole sources. The temperature in the corona is T(sub e) 10 keV in a boundary layer near the disk and rises smoothly to T(sub e) greater than or approximately 100 keV in low-density regions far above the disk. Even as the disk's reflection edge varies from the horizon out to approximately equal to 6M as the accretion rate decreases, we find that the shape of the Fe Ka line is remarkably constant. This is because photons emitted from the plunging region are strongly beamed into the horizon and never reach the observer. We have also carried out a basic timing analysis of the spectra and find that the fractional variability increases with photon energy and viewer inclination angle, consistent with the coronal hot spot model for X-ray fluctuations.

  9. Quasi-static model of collimated jets and radio lobes. I. Accretion disk and jets

    Energy Technology Data Exchange (ETDEWEB)

    Colgate, Stirling A.; Li, Hui [Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545 (United States); Fowler, T. Kenneth [University of California, Berkeley, CA 94720 (United States); Pino, Jesse [Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States)

    2014-07-10

    This is the first of a series of papers showing that when an efficient dynamo can be maintained by accretion disks around supermassive black holes in active galactic nuclei, it can lead to the formation of a powerful, magnetic helix that could explain both the observed radio jet/lobe structures on very large scales and ultimately the enormous power inferred from the observed ultra-high-energy cosmic rays. In this work, we solve a set of one-dimensional equations similar to the steady-state standard accretion disk model, but now including the large-scale magnetic fields giving rises to jets. We find that the frequently made assumption that large-scale fields are frozen into the disk is fundamentally incorrect, due to the necessity for current and the accreting mass to flow perpendicular to magnetic flux surfaces. A correct treatment greatly simplifies the calculations, yielding fields that leave the disk nearly vertically with magnetic profiles uniquely determined by disk angular momentum conservation. Representative solutions of the magnetic fields in different radial regions of the disk surface are given, and they determine the overall key features in the jet structure and its dissipation, which will be the subjects of later papers.

  10. Hot-spot model for accretion disc variability as random process

    CERN Document Server

    Pechacek, T; Czerny, B

    2008-01-01

    Theory of random processes provides an attractive mathematical tool to describe the fluctuating signal from accreting sources, such as active galactic nuclei and Galactic black holes observed in X-rays. These objects exhibit featureless variability on different timescales, probably originating from an accretion disc. We study the basic features of the power spectra in terms of a general framework, which permits semi-analytical determination of the power spectral density (PSD) of the resulting light curve. We consider the expected signal generated by an ensemble of spots randomly created on the accretion disc surface. Spot generation is governed by Poisson or by Hawkes processes. We include general relativity effects shaping the signal on its propagation to a distant observer. We analyse the PSD of a spotted disc light curve and show the accuracy of our semi-analytical approach by comparing the obtained PSD with the results of Monte Carlo simulations. The asymptotic slopes of PSD are 0 at low frequencies and t...

  11. Interference as an origin of the peaked noise in accreting X-ray binaries

    CERN Document Server

    Veledina, Alexandra

    2016-01-01

    We propose a physical model for the peaked noise in the X-ray power density spectra of accreting X-ray binaries. We interpret its appearance as an interference of two Comptonization continua: one coming from the up-scattering of seed photons from the cold thin disk and the other fed by the synchrotron emission of the hot flow. Variations of both X-ray components are caused by fluctuations in mass accretion rate, but there is a delay between them corresponding to the propagation timescale from the disk Comptonization radius to the region of synchrotron Comptonization. If the disk and synchrotron Comptonization are correlated, the humps in the power spectra are harmonically related and the dips between them appear at frequencies related as odd numbers 1:3:5. If they are anti-correlated, the humps are related as 1:3:5, but the dips are harmonically related. Similar structures are expected to be observed in accreting neutron star binaries and supermassive black holes. The delay can be easily recovered from the fr...

  12. Discovery of the Onset of Rapid Accretion by a Dormant Massive Black Hole

    CERN Document Server

    Burrows, D N; Ghisellini, G; Mangano, V; Zhang, B; Page, K L; Eracleous, M; Romano, P; Sakamoto, T; Falcone, A D; Osborne, J P; Campana, S; Beardmore, A P; Breeveld, A A; Chester, M M; Corbet, R; Covino, S; Cummings, J R; D'Avanzo, P; D'Elia, V; Esposito, P; Evans, P A; Fugazza, D; Gelbord, J M; Hiroi, K; Holland, S T; Huang, K Y; Im, M; Israel, G; Jeon, Y; Jeon, Y -B; Kawai, N; Krimm, H A; Mészáros, P; Negoro, H; Omodei, N; Park, W -K; Perkins, J S; Sugizaki, M; Sung, H -I; Tagliaferri, G; Troja, E; Ueda, Y; Urata, Y; Usui, R; Antonelli, L A; Barthelmy, S D; Cusumano, G; Giommi, P; Marshall, F E; Melandri, A; Perri, M; Racusin, J L; Sbarufatti, B; Siegel, M H; Gehrels, N

    2011-01-01

    Massive black holes are believed to reside at the centres of most galaxies. They can be- come detectable by accretion of matter, either continuously from a large gas reservoir or impulsively from the tidal disruption of a passing star, and conversion of the gravitational energy of the infalling matter to light. Continuous accretion drives Active Galactic Nuclei (AGN), which are known to be variable but have never been observed to turn on or off. Tidal disruption of stars by dormant massive black holes has been inferred indirectly but the on- set of a tidal disruption event has never been observed. Here we report the first discovery of the onset of a relativistic accretion-powered jet in the new extragalactic transient, Swift J164449.3+573451. The behaviour of this new source differs from both theoretical models of tidal disruption events and observations of the jet-dominated AGN known as blazars. These differences may stem from transient effects associated with the onset of a powerful jet. Such an event in th...

  13. 3D Dynamical Modeling of Wind Accretion in Cyg X-3

    CERN Document Server

    Okazaki, Atsuo T

    2014-01-01

    Cyg X-3 is a high mass X-ray binary consisting of a Wolf-Rayet star and a compact object in a very short orbital period of 4.8h. The only confirmed microquasar with high energy gamma-ray emission, Cyg X-3 provides a unique opportunity to study the relationship between the accretion power and the power in high energy emission. Because of a compact orbit and a slow Wolf-Rayet wind, the flow structure around the compact object is thought to be strongly affected by the orbital motion, details of which can be obtained only by numerical simulations. In this paper, we report on the results from 3D hydrodynamic simulations of the wind accretion in Cyg X-3. For simplicity we adopt an anti-gravity-like force that emulates the radiative acceleration consistent with the beta-velocity wind. Due to the rapid orbital motion, the flow around the compact object has large density gradients. As a result, the accretion rate onto the compact object is significantly lower than that of the Bondi-Hoyle-Lyttleton rate. We also calcul...

  14. Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs

    CERN Document Server

    Dexter, Jason; Markoff, Sera; Tchekhovskoy, Alexander

    2013-01-01

    Magnetically arrested accretion discs (MADs), where the magnetic pressure in the inner disc is dynamically important, provide an alternative mechanism for regulating accretion to what is commonly assumed in black hole systems. We show that a global magnetic field inversion in the MAD state can destroy the jet, significantly increase the accretion rate, and move the effective inner disc edge in to the marginally stable orbit. Reconnection of the MAD field in the inner radii launches a new type of transient outflow containing hot plasma generated by magnetic dissipation. This transient outflow can be as powerful as the steady magnetically-dominated Blandford-Znajek jet in the MAD state. The field inversion qualitatively describes many of the observational features associated with the high luminosity hard to soft state transition in black hole X-ray binaries: the jet line, the transient ballistic jet, and the drop in rms variability. These results demonstrate that the magnetic field configuration can influence t...

  15. Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries

    Science.gov (United States)

    Weston, Jennifer Helen Seng; E-Nova Project

    2017-01-01

    In this dissertation, I use radio observations with the Karl G. Jansky Very Large Array (VLA) to reveal that colliding flows within the ejecta from nova explosions can lead to shocks that accelerate particles and produce radio synchrotron emission. In both novae V1723 Aql and V5589 Sgr, radio emission within the first one to two months deviated strongly from the classic thermal model for radio emission from novae. Three years of radio observations of V1723 Aql show that multiple outflows from the system collided to create non-thermal shocks with a brightness temperature of >106 K. After these shocks faded, the radio light curve became roughly consistent with an expanding thermal shell. However, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months. In the case of nova V5589 Sgr, I show that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTx > 33 keV) X-rays. These findings have important implications for understanding how normal novae generate GeV gamma-rays.Additionally, I present VLA observations of the symbiotic star CH Cyg and two small surveys of symbiotic binaries. Radio observations of CH Cyg tie the ejection of a collimated jet to a change of state in the accretion disk, strengthening the link between bipolar outflows from accreting white dwarfs and other types of accreting compact objects. Next, I use a survey of eleven accretion-driven symbiotic binaries to determine that the radio brightness of a symbiotic system could potentially be used as an indicator of whether it is powered predominantly by shell burning on the surface of the white dwarf or by accretion. This survey also produces the first radio detections of seven of the target systems. In the second survey of seventeen symbiotic binaries, I spatially resolve extended radio emission in several systems for the first time. The results from these surveys provide some support for the

  16. Measuring Mass Accretion Rate onto the Supermassive Black Hole in M 87 Using Faraday Rotation Measure with the Submillimeter Array

    CERN Document Server

    Kuo, C Y; Rao, R; Nakamura, M; Algaba, J C; Liu, H B; Inoue, M; Koch, P M; Ho, P T P; Matsushita, S; Pu, H -Y; Akiyama, K; Nishioka, H; Pradel, N

    2014-01-01

    We present the first constraint on Faraday rotation measure (RM) at submillimeter wavelengths for the nucleus of M 87. By fitting the polarization position angles ($\\chi$) observed with the SMA at four independent frequencies around $\\sim$230 GHz and interpreting the change in $\\chi$ as a result of \\emph{external} Faraday rotation associated with accretion flow, we determine the rotation measure of the M 87 core to be between $-$7.5$\\times$10$^{5}$ and 3.4$\\times$10$^{5}$ rad/m$^{2}$. Assuming a density profile of the accretion flow that follows a power-law distribution and a magnetic field that is ordered, radial, and has equipartition strength, the limit on the rotation measure constrains the mass accretion rate $\\dot{M}$ to be below 9.2$\\times$10$^{-4}$ M$_{\\odot}$~yr$^{-1}$ at a distance of 21 Schwarzchild radii from the central black hole. This value is at least two orders of magnitude smaller than the Bondi accretion rate, suggesting significant suppression of the accretion rate in the inner region of t...

  17. As above, so below: exploiting mass scaling in black hole accretion to break degeneracies in spectral interpretation

    CERN Document Server

    Markoff, Sera; Gallo, Elena; Hynes, Robert; Wilms, Jörn; Plotkin, Richard M; Maitra, Dipankar; Silva, Catia V; Drappeau, Samia

    2015-01-01

    Over the last decade, the evidence is mounting that several aspects of black hole accretion physics proceed in a mass-invariant way. One of the best examples of this scaling is the empirical "Fundamental Plane of Black Hole Accretion" relation linking mass, radio and X-ray luminosity over eight orders of magnitude in black hole mass. The currently favored theoretical interpretation of this relation is that the physics governing power output in weakly accreting black holes depends more on relative accretion rate than on mass. In order to test this theory, we explore whether a mass-invariant approach can simultaneously explain the broadband spectral energy distributions from two black holes at opposite ends of the mass scale but at similar Eddington accretion fractions. We find that the same model, with the same value of several fitted physical parameters expressed in mass-scaling units to enforce self-similarity, can provide a good description of two datasets from V404 Cyg and M81*, a stellar and supermassive ...

  18. Linear Two-Dimensional MHD of Accretion Disks: Crystalline structure and Nernst coefficient

    CERN Document Server

    Montani, Giovanni

    2009-01-01

    We analyse the two-dimensional MHD configurations characterising the steady state of the accretion disk on a highly magnetised neutron star. The model we describe has a local character and represents the extension of the crystalline structure outlined in Coppi (2005), dealing with a local model too, when a specific accretion rate is taken into account. We limit our attention to the linearised MHD formulation of the electromagnetic back-reaction characterising the equilibrium, by fixing the structure of the radial, vertical and azimuthal profiles. Since we deal with toroidal currents only, the consistency of the model is ensured by the presence of a small collisional effect, phenomenologically described by a non-zero constant Nernst coefficient (thermal power of the plasma). Such an effect provides a proper balance of the electron force equation via non zero temperature gradients, related directly to the radial and vertical velocity components. We show that the obtained profile has the typical oscillating feat...

  19. Resolving the Bondi Accretion Flow toward the Supermassive Black Hole of NGC 3115 with Chandra

    Science.gov (United States)

    Wong, Ka-Wah; Irwin, J.; Million, E.; Yukita, M.; Mathews, W.; Bregman, J.

    2011-09-01

    Gas undergoing Bondi accretion on to a supermassive black hole (SMBH) becomes hotter toward smaller radii. We searched for this signature with a Chandra observation of the hot gas in NGC 3115, which optical observation show has a very massive SMBH. Our observations show that the gas temperature rises toward the galaxy center as expected in all accretion models in which the black hole is gravitationally capturing the ambient gas. The data support that the Bondi radius is at least about 4-5 arcsec (188-235 pc), suggesting a supermassive blackhole of two billion solar masses that is consistent with the upper end of the optical results. The density profile within the Bondi radius has a power law index of 1.03, and we will discuss the interpretations of the results.

  20. Accretion disk winds in active galactic nuclei: X-ray observations, models, and feedback

    CERN Document Server

    Tombesi, Francesco

    2016-01-01

    Powerful winds driven by active galactic nuclei (AGN) are often invoked to play a fundamental role in the evolution of both supermassive black holes (SMBHs) and their host galaxies, quenching star formation and explaining the tight SMBH-galaxy relations. A strong support of this "quasar mode" feedback came from the recent X-ray observation of a mildly relativistic accretion disk wind in a ultraluminous infrared galaxy (ULIRG) and its connection with a large-scale molecular outflow, providing a direct link between the SMBH and the gas out of which stars form. Spectroscopic observations, especially in the X-ray band, show that such accretion disk winds may be common in local AGN and quasars. However, their origin and characteristics are still not fully understood. Detailed theoretical models and simulations focused on radiation, magnetohydrodynamic (MHD) or a combination of these two processes to investigate the possible acceleration mechanisms and the dynamics of these winds. Some of these models have been dir...

  1. Inefficient highly eccentric accretion and the low luminosity of stellar tidal disruption events

    CERN Document Server

    Svirski, Gilad; Krolik, Julian

    2015-01-01

    Models for tidal disruption events (TDEs) in which a supermassive black hole disrupts a star commonly assume that the highly eccentric streams of bound stellar debris promptly form a circular accretion disk at the pericenter scale. However, the bolometric peak luminosity of most TDE candidates, $\\sim10^{44}\\,\\rm{erg\\,s^{-1}}$, implies that we observe only $\\sim1\\%$ of the energy expected from accretion. Moreover, recent numerical simulations (Shiokawa et al. 2015) have shown that dissipation via hydrodynamical shocks is insufficient to circularize debris orbits on the pericenter scale, and the debris flow retains its initial semi-major axis scale throughout the first $\\sim10$ orbits of the event. Motivated by these numerical results, Piran et al. (2015) suggested that the observed optical TDE emission is powered by shocks at the apocenter between freshly infalling material and earlier-arriving matter. This model explains the small radiated energy, the low temperature, and the large radius implied by the obser...

  2. Signatures of Accretion Shocks in Broadband Spectrum of Advective Flows Around Black Holes

    CERN Document Server

    Mandal, S; Mandal, Samir; Chakrabarti, Sandip K.

    2005-01-01

    We compute the effects of the centrifugal pressure supported shock waves on the emitted spectrum from an accretion disk primarily consisting of low angular momentum matter. Electrons are very efficiently accelerated by the accretion shock and acquire power-law distribution. The accelerated particles in turn emit synchrotron radiation in presence of a stochastic magnetic field in equipartition with the gas. Efficient cooling of the electrons by these soft photons reduces its temperature in comparison to the protons. We explore the nature of the broadband spectra by using Comptonization, bremsstrahlung and synchrotron emission. We then show that there could be two crossing points in a broadband spectrum, one near $\\sim 10 keV$ and the other $\\sim 300-400$KeV.

  3. Signature of Accretion Shocks in Emitted Radiation From a Two Temperature Advective Flows Around Black Holes

    CERN Document Server

    Mandal, S; Mandal, Samir; Chakrabarti, Sandip K.

    2005-01-01

    Centrifugal barrier supported boundary layer (CENBOL) of a black hole affects the spectrum exactly in the same way the boundary layer of a neutron star does. The CENBOL is produced due to standing or oscillating shock waves and these shocks accelerate electrons very efficiently and produce a power-law distribution. The accelerated particles in turn emit synchrotron radiation in presence of the magnetic field. We study the spectral properties of an accretion disk as a function of the shock strength, compression ratio, flow accretion rate and flow geometry. In the absence of a satisfactory description of magnetic fields inside the advective disk, we consider the presence of only stochastic fields and use the ratio of the field energy density to the gravitational energy density to be a parameter. Not surprisingly, stronger fields produce stronger humps due to synchrotron radiation. We not only include `conventional' synchrotron emission and Comptonization due to Maxwell-Bolzmann electrons in the gas, we also com...

  4. The jets-accretion relation, mass-luminosity relation in Fermi blazars

    Science.gov (United States)

    Yu, Xiaoling; Zhang, Xiong; Zhang, Haojing; Xiong, Dingrong; Li, Bijun; Cha, Yongjuan; Chen, Yongyun; Huang, Xia; Wang, Yuwei

    2015-05-01

    A sample of 111 Fermi blazars each with a well-established radio core luminosity, broad-line luminosity, bolometric luminosity and black hole mass has been compiled from the literatures. We present a significant correlation between radio core and broad-line emission luminosities that supports a close link between accretion processes and relativistic jets. Analysis reveals a relationship of which is consistant with theoretical predicted coefficient and supports that blazar jets are powered by energy extraction from a rapidly spinning Kerr black hole through the magnetic field provided by the accretion disk. Through studying the correlation between the intrinsic bolometric luminosity and the black hole mass, we find a relationship of which supports mass-luminosity relation for Fermi blazars derived in this work is a powerlaw relation similar to that for main-sequence stars. Finally, EVOLUTIONARY SEQUENCE OF BLAZARS is discussed.

  5. Spectroscopy of Low Mass X-Ray Binaries: New Insights into Accretion. Revised

    Science.gov (United States)

    DilVrtilek, Saeqa; Mushotzky, Richard (Technical Monitor)

    2001-01-01

    This project is to observe two low mass X-ray binaries, chosen for their X-ray brightness, low column density, and diversity of accretion behavior. The high spectral resolution of the RGS, the broad energy range and tremendous collecting power of EPIC, and simultaneous optical monitoring with the OM are particularly well-suited to these studies. observation of one of the two objects has taken place and the data were received in late November. The second object is yet to be observed. Over the next year we will: investigate the physical conditions of the emitting gas using emission and recombination line diagnostics to determine temperatures, densities, elemental abundances, and ionization structure; study the behavior of emission features as a function of binary orbit; and test and improve models of X-ray line emission developed by us over the past decade. We will gain insight on both the geometry of the accretion flow and on the evolutionary history of LMXBs.

  6. Spectroscopy of Low Mass X-Ray Binaries: New Insights into Accretion

    Science.gov (United States)

    Vrtilek, Saeqa Dil; Mushotzky, Richard F. (Technical Monitor)

    2002-01-01

    This project is to observe two low mass X-ray binaries, chosen for their X-ray brightness, low column density, and diversity of accretion behavior. The high spectral resolution of the RGS, the broad energy range and tremendous collecting power of EPIC, and simultaneous optical monitoring with the OM are particularly well-suited to these studies. The second of two objects was not observed until September of 2002. Data analysis for the new observation is underway. over the next year we will: investigate the physical conditions of the emitting gas using emission and recombination line diagnostics to determine temperatures, densities, elemental abundances, and ionization structure; study the behavior of emission features as a function of binary orbit; and test and improve models of X-ray line emission developed by us over the past decade. We will gain insight on both the geometry of the accretion flow and on the evolutionary history of LMXBs.

  7. Crossing the Eddington Limit: Examining Disk Spectra at High Accretion Rates

    Science.gov (United States)

    Sutton, Andrew D.; Swartz, Douglas A.; Roberts, Timothy P.; Middleton, Matthew J.; Soria, Roberto; Done, Chris

    2017-02-01

    The faintest ultraluminous X-ray sources (ULXs), those with 0.3–10 keV luminosities 1thermal-dominant black hole binaries, which have Eddington ratios constrained to moderate values, also show BD spectra in the 0.3–10 keV band at an order of magnitude lower luminosities. This broadening would be missed in studies that only look above ∼ 2 {keV}. While this may suggest that BD ULXs could be powered by accretion onto massive stellar remnant black holes with close to maximal spin, we argue in favor of a scenario where they are at close to the Eddington luminosity, such that radiation pressure would be expected to result in geometrically slim, advective accretion disks. However, this implies that an additional physical mechanism is required to produce the observed broad spectra at low Eddington ratios.

  8. Stochastic Wobble of Accretion Discs and Jets from Turbulent Rocket Torques

    CERN Document Server

    Pettibone, Ryan

    2008-01-01

    Models of accretion discs and their associated outflows often incorporate assumptions of axisymmetry and symmetry across the disc plane. However, because discs are likely turbulent, these symmetries do not apply locally. The local asymmetry may induce local imbalances in outflow power across the disc mid-plane, which can in turn induce local tilting torques. Here we calculate the effect of the resulting stochastic torques on disc annuli. The torques induce a random walk of the vector perpendicular to the plane of each averaged annulus. This random walk is characterized by a radially dependent diffusion coefficient. We calculate the diffusion coefficient for small angle tilt and use it to obtain a radially dependent time scale for annular tilt and associated jet wobble. To be relevant, the tilt time scale must be less than the disc age, which favors binary accretion systems due to the prolonged mass supply. Accordingly, in crudely applying our results to blazars, young stellar objects and the binary engines of...

  9. Simulations of flux variability of oscillating accretion fluid tori around Kerr black holes

    CERN Document Server

    Bakala, Pavel; Šrámková, Eva; Kotrlová, Andrea; Török, Gabriel; Vincent, Frederic H; Abramowicz, Marek A

    2014-01-01

    High frequency quasi-periodic oscillations (HF QPOs) are observed in the X-ray power-density spectra (PDS) of several microquasars and low mass X-ray binaries. Many proposed QPO models are based on oscillations of accretion toroidal fluid structures orbiting in the vicinity of a compact object. We study oscillating accretion tori orbiting in the vicinity of a Kerr black hole. We demonstrate that significant variation of the observed flux can be caused by the combination of radial and vertical oscillation modes of a slender, polytropic, perfect fluid, non-self-graviting torus with constant specific angular momentum. We investigate two combinations of the oscillating modes corresponding to the direct resonance QPO model and the modified relativistic precession QPO model.

  10. Multi-dimensional structure of accreting young stars

    CERN Document Server

    Geroux, C; Viallet, M; Goffrey, T; Pratt, J; Constantino, T; Folini, D; Popov, M V; Walder, R

    2016-01-01

    This work is the first attempt to describe the multi-dimensional structure of accreting young stars based on fully compressible time implicit multi-dimensional hydrodynamics simulations. One major motivation is to analyse the validity of accretion treatment used in previous 1D stellar evolution studies. We analyse the effect of accretion on the structure of a realistic stellar model of the young Sun. Our work is inspired by the numerical work of Kley \\& Lin (1996, ApJ, 461, 933) devoted to the structure of the boundary layer in accretion disks. We analyse the redistribution of accreted material with a range of values of specific entropy relative to the bulk specific entropy of the material in the accreting object's convective envelope. A primary goal is to understand whether and how accreted energy deposited onto a stellar surface is redistributed in the interior. This study focusses on the high accretion rates characteristic of FU Ori systems. We find that the highest entropy cases produce a distinctive ...

  11. X-ray Photoevaporation-starved T Tauri Accretion

    CERN Document Server

    Drake, Jeremy J; Flaccomio, Ettore; Micela, Giusi

    2009-01-01

    X-ray luminosities of accreting T Tauri stars are observed to be systematically lower than those of non-accretors. There is as yet no widely accepted physical explanation for this effect, though it has been suggested that accretion somehow suppresses, disrupts or obscures coronal X-ray activity. Here, we suggest that the opposite might be the case: coronal X-rays modulate the accretion flow. We re-examine the X-ray luminosities of T Tauri stars in the Orion Nebula Cluster and find that not only are accreting stars systematically fainter, but that there is a correlation between mass accretion rate and stellar X-ray luminosity. We use the X-ray heated accretion disk models of Ercolano et al. to show that protoplanetary disk photoevaporative mass loss rates are strongly dependent on stellar X-ray luminosity and sufficiently high to be competitive with accretion rates. X-ray disk heating appears to offer a viable mechanism for modulating the gas accretion flow and could be at least partially responsible for the o...

  12. On the high frequency spectrum of a classical accretion disc

    CERN Document Server

    Balbus, Steven A

    2014-01-01

    We derive simple and explicit expressions for the high frequency spectrum of a classical accretion disc. Both stress-free and finite stress inner boundaries are considered. A classical accretion disc spectrum with a stress-free inner boundary departs from a Wien spectrum at large $\

  13. Dynamic processes during accretion into a black hole

    Directory of Open Access Journals (Sweden)

    G. S. Bisonvatyi-kogan

    2001-01-01

    Full Text Available Accretion disc theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disc. One of the most important new invention of this theory was a phenomenological treatment of the turbulent viscosity, known as “alpha” prescription, when the (rϕ component of the stress tensor was approximated by (αP with a unknown constant α This prescription played the role in the accretion disc theory as well important as the mixing-length theory of convection for stellar evolution. Sources of turbulence in the accretion disc are discussed, including nonlinear hydrodynamic turbulence, convection and magnetic filed role. In parallel to the optically thick geometrically thin accretion disc models, a new branch of the optically thin accretion disc models was discovered, with a larger thickness for the same total luminosity. The choice between these solutions should be done of the base of stability analysis. The ideas underlying the necessity to include advection into the accretion disc theory are presented and first models with advection are reviewed. The present status of the solution for a low-luminous optically thin accretion disc model with advection is discussed and the limits for an advection dominated accretion flows (ADAF imposed by the presence of magnetic field are analyzed.

  14. Correlation analysis of radio properties and accretion-disk luminosity for low luminosity AGNs

    CERN Document Server

    Su, Renzhi; Zhang, Zhen

    2016-01-01

    The correlation between the jet power and accretion disk luminosity is investigated and analyzed with our model for 7 samples of low luminosity active galactic nuclei (LLAGNs). The main results are: (1) the power-law correlation index ($P_{jet} \\propto L_{disk}^{\\mu}$) typically ranges $\\mu=0.4-0.7$ for the LLAGN samples, and there is a hint of steep index for the LLAGN sample which hosted by a high fraction of elliptical galaxies, and there are no significant correlation between the $\\mu$ and the LLAGN types (Seyfert, LINER); (2) for $\\mu \\approx$1, as noted in Liu et al., the accretion disk dominates the jet power and the black hole (BH) spin is not important, for the LLAGN samples studied in this paper we find that the $\\mu$ is significantly less than unity, implying that BH spin may play a significant role in the jet power of LLAGNs; (3) the BH spin-jet power is negatively correlated with the BH mass in our model, which means a high spin-jet efficiency in the `low' BH-mass LLAGNs; (4) an anti-correlation ...

  15. Rossby Wave Instability in the Accretion Flows around Black Holes

    Science.gov (United States)

    Gholipour, Mahmoud

    2017-01-01

    The roles of the Rossby wave instability (RWI) have been significantly developed in some important processes, such as planet formation and angular momentum transport through thin accretion disks. However, their development on accretion flows with advection is insignificant. In this paper, we investigate the effect of advection in the occurrence of RWI through accretion flows around black holes (BHs). In the absence of advection, the occurrence of RWI is extremely low because of high viscosity in the accretion flows around BHs. The results of this paper show that there is a significant chance for the occurrence of RWI in some wavelengths if we consider advection even in low amounts. Therefore, the RWI can be a suitable candidate for angular momentum transport in the accretion flows around BHs. Also, the results show that the advection parameter and the ratio of heat capacity, which are special characters of advection flows, play important roles in the occurrence of RWI.

  16. Accretion onto a noncommutative geometry inspired black hole

    Science.gov (United States)

    Kumar, Rahul; Ghosh, Sushant G.

    2017-09-01

    The spherically symmetric accretion onto a noncommutative (NC) inspired Schwarzschild black hole is treated for a polytropic fluid. The critical accretion rate \\dot{M}, sonic speed a_s and other flow parameters are generalized for the NC inspired static black hole and compared with the results obtained for the standard Schwarzschild black holes. Also explicit expressions for gas compression ratios and temperature profiles below the accretion radius and at the event horizon are derived. This analysis is a generalization of Michel's solution to the NC geometry. Owing to the NC corrected black hole, the accretion flow parameters also have been modified. It turns out that \\dot{M} ≈ {M^2} is still achievable but r_s seems to be substantially decreased due to the NC effects. They in turn do affect the accretion process.

  17. On the Eddington limit for relativistic accretion discs

    CERN Document Server

    Abolmasov, Pavel

    2015-01-01

    Standard accretion disc model relies upon several assumptions, the most important of which is geometrical thinness. Whenever this condition is violated, new physical effects become important such as radial energy advection and mass loss from the disc. These effects are important, for instance, for large mass accretion rates when the disc approaches its local Eddington limit. In this work, we study the upper limits for standard accretion disc approximation and find the corrections to the standard model that should be considered in any model aiming on reproducing the transition to super-Eddington accretion regime. First, we find that for thin accretion disc, taking into account relativistic corrections allows to increase the local Eddington limit by about a factor of two due to stronger gravity in General Relativity (GR). However, violation of the local Eddington limit also means large disc thickness. To consider consequently the disc thickness effects, one should make assumptions upon the two-dimensional rotat...

  18. Accreting planets as dust dams in `transition' discs

    CERN Document Server

    Owen, James E

    2014-01-01

    We investigate under what circumstances an embedded planet in a protoplanetary disc may sculpt the dust distribution such that it observationally presents as a `transition' disc. We concern ourselves with `transition' discs that have large holes ($\\gtrsim 10$ AU) and high accretion rates ($\\sim 10^{-9}-10^{-8}$ M$_\\odot$ yr$^{-1}$). Particularly, those discs which photoevaporative models struggle to explain. Assuming the standard picture for how massive planets sculpt their parent discs, along with the observed accretion rates in `transition' discs, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small ($s\\lesssim 1\\mu$m) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disc with an embedded, accreting planet, show that only ...

  19. Accretion onto the First Stellar Mass Black Holes

    CERN Document Server

    Alvarez, Marcelo A; Abel, Tom

    2008-01-01

    The first stars in the universe, forming at redshifts z>15 in minihalos with masses of order 10^6 Msun, may leave behind black holes as their remnants. These objects could conceivably serve as "seeds" for much larger black holes observed at redshifts z~6. We study the growth of the remnant black holes through accretion including for the first time the emitted accretion radiation with adaptive mesh refinement cosmological radiation-hydrodynamical simulations. The effects of photo-ionization and heating dramatically affect the accretion flow from large scales, resulting in negligible mass growth of the black hole. We compare cases with the accretion luminosity included and neglected to show that the accretion radiation drastically changes the environment within 100 pc of the black hole, where gas temperatures are increased by an order of magnitude. The gas densities are reduced and further star formation in the same minihalo prevented for the two hundred million years of evolution we followed. These calculation...

  20. X-ray spectra of hot accretion flows

    Science.gov (United States)

    Niedźwiecki, Andrzej; Xie, Fu-Guo; Stȩpnik, Agnieszka

    2014-09-01

    We study radiative properties of hot accretion flows in a general relativistic model with an exact treatment of global Comptonization, developed in our recent works. We note a strong dependence of electron temperature on the strength of magnetic field and we clarify that the underlying mechanism involves the change of the flow structure, with more strongly magnetized flows approaching the slab geometry more closely. We find that the model with thermal synchrotron radiation being the main source of seed photons agrees with the spectral index versus Eddington ratio relation observed in black hole transients below 1 per cent of the Eddington luminosity, LEdd, and models with a weak direct heating of electrons (small δ) are more consistent with observations. Models with large δ predict slightly too soft spectra, furthermore, they strongly overpredict electron temperatures at ˜0.01LEdd. The low-luminosity spectra, at ≲ 0.001LEdd, deviate from a power-law shape in the soft X-ray range, and we note that the first-scattering bump often resembles a thermal like component, with the temperature of a few hundred eV, superimposed on a power-law spectrum. The model with thermal Comptonization of thermal synchrotron radiation does not agree with well-studied AGNs observed below ˜0.01LEdd, for which there is a substantial evidence for the lack of an inner cold disc. This indicates that the model of hot flows powering AGNs should be revised, possibly by taking into account an additional (but internal to the flow) source of seed photons.