WorldWideScience

Sample records for accretion disc theory

  1. Accretion Discs Around Black Holes: Developement of Theory

    Bisnovatyi-Kogan, G. S.

    1999-01-01

    Standard accretion disk theory is formulated which is based on the local heat balance. The energy produced by a turbulent viscous heating is supposed to be emitted to the sides of the disc. Sources of turbulence in the accretion disc are connected with nonlinear hydrodynamic instability, convection, and magnetic field. In standard theory there are two branches of solution, optically thick, and optically thin. Advection in accretion disks is described by the differential equations what makes t...

  2. Some challenges and directions for next generation accretion disc theory

    Blackman, Eric G

    2015-01-01

    Accretion disc theory is far less developed than that of stellar evolution, although a similarly mature phenomenological picture is ultimately desired. While conceptual progress from the interplay of theory and numerical simulations has amplified awareness of the role of magnetic fields in angular momentum transport, there remains a significant gap between the output of magneto-rotational instability (MRI) simulations and the synthesis of lessons learned into improved practical models. If discs are turbulent, then axisymmetric models must be recognized to be sensible only as mean field theories. Such is the case for the wonderfully practical and widely used framework of Shakura-Sunyaev (SS73). This model is most justifiable when the radial angular momentum transport dominates in discs and the transport is assumed to take the form of a local viscosity. However, the importance of large scale fields in coronae and jets and numerical evidence from MRI simulations points to a significant fraction of transport bein...

  3. An accurate geometric distance to the compact binary SS Cygni vindicates accretion disc theory

    Miller-Jones, J C A; Knigge, C; Körding, E G; Templeton, M; Waagen, E O

    2013-01-01

    Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 +/- 12 pc measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source significantly closer at 114 +/- 2 pc. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.

  4. Comparisons and Connections between Mean Field Dynamo Theory and Accretion Disc Theory

    Blackman, Eric G

    2009-01-01

    The origin of large scale magnetic fields in astrophysical rotators, and the conversion of gravitational energy into radiation near stars and compact objects via accretion have been subjects of active research for a half century. Magnetohydrodynamic turbulence makes both problems highly nonlinear, so both subjects have benefitted from numerical simulations.However, understanding the key principles and practical modeling of observations warrants testable semi-analytic mean field theories that distill the essential physics. Mean field dynamo (MFD) theory and alpha-viscosity accretion disc theory exemplify this pursuit. That the latter is a mean field theory is not always made explicit but the combination of turbulence and global symmetry imply such. The more commonly explicit presentation of assumptions in 20th century textbook MFDT has exposed it to arguably more widespread criticism than incurred by 20th century alpha-accretion theory despite complementary weaknesses. In the 21st century however, MFDT has exp...

  5. Jet launching theory and the vertical structure of a magnetically-dominated thin accretion disc

    Rudiger, G.; Shalybkov, D. A.

    2001-01-01

    The presence of an imposed external magnetic field may drastically influence the structure of thin accretion discs. The magnetic field energy is here assumed to be in balance with the thermal energy of the accretion flow. The vertical magnetic field, its toroidal component B^tor at the disc surface (due to different rotation rates between disc and its magnetosphere), the turbulent magnetic Prandtl number and the viscosity-alpha are the key parameters of our model. Inside the corotation radius...

  6. How do accretion discs break?

    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.

  7. Broken discs: warp propagation in accretion discs

    Nixon, Christopher J.; King, Andrew R.

    2012-04-01

    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 non-linear fluid effects, which reduce the effective viscosities in warped regions, can promote breaking of the disc into two distinct planes. This occurs when the Shakura & Sunyaev dimensionless viscosity parameter α is ≲0.3 and the initial angle of misalignment between the disc and hole is ≳45°. The break can be a long-lived feature, propagating outwards in the disc on the usual alignment time-scale, after which the disc is fully co-aligned or counter-aligned with the hole. Such a break in the disc may be significant in systems where we know the inclination of the outer accretion disc to the line of sight, such as some X-ray binaries: the inner disc, and so any jets, may be noticeably misaligned with respect to the orbital plane.

  8. Black hole accretion disc impacts

    Pihajoki, P.

    2016-04-01

    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 λ = 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.

  9. Black hole accretion disc impacts

    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.

  10. Counter-Rotating Accretion Discs

    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...

  11. Black hole feedback from thick accretion discs

    Sadowski, Aleksander; Lasota, Jean-Pierre; 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 accretio...

  12. Accretion and plasma outflow from dissipationless discs

    Bogovalov, Sergei; Kelner, Stanislav

    2008-01-01

    We consider an extreme case of disc accretion onto a gravitating centre when the viscosity in the disc is negligible. The angular momentum and the rotational energy of the accreted matter is carried out by a magnetized wind outflowing from the disc. The outflow of matter from the disc occurs due to the Blandford & Payne(1982) centrifugal mechanism. The disc is assumed to be cold. Accretion and outflow are connected by the conservation of the energy, mass and the angular momentum. The basic pr...

  13. Black hole feedback from thick accretion discs

    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.

  14. Strongly magnetized accretion discs require poloidal flux

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

    2016-05-01

    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.

  15. Strongly magnetized accretion discs require poloidal flux

    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. Strongly magnetized accretion discs require poloidal flux

    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.

  17. Magnetohydrodynamic turbulence in warped accretion discs

    Torkelsson, U; Brandenburg, A; Pringle, J E; Nordlund, A A; Stein, R F; Nordlund, AA.

    2001-01-01

    Warped, precessing accretion discs appear in a range of astrophysical systems, for instance the X-ray binary Her X-1 and in the active nucleus of NGC4258. In a warped accretion disc there are horizontal pressure gradients that drive an epicyclic motion. We have studied the interaction of this epicyclic motion with the magnetohydrodynamic turbulence in numerical simulations. We find that the turbulent stress acting on the epicyclic motion is comparable in size to the stress that drives the accretion, however an important ingredient in the damping of the epicyclic motion is its parametric decay into inertial waves.

  18. On the diffusive propagation of warps in thin accretion discs

    LODATO G; Price, D.

    2010-01-01

    In this paper we revisit the issue of the propagation of warps in thin and viscous accretion discs. In this regime warps are know to propagate diffusively, with a diffusion coefficient approximately inversely proportional to the disc viscosity. Previous numerical investigations of this problem (Lodato & Pringle 2007) did not find a good agreement between the numerical results and the predictions of the analytic theories of warp propagation, both in the linear and in the non-linear case. Here,...

  19. Accretion Discs Show Their True Colours

    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

  20. Eclipse Mapping: Astrotomography of Accretion Discs

    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...

  1. Modeling quasar accretion disc temperature profiles

    Hall, Patrick B; Chajet, L S; Weiss, E; Nixon, C J

    2013-01-01

    Microlensing observations indicate that quasar accretion discs have half-light radii larger than expected from standard theoretical predictions based on quasar fluxes or black hole masses. Blackburne and colleagues have also found a very weak wavelength dependence of these half-light radii. We consider disc temperature profile models that might match these observations. Nixon and colleagues have suggested that misaligned accretion discs around spinning black holes will be disrupted at radii small enough for the Lense-Thirring torque to overcome the disc's viscous torque. Gas in precessing annuli torn off a disc will spread radially and intersect with the remaining disc, heating the disc at potentially large radii. However, if the intersection occurs at an angle of more than a degree or so, highly supersonic collisions will shock-heat the gas to a Compton temperature of T~10^7 K, and the spectral energy distributions (SEDs) of discs with such shock-heated regions are poor fits to observations of quasar SEDs. T...

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

    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

  3. Magnetic activity in accretion disc boundary layers

    Armitage, Philip J.

    2002-03-01

    We use three-dimensional magnetohydrodynamic simulations to study the structure of the boundary layer between an accretion disc and a non-rotating, unmagnetized star. Under the assumption that cooling is efficient, we obtain a narrow but highly variable transition region in which the radial velocity is only a small fraction of the sound speed. A large fraction of the energy dissipation occurs in high-density gas adjacent to the hydrostatic stellar envelope, and may therefore be reprocessed and largely hidden from view of the observer. As suggested by Pringle, the magnetic field energy in the boundary layer is strongly amplified by shear, and exceeds that in the disc by an order of magnitude. These fields may play a role in generating the magnetic activity, X-ray emission and outflows in disc systems where the accretion rate is high enough to overwhelm the stellar magnetosphere.

  4. Satellites in discs regulating the accretion luminosity

    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.

  5. Inhomogeneous accretion discs and the soft states of black hole X-ray binaries

    Dexter, Jason; Quataert, Eliot

    2012-01-01

    Observations of black hole binaries (BHBs) have established a rich phenomenology of X-ray states. The soft states range from the low variability, accretion disc dominated thermal state (TD) to the higher variability, non-thermal steep power law state (SPL). The disc component in all states is typically modeled with standard thin disc accretion theory. However, this theory is inconsistent with optical/UV spectral, variability, and gravitational microlensing observations of active galactic nucl...

  6. Magnetised accretion discs in Kerr spacetimes

    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...

  7. The dynamics of eccentric accretion discs in superhump systems

    Goodchild, S; Goodchild, Simon; Ogilvie, Gordon

    2006-01-01

    We have applied an eccentric accretion disc theory in simplified form to the case of an accretion disc in a binary system, where the disc contains the 3:1 Lindblad resonance. This is relevant to the case of superhumps in SU Ursae Majoris cataclysmic variables and other systems, where it is thought that this resonance leads to growth of eccentricity and a modulation in the light curve due to the interaction of a precessing eccentric disc with tidal stresses. A single differential equation is formulated which describes the propagation, resonant excitation and viscous damping of eccentricity. The theory is first worked out in the simple case of a narrow ring and leads to the conclusion that the eccentricity distribution is locally suppressed by the presence of the resonance, creating a dip in the eccentricity at the resonant radius. Application of this theory to the superhump case confirms this conclusion and produces a more accurate expression for the precession rate of the disc than has been previously accompl...

  8. Virial theorem for radiating accretion discs

    Mach, Patryk

    2011-01-01

    A continuum version of the virial theorem is derived for a radiating self-gravitating accretion disc around a compact object. The central object is point-like, but we can avoid the regularization of its gravitational potential. This is achieved by applying a modified Pohozaev-Rellich identity to the gravitational potential of the disk only. The theorem holds for general stationary configurations, including discontinuous flows (shock waves, contact discontinuities). It is used to test numerica...

  9. Magnetically driven accretion in protoplanetary discs

    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...

  10. Tidally distorted accretion discs in binary stars

    Ogilvie, G. I.

    2002-03-01

    The non-axisymmetric features observed in the discs of dwarf novae in outburst are usually considered to be spiral shocks, which are the non-linear relatives of tidally excited waves. This interpretation suffers from a number of problems. For example, the natural site of wave excitation lies outside the Roche lobe, the disc must be especially hot, and most treatments of wave propagation do not take into account the vertical structure of the disc. In this paper I construct a detailed semi-analytical model of the non-linear tidal distortion of a thin, three-dimensional accretion disc by a binary companion on a circular orbit. The analysis presented here allows for vertical motion and radiative energy transport, and introduces a simple model for the turbulent magnetic stress. The m=2 inner vertical resonance has an important influence on the amplitude and phase of the tidal distortion. I show that the observed patterns find a natural explanation if the emission is associated with the tidally thickened sectors of the outer disc, which may be irradiated from the centre. According to this hypothesis, it may be possible to constrain the physical parameters of the disc through future observations.

  11. What can we learn from Accretion Disc Eclipse Mapping experiments?

    Baptista, Raymundo

    2004-01-01

    The accretion disc eclipse mapping method is an astrotomographic 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 cataclysmic variables. This paper presents examples of eclipse mapping results that have been key to improve our understanding of accretion physics.

  12. Theory of wind accretion

    Shakura N.I.

    2014-01-01

    Full Text Available A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

  13. Theory of wind accretion

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.

    2014-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

  14. Type I migration in optically thick accretion discs

    Yamada, K; Inaba, S.

    2012-01-01

    We study the torque acting on a planet embedded in an optically thick accretion disc, using global two-dimensional hydrodynamic simulations. The temperature of an optically thick accretion disc is determined by the energy balance between the viscous heating and the radiative cooling. The radiative cooling rate depends on the opacity of the disc. The opacity is expressed as a function of the temperature. We find the disc is divided into three regions that have different temperature distributio...

  15. Super-Eddington accretion disc around a Kerr black hole

    Beloborodov, Andrei M.

    1998-01-01

    We calculate the structure of accretion disc around a rapidly rotating black hole with a super-Eddington accretion rate. The luminosity and height of the disc are reduced by the advection effect. In the case of a large viscosity parameter, alpha > 0.03, the accretion flow strongly deviates from thermodynamic equilibrium and overheats in the central region. With increasing accretion rate, the flow temperature steeply increases, reaches a maximum, and then falls off. The maximum is achieved in ...

  16. Bulk Comptonization by turbulence in accretion discs

    Kaufman, J.; Blaes, O. M.

    2016-06-01

    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, radiation viscous dissipation is suppressed, and the evolution of local photon spectra can be understood in terms of compression and expansion of the strongly coupled photon and gas fluids. We discuss the consequences of these effects for self-consistently resolving and interpreting turbulent Comptonization in spectral calculations in radiation magnetohydrodynamic simulations of high luminosity accretion flows.

  17. Observations of accretion discs in interacting binaries

    Honey, William Bruce

    Cataclysmic and X-ray binaries (CV and LMXB) are considered, and new observations of both types of source are considered. Chapter 1 gives an introduction to the subject and presents a study of the evolution and period relationships of these objects. Chapter 2 studies the superoutburst of a system. The observational data presented in the Chapter are used to place constraints on the geometry of the system, and also upon the theoretical models examined; only eccentric disc models are found to be acceptable. A tidally dominated eccentric accretion disc is considered, and good agreement between the observations and a tidally distorted disc simulation is achieved. In Chapter 3, a search for the superhump phenomenon is conducted. No such superhump behavior was found. These observations support the ideas first raised in Chapter 2 of the importance of tidal behavior in dwarf novae. Chapter 4 reviews observations of black hole candidates, and lists the generally expected 'fingerprint' thought to be associated with black holes in binary systems. Chapter 5 reports on observations of the LMXB GX339-4 and the discovery of the period for the system. Constraints on the system parameters are given and a model is presented that is compatible with the observations. Chapter 6 reviews the work done and considers other important observational evidence that is to be found in the literature and is pertinent to the work in this thesis.

  18. Outflows from dynamo-active protostellar accretion discs

    Von Rekowski, B; Dobler, W; Shukurov, A M; Brandenburg, Axel; Dobler, Wolfgang; Rekowski, Brigitta von; Shukurov, Anvar

    2003-01-01

    An axisymmetric model of a cool, dynamo-active accretion disc is applied to protostellar discs. Thermally and magnetically driven outflows develop that are not collimated within 0.1 AU. In the presence of a central magnetic field from the protostar, accretion onto the protostar is highly episodic, which is in agreement with earlier work.

  19. Inhomogeneous accretion discs and the soft states of black hole X-ray binaries

    Dexter, Jason; Quataert, Eliot

    2012-10-01

    Observations of black hole binaries (BHBs) have established a rich phenomenology of X-ray states. The soft states range from the low variability, accretion disc dominated thermal (TD) state to the higher variability, non-thermal steep power law (SPL) state. The disc component in all states is typically modelled with standard thin disc accretion theory. However, this theory is inconsistent with optical/UV spectral, variability and gravitational microlensing observations of active galactic nuclei (AGNs), the supermassive analogues of BHBs. An inhomogeneous disc (ID) model with large (≃0.4 dex) temperature fluctuations in each radial annulus can qualitatively explain all of these AGN observations. The inhomogeneity may be a consequence of instabilities in radiation-dominated discs, and therefore may be present in BHBs as well. We show that ID models can explain many features of the TD and SPL states of BHBs. The observed relationships between spectral hardness, disc fraction and rms variability amplitude in BHBs are reproduced with temperature fluctuations similar to those inferred in AGNs, suggesting a unified picture of luminous accretion discs across orders of magnitude in black hole mass. This picture can be tested with spectral fitting of ID models, X-ray polarization observations and radiation magnetohydrodynamic simulations. If BHB accretion discs are indeed inhomogeneous, only the most disc-dominated states (disc fraction ≳0.95) can be used to robustly infer black hole spin using current continuum fitting methods.

  20. Inhomogeneous accretion discs and the soft states of black hole X-ray binaries

    Dexter, Jason

    2012-01-01

    Observations of black hole binaries (BHBs) have established a rich phenomenology of X-ray states. The soft states range from the low variability, accretion disc dominated thermal state (TD) to the higher variability, non-thermal steep power law state (SPL). The disc component in all states is typically modeled with standard thin disc accretion theory. However, this theory is inconsistent with optical/UV spectral, variability, and gravitational microlensing observations of active galactic nuclei (AGNs), the supermassive analogs of BHBs. An inhomogeneous disc (ID) model with large (~0.4 dex) temperature fluctuations in each radial annulus can qualitatively explain all of these AGN observations. The inhomogeneity may be a consequence of instabilities in radiation dominated discs, and therefore may be present in BHBs as well. We show that ID models can explain many features of the TD and SPL states of BHBs. The observed relationships between spectral hardness, disc fraction, and rms variability amplitude in BHBs ...

  1. The evolution of misaligned accretion discs and spinning black holes

    Pringle, J E

    2006-01-01

    In this paper we consider the process of alignment of a spinning black hole and a surrounding misaligned accretion disc. We use a simplified set of equations, that describe the evolution of the system in the case where the propagation of warping disturbances in the accretion disc occurs diffusively, a situation likely to be common in the thin discs in Active Galactic Nuclei (AGN). We also allow the direction of the hole spin to move under the action of the disc torques. In such a way, the evolution of the hole-disc system is computed self-consistently. We consider a number of different situations and we explore the relevant parameter range, by varying the location of the warp radius $R_{\\rm w}$ and the propagation speed of the warp. We find that the dissipation associated with the twisting of the disc results in a large increase in the accretion rate through the disc, so that AGN accreting from a misaligned disc are likely to be significantly more luminous than those accreting from a flat disc. We compute exp...

  2. The evolution of misaligned accretion discs and spinning black holes

    LODATO G; Pringle, J. E.

    2006-01-01

    In this paper we consider the process of alignment of a spinning black hole and a surrounding misaligned accretion disc. We use a simplified set of equations, that describe the evolution of the system in the case where the propagation of warping disturbances in the accretion disc occurs diffusively, a situation likely to be common in the thin discs in Active Galactic Nuclei (AGN). We also allow the direction of the hole spin to move under the action of the disc torques. In such a way, the evo...

  3. On the diffusive propagation of warps in thin accretion discs

    Lodato, G

    2010-01-01

    In this paper we revisit the issue of the propagation of warps in thin and viscous accretion discs. In this regime warps are know to propagate diffusively, with a diffusion coefficient approximately inversely proportional to the disc viscosity. Previous numerical investigations of this problem (Lodato & Pringle 2007) did not find a good agreement between the numerical results and the predictions of the analytic theories of warp propagation, both in the linear and in the non-linear case. Here, we take advantage of a new, low-memory and highly efficient SPH code to run a large set of very high resolution simulations (up to 20 million SPH particles) of warp propagation, implementing an isotropic disc viscosity in different ways, to investigate the origin of the discrepancy between the theory and the numerical results. Our new and improved analysis now shows a remarkable agreement with the analytic theory both in the linear and in the non-linear regime, in terms of warp diffusion coefficient and precession ra...

  4. Convection in axially symmetric accretion discs with microscopic transport coefficients

    Malanchev, K L; Shakura, N I

    2016-01-01

    The vertical structure of stationary thin accretion discs is calculated from the energy balance equation with heat generation due to microscopic ion viscosity {\\eta} and electron heat conductivity {\\kappa}, both depending on temperature. In the optically thin discs it is found that for the heat conductivity increasing with temperature, the vertical temperature gradient exceeds the adiabatic value at some height, suggesting convective instability in the upper disc layer. There is a critical Prandtl number, Pr = 4/9, above which a Keplerian disc become fully convective. The vertical density distribution of optically thin laminar accretion discs as found from the hydrostatic equilibrium equation cannot be generally described by a polytrope but in the case of constant viscosity and heat conductivity. In the optically thick discs with radiation heat transfer, the vertical disc structure is found to be convectively stable for both absorption dominated and scattering dominated opacities, unless a very steep dependen...

  5. Theory of wind accretion

    Shakura N.I.; Postnov K.A.; Kochetkova A.Yu.; Hjalmarsdotter L.

    2013-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about $4\\...

  6. Observational constraints on viscosity in AGN accretion discs

    The optical/UV/soft X-ray big bump can be modelled as thermal emission from an accretion disc. The observed UV variability in AGN spectra may be caused by accretion-disc instabilities, and can be used to constrain the viscosity. The comparison of thermal time-scales with the observed time-scales of variability in 10 Seyfert galaxies and 16 QSOs indicates values for the parameter α of the order of 0.01 for most cases. (author)

  7. Black hole accretion discs and screened scalar hair

    Davis, Anne-Christine; Jha, Rahul

    2016-01-01

    We present a novel way to investigate scalar field profiles around black holes with an accretion disc for a range of models where the Compton wavelength of the scalar is large compared to other length scales. By analysing the problem in "Weyl" coordinates, we are able to calculate the scalar profiles for accretion discs in the static Schwarzschild, as well as rotating Kerr, black holes. We comment on observational effects.

  8. Corotation resonance and overstable oscillations in black-hole accretion discs: general-relativistic calculations

    Horak, Jiri

    2013-01-01

    We study the dynamics of spiral waves and oscillation modes in relativistic rotating discs around black holes. Generalizing the Newtonian theory, we show that wave absorption can take place at the corotation resonance, where the pattern frequency of the wave matches the background disc rotation rate. We derive the general relativistic expression for the disc vortensity (vorticity divided by surface density), which governs the behaviour of density perturbation near corotation. Depending on the gradient of the generalized disc vortensity, corotational wave absorption can lead to the amplification or damping of the spiral wave. We apply our general theory of relativistic wave dynamics to calculate the non-axisymmetric inertial-acoustic modes (also called p-modes) trapped in the inner-most region of a black hole accretion disc. Because general relativity changes the profiles of the radial epicyclic frequency and disc vortensity near the inner disc edge close to the black hole, these p-modes can become overstable ...

  9. Review: Accretion Disk Theory

    Montesinos, Matias

    2012-01-01

    In this paper I review and discuss the basic concepts of accretion disks, focused especially on the case of accretion disks around black holes. The well known alpha-model is revisited, showing the strengths and weaknesses of the model. Other turbulent viscosity prescription, based on the Reynolds number, that may improve our understanding of the accretion paradigm is discussed. A simple but efficient mathematical model of a self-gravitating accretion disk, as well as observational evidence of...

  10. On the properties of discs around accreting brown dwarfs

    Mayne, Nathan

    2010-01-01

    We present a grid of models of accreting brown dwarf systems with circumstellar discs. The calculations involve a self-consistent solution of both vertical hydrostatic and radiative equilibrium along with a sophisticated treatment of dust sublimation. We have simulated observations of the spectral energy distributions and several broadband photometric systems. Analysis of the disc structures and simulated observations reveal a natural dichotomy in accretion rates, with \\logmdot $>-$9 and $\\leq -$9 classed as extreme and typical accretors respectively. Derivation of ages and masses from our simulated photometry using isochrones is demonstrated to be unreliable even for typical accretors. Although current brown dwarf disc candidate selection criteria have been shown to be largely reliable when applied to our model grid we suggest improved selection criteria in several colour indices. We show that as accretion rates increase brown dwarf disc systems are less likely to be correctly identified. This suggests that,...

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

    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.

  12. Migration of accreting planets in radiative discs from dynamical torques

    Pierens, Arnaud

    2016-01-01

    We present the results of hydrodynamical simulations of the orbital evolution of planets undergoing runaway gas accretion in radiative discs. We consider accreting disc models with constant mass flux through the disc, and where radiative cooling balances the effect of viscous heating and stellar irradiation. We assume that 20-30 $M_\\oplus$ giant planet cores are formed in the region where viscous heating dominates and migrate outward under the action of a strong corotation torque. In the case where gas accretion is neglected, we find evidence for strong dynamical torques in accreting discs with accretion rates ${\\dot M}\\gtrsim 7\\times 10^{-8} \\;M_\\odot/yr$. Their main effect is to increase outward migration rates by a factor of $\\sim 2$ typically. In the presence of gas accretion, however, runaway outward migration is observed with the planet passing through the zero-torque radius and the transition between the viscous heating and stellar heating dominated regimes. The ability for an accreting planet to enter...

  13. Accretion Disc Evolution in Single and Binary T Tauri Stars

    Armitage, P J; Tout, C A; Armitage, Philip J.

    1998-01-01

    We present theoretical models for the evolution of T Tauri stars surrounded by circumstellar discs. The models include the effects of pre-main-sequence stellar and time dependent disc evolution, and incorporate the effects of stellar magnetic fields acting on the inner disc. For single stars, consistency with observations in Taurus-Auriga demands that disc dispersal occurs rapidly, on much less than the viscous timescale of the disc, at roughly the epoch when heating by stellar radiation first dominates over internal viscous dissipation. Applying the models to close binaries, we find that because the initial conditions for discs in binaries are uncertain, studies of extreme mass ratio systems are required to provide a stringent test of theoretical disc evolution models. We also note that no correlation of the infra-red colours of T Tauri stars with their rotation rate is observed, in apparent contradiction to the predictions of simple magnetospheric accretion models.

  14. Massive star formation by accretion I. Disc accretion

    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...

  15. Tilted black hole accretion disc models of Sagittarius A*: time-variable millimetre to near-infrared emission

    Dexter, Jason; Fragile, P. Chris

    2012-01-01

    High-resolution, multi-wavelength, and time-domain observations of the Galactic centre black hole candidate, Sgr A*, allow for a direct test of contemporary accretion theory. To date, all models have assumed alignment between the accretion disc and black hole angular momentum axes, but this is unjustified for geometrically thick accretion flows like that onto Sgr A*. Instead, we calculate images and spectra from a set of simulations of accretion flows misaligned ('tilted') by 15 degrees from ...

  16. Theory of wind accretion

    Shakura, N I; Kochetkova, A Yu; Hjalmarsdotter, L

    2013-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about $4\\times10^{36}$ erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the ...

  17. Ubiquitous equatorial accretion disc winds in black hole soft states

    Ponti, G.; Fender, R. P.; Begelman, M. C.; Dunn, R. J. H.; Neilsen, J.; Coriat, M.

    2012-01-01

    High resolution spectra of Galactic Black Holes (GBH) reveal the presence of highly ionised absorbers. In one GBH, accreting close to the Eddington limit for more than a decade, a powerful accretion disc wind is observed to be present in softer X-ray states and it has been suggested that it can carry away enough mass and energy to quench the radio jet. Here we report that these winds, which may have mass outflow rates of the order of the inner accretion rate or higher, are an ubiquitous compo...

  18. Polarization signatures of strong gravity by reflection from accretion discs

    Dovčiak, Michal; Goosmann, René; Karas, Vladimír; Matt, G.; Muleri, F.; Svoboda, Jiří

    Melville: American Institute of Physics, 2010 - (Zatloukal, M.), s. 419-420. (AIP Conference Proceedings. 1248). ISBN 978-0-7354-0795-4. ISSN 0094-243X. [X-ray astronomy 2009. Bologna (IT), 07.09.2009-11.09.2009] Institutional research plan: CEZ:AV0Z10030501 Keywords : black hole * accretion disc * polarization Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  19. Stability of slim accretion discs - effects of central mass and viscosity

    Slim accretion discs have a total luminosity of the order L/LE = m ∝ 1, where LE is the Eddington luminosity and m = M/Mc, where Mc is a critical accretion rate, related to the Teddington one. The local stability properties of such discs are examined, in the three-dimensional parameter space spanned by the (α, μ, m) axes, where α and μ are two viscosity parameters, and m = M/Msun the central mass. We suggest that various types of observed quasi-periodic behaviour may be connected with slim disc instabilities. If this turns out to be correct, the so-called normal and horizontal branch oscillations could be due to unstable thermal and acoustic modes, respectively. It is subsequently shown that some of the observed short-term (quasi-periodic) variability in active galactic nuclei may also originate from short-wavelength acoustic modes in the innermost region of the disc. Consequently, observational characteristics, in connection with stability theory, may yield estimates of basic accretion parameters. In the case of the Seyfert galaxy NGC 6814, this process seems to favour (α, μ, m, m) ≅ (0.5, 0, 106, 10-2). We finally conclude that this line of work may provide additional evidence for both accretion discs and black holes, in various compact sources. (orig.)

  20. Limits on luminosity and mass accretion rate of a radiation pressure dominated accretion disc

    Cao, Xinwu

    2015-01-01

    There is a maximum for the gravity of a black hole in the vertical direction in the accretion disc. Outflows may probably be driven from the disc if the radiation flux of the disc is greater than a critical value corresponding to the maximal vertical gravity. We find that outflows are driven by the radiation force from the disc if the accretion rate is greater than the Eddington rate. The radiation of the disc is therefore limited by such outflows. The disc luminosity, L=L_Edd\\propto ln mdot, at large-mdot cases. The Eddington ratio of the disc is ~3 for mdot~100, which is significantly lower than that of a conventional slim disc without outflows. This implies that the emission from some ultra-luminous X-ray sources with highly super Eddington luminosity should be Doppler beamed, or intermediate mass black holes are in these sources instead of stellar mass black holes. The spectra of the discs with outflows are saturated in the high frequency end provided mdot>2. We suggest that the saturated emission can be ...

  1. Thin accretion discs are stabilized by a strong magnetic field

    Sądowski, Aleksander

    2016-07-01

    By studying three-dimensional, radiative, global simulations of sub-Eddington, geometrically thin (H/R ≈ 0.15) black hole accretion flows we show that thin discs which are dominated by magnetic pressure are stable against thermal instability. Such discs are thicker than predicted by the standard model and show significant amount of dissipation inside the marginally stable orbit. Radiation released in this region, however, does not escape to infinity but is advected into the black hole. We find that the resulting accretion efficiency (5.5 ± 0.5 per cent for the simulated 0.8dot{M}_Edd disc) is very close to the predicted by the standard model (5.7 per cent).

  2. Standing Rankine-Hugoniot Shocks in Black Hole Accretion Discs

    GU Wei-Min; LU Ju-Fu

    2004-01-01

    @@ We study the problem of standing shocks in viscous disc-like accretion flows around black holes. For the first time we parametrize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K. By providing the global dependence of shock formation in the j - K parameter space, we show that a significant parameter region can ensure solutions with Rankine-Hugoniot shocks; and that the possibilities of shock formation are the largest for inviscid flows, decreasing with increasing viscosity, and ceasing to exist for a strong enough viscosity. Our results support the view that the standing shock is an essential ingredient in black hole accretion discs and is a general phenomenon in astrophysics, and that there should be a continuous change from the properties of inviscid flows to those of viscous ones.

  3. Evolution of linear warps in accretion discs and applications to protoplanetary discs in binaries

    Foucart, Francois

    2014-01-01

    The existence of warped accretion discs is expected in a wide variety of astrophysical systems, including circumstellar discs in binaries and discs around binary protostars. A common feature of these discs is that they are perturbed by a misaligned external potential. In this paper, we study the long-term evolution of the disc warp and precession in the case of thick discs (with the dimensionless thickness $H/r$ larger than the viscosity parameter $\\alpha$) in which bending waves can propagate. For small warps, such discs undergo approximately rigid-body precession with a coherent global frequency. We derive the analytical expressions for the warp/twist profiles of the disc and the alignment timescale for a variety of disc models/parameters. Applying our results to circumbinary discs, we find that these discs align with the orbital plane of the binary on a timescale comparable to the global precession time of the disc, and typically much smaller than its viscous timescale. The development of parametric instab...

  4. Face-on accretion onto a protoplanetary disc

    Wijnen, T P G; Pelupessy, F I; Zwart, S Portegies

    2016-01-01

    Globular clusters (GCs) are known to harbor multiple stellar populations. To explain these observations Bastian et al. suggested a scenario in which a second population is formed by the accretion of enriched material onto the low-mass stars in the initial GC population. The idea is that the low-mass, pre-main sequence stars sweep up gas expelled by the massive stars of the same generation into their protoplanetary disc as they move through the GC core. We perform simulations with 2 different smoothed particle hydrodynamics codes to investigate if a low-mass star surrounded by a protoplanetary disc can accrete the amount of enriched material required in this scenario. We focus on the gas loading rate onto the disc and star as well as on the lifetime of the disc. We find that the gas loading rate is a factor of 2 smaller than the geometric rate, because the effective cross section of the disc is smaller than its surface area. The loading rate is consistent for both codes, irrespective of resolution. The disc ga...

  5. Stability of helium accretion discs in ultracompact binaries

    Lasota, Jean-Pierre; Kruk, Katarzyna

    2008-01-01

    Stellar companions of accreting neutron stars in Ultra Compact X-ray binaries (UCXBs) are hydrogen-deficient. Their helium or C/O accretion discs are strongly X-ray irradiated. Both the chemical composition and irradiation determine the disc stability with respect to thermal and viscous perturbations. At shorter periods UCXBs are persistent whereas longer-period systems are mostly transient. To understand this behaviour one has to derive the stability criteria for X-ray irradiated hydrogen-poor accretion discs. We modify and update the code of Hameury et al. (1998), Dubus et al. (1999; 2001). We obtain the relevant stability criteria and compare the results to observed properties of UCXBs. Although the general trend of the stability behaviour of UCXBs is consistent with the prediction of the disc instability model, in few cases the the inconsistency of theoretical predictions with the system observed properties is weak enough to be attributed to observational and/or theoretical uncertainties but for two syste...

  6. Wind accretion: Theory and observations

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.; Sidoli, L.; Paizis, A.

    2015-07-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass

  7. Accretion Discs with an Inner Spiral Density Wave

    Montgomery, M M

    2010-01-01

    In Montgomery (2009a), we show that accretion discs in binary systems could retrogradely precess by tidal torques like the Moon and the Sun on a tilted, spinning, non-spherical Earth. In addition, we show that the state of matter and the geometrical shape of the celestial object could significantly affect the precessional value. For example, a Cataclysmic Variable (CV) Dwarf Novae (DN) non-magnetic system that shows negative superhumps in its light curve can be described by a retrogradely precessing, differentially rotating, tilted disc. Because the disc is a fluid and because the gas stream overflows the tilted disc and particles can migrate into inner disc annuli, coupled to the disc could be a retrogradely precessing inner ring that is located near the innermost annuli of the disc. However, numerical simulations by Bisikalo et al. (2003, 2004) and this work show that an inner spiral density wave can be generated instead of an inner ring. Therefore, we show that retrograde precession in non-magnetic, spinni...

  8. Numerical simulations of thin accretion discs with PLUTO

    Parthasarathy, Varadarajan; Kluzniak, Wlodek

    2014-01-01

    Our goal is to perform global simulations of thin accretion discs around compact bodies like neutron stars with dipolar magnetic profile and black holes by exploiting the facilities provided by state-of-the-art grid-based, high resolution shock capturing (HRSC) and finite volume codes. We have used the Godunov-type code PLUTO to simulate a thin disc around a compact object prescribed with a pseudo-Newtonian potential in a purely hydrodynamical (HD) regime, with numerical viscosity as a first ...

  9. Numerical simulations of thin accretion discs with PLUTO

    Parthasarathy, Varadarajan

    2014-01-01

    Our goal is to perform global simulations of thin accretion discs around compact bodies like neutron stars with dipolar magnetic profile and black holes by exploiting the facilities provided by state-of-the-art grid-based, high resolution shock capturing (HRSC) and finite volume codes. We have used the Godunov-type code PLUTO to simulate a thin disc around a compact object prescribed with a pseudo-Newtonian potential in a purely hydrodynamical (HD) regime, with numerical viscosity as a first step towards achieving our goal as mentioned above.

  10. Beyond the standard model of the disc-line spectral profiles from black hole accretion discs

    Sochora, Vjačeslav; Karas, Vladimír; Svoboda, Jiří; Dovčiak, Michal

    2014-01-01

    Roč. 54, č. 4 (2014), s. 301-304. ISSN 1210-2709 R&D Projects: GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : accretion discs * black hole physics * galactic nuclei Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  11. Retrograde binaries of massive black holes in circumbinary accretion discs

    Amaro-Seoane, Pau; Maureira-Fredes, Cristián; Dotti, Massimo; Colpi, Monica

    2016-06-01

    Context. We explore the hardening of a massive black hole binary embedded in a circumbinary gas disc under a specific circumstance: when the binary and the gas are coplanar and the gas is counter-rotating. The binary has unequal mass and the interaction of the gas with the lighter secondary black hole is the main cause of the braking torque on the binary that shrinks with time. The secondary black hole, revolving in the direction opposite to the gas, experiences a drag from gas-dynamical friction and from direct accretion of part of it. Aims: In this paper, using two-dimensional (2D) hydrodynamical grid simulations we investigate the effect of changing the accretion prescriptions on the dynamics of the secondary black hole, which in turn affect the binary hardening and eccentricity evolution. Methods: We find that realistic accretion prescriptions lead to results that differ from those inferred assuming accretion of all the gas within the Roche Lobe of the secondary black hole. Results: When considering gas accretion within the gravitational influence radius of the secondary black hole (which is smaller than the Roche Lobe radius) to better describe gas inflows, the shrinking of the binary is slower. In addition, in this case, a smaller amount of accreted mass is required to reduce the binary separation by the same amount. Different accretion prescriptions result in different discs' surface densities, which alter the black hole's dynamics back. Full 3D Smoothed-particle hydrodynamics realizations of a number of representative cases, run over a shorter interval of time, validate the general trends observed in the less computationally demanding 2D simulations. Conclusions: Initially circular black hole binaries increase their eccentricity only slightly, which then oscillates around small values (<0.1) while they harden. By contrast, initially eccentric binaries become more and more eccentric. A semi-analytical model describing the black hole's dynamics under

  12. Ubiquitous equatorial accretion disc winds in black hole soft states

    Ponti, G; Begelman, M C; Dunn, R J H; Neilsen, J; Coriat, M

    2012-01-01

    High resolution spectra of Galactic Black Holes (GBH) reveal the presence of highly ionised absorbers. In one GBH, accreting close to the Eddington limit for more than a decade, a powerful accretion disc wind is observed to be present in softer X-ray states and it has been suggested that it can carry away enough mass and energy to quench the radio jet. Here we report that these winds, which may have mass outflow rates of the order of the inner accretion rate or higher, are an ubiquitous component of the jet-free soft states of all GBH. We furthermore demonstrate that these winds have an equatorial geometry with opening angles of few tens of degrees, and so are only observed in sources in which the disc is inclined at a large angle to the line of sight. The decrease in Fe XXV / Fe XXVI line ratio with Compton temperature, observed in the soft state, suggests a link between higher wind ionisation and harder spectral shapes. Although the physical interaction between the wind, accretion flow and jet is still not ...

  13. Mass flow rates in and outflow rates from AGN accretion discs

    Blank, Marvin; Duschl, Wolfgang J.

    2012-01-01

    We derive analytical expressions for the mass flow rates in and from accretion discs, taking into account the Eddington limit. This allows us to connect the basic properties of outflows with those of the accretion flow. As an example, we derive the radial surface density distribution in the accretion disc of Mrk 231.

  14. Slim accretion discs a model for ADAF-SLE transitions

    Igumenshchev, I V; Novikov, I D

    1997-01-01

    We numerically construct slim, global, vertically integrated models of optically thin, transonic accretion discs around black holes, assuming a regularity condition at the sonic radius and boundary conditions at the outer radius of the disc and near the black hole. In agreement with several previous studies, we find two branches of shock-free solutions, in which the cooling is dominated either by advection, or by local radiation. We also confirm that the part of the accretion flow where advection dominates is in some circumstances limited in size: it does not extend beyond a certain outer limiting radius. New results found in our paper concern the location of the limiting radius and properties of the flow near to it. In particular, we find that beyond the limiting radius, the advective dominated solutions match on to Shapiro, Lightman & Eardley (SLE) discs through a smooth transition region. Therefore, the full global solutions are shock-free and unlimited in size. There is no need for postulating an extr...

  15. On the Stability of Elliptical Vortices in Accretion Discs

    Lesur, G

    2009-01-01

    (Abriged) The existence of large-scale and long-lived 2D vortices in accretion discs has been debated for more than a decade. They appear spontaneously in several 2D disc simulations and they are known to accelerate planetesimal formation through a dust trapping process. However, the issue of the stability of these structures to the imposition of 3D disturbances is still not fully understood, and it casts doubts on their long term survival. Aim: We present new results on the 3D stability of elliptical vortices embedded in accretion discs, based on a linear analysis and several non-linear simulations. Methods: We derive the linearised equations governing the 3D perturbations in the core of an elliptical vortex, and we show that they can be reduced to a Floquet problem. We solve this problem numerically in the astrophysical regime and we present several analytical limits for which the mechanism responsible for the instability can be explained. Finally, we compare the results of the linear analysis to some high ...

  16. On the thermal stability of transonic accretion discs

    Szuszkiewicz, E; Szuszkiewicz, Ewa; Miller, John C.

    1997-01-01

    Nonlinear time-dependent calculations have been carried out in order to study the evolution of the thermal instability for optically thick, transonic, slim accretion discs around black holes. In the present calculations we have investigated only the original version of the slim disc model with low viscosity. This version does not yet contain several important non-local effects but our aim is to use it as a standard reference against which to compare the results from forthcoming studies in which additional effects will be added one by one thus giving a systematic way of understanding the contribution from each of them. A range of results for different cases is presented showing a number of interesting features. One preliminary conclusion is that the stabilizing effect of advection seems not to be strong enough in these low viscosity models to allow for limit cycle behaviour to occur.

  17. Accretion discs around black holes two dimensional, advection cooled flows

    Igumenshchev, I V; Abramowicz, M A; Igumenshchev, Igor V; Chen, Xingming; Abramowicz, Marek Artur

    1995-01-01

    Two-dimensional accretion flows near black holes have been investigated by time-dependent hydrodynamical calculations. We assume that the flow is axisymmetric and that radiative losses of internal energy are negligible, so that the disc is geometrically thick and hot. Accretion occurs due to the overflow of the effective potential barrier near the black hole, similar to the case of the Roche lobe overflowing star in a binary system. We make no pre-assumptions on the properties of the flow, instead our models evolve self-consistently from an initially non-accreting state. The viscosity is due to the the small-scale turbulence and it is described by the \\alpha-viscosity prescription. We confirm earlier suggestions that viscous accretion flows are convectively unstable. We found that the instability produces transient eddies of various length-scales. The eddies contribute to the strength of the viscosity in the flow by redistributing the angular momentum. They also introduce low amplitude oscillatory variations ...

  18. Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

    Moss, David; Suleimanov, Valery

    2016-01-01

    Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. We consider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models ...

  19. Constraints on active galactic nucleus accretion disc viscosity derived from continuum variability

    R.L.C. Starling; A. Siemiginowska; P. Uttley; R. Soria

    2004-01-01

    We estimate a value of the viscosity parameter in active galactic nucleus (AGN) accretion discs for the Palomar-Green quasar sample. We assume that optical variability on time-scales of months to years is caused by local instabilities in the inner accretion disc. Comparing the observed variability t

  20. The growth of planets by pebble accretion in evolving protoplanetary discs

    Bitsch, Bertram; Johansen, Anders

    2015-01-01

    The formation of planets depends on the underlying protoplanetary disc structure, which influences both the accretion and migration rates of embedded planets. The disc itself evolves on time-scales of several Myr during which both temperature and density profiles change as matter accretes onto the central star. Here we use a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model the emergence of planetary embryos pr...

  1. Hydrodynamical stability of thin accretion discs: transient growth of global axisymmetric perturbations

    Umurhan, O. M.; Nemirovsky, A.; Regev, O.; Shaviv, G.

    2005-01-01

    The purpose of this paper is to explore how accretion discs manifest the phenomenon of transient growth on a global scale. We investigate analytically the time response of a thin accretion disc to particular axisymmetric perturbations. To facilitate an analytical treatment we replace the energy equation with a general polytropic assumption. The asymptotic expansion of Klu\\'zniak & Kita (2000), which extended the method of Regev (1983) to a full steady polytropic disc (with $n=3/2$), is furthe...

  2. Electromagnetic vs. Lense-Thirring alignment of black hole accretion discs

    Polko, Peter

    2015-01-01

    Accretion discs and black holes (BHs) have angular momenta that are generally misaligned with respect to each other, which can lead to warps in the discs and bends in any jets produced. We consider a disc that is misaligned at large radii and torqued by Lense-Thirring (LT) precession and a Blandford-Znajek (BZ) jet torque. We consider a variety of disc states that include radiatively inefficient thick discs, radiatively efficient thin discs, and super-Eddington accretion discs. The magnetic field strength of the BZ jet is chosen as either from standard equipartition arguments or from magnetically arrested disc (MAD) simulations. We show that standard thin accretion discs can reach spin-disc alignment out to large radii long before LT would play a role, as caused by the slow infall time that gives even a weak BZ jet time to align the disc. We show that geometrically thick radiatively inefficient discs and super-Eddington discs in the MAD state reach disc-spin alignment near the black hole when density profiles...

  3. Wind accretion: Theory and Observations

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.; Sidoli, L.; Paizis, A.

    2014-01-01

    A review of wind accretion in HMXB is presented. We focus on different regimes of quasi-spherical accretion onto a NS: supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when the plasma remains hot until it meets the magnetospheric boundary. The two regimes are separated by a limit in X-ray luminosity at about 4 10^{36} erg/s. In subsonic accretion, wh...

  4. Magnetised Accretion Discs in Kerr Spacetimes II: Hot Spots

    García, Federico; Johannsen, Tim

    2015-01-01

    Context. Quasi-periodic variability has been observed in a number of X-ray binaries harboring black hole candidates. In general relativity, black holes are uniquely described by the Kerr metric and, according to the cosmic censorship conjecture, curvature singularities always have to be clothed by an event horizon. Aims. In this paper, we study the effect of an external magnetic field on the observed light curves of orbiting hot spots in thin accretion discs around Kerr black holes and naked singularities. Methods. We employ a ray-tracing algorithm to calculate the light curves and power spectra of such hot spots as seen by a distant observer for uniform and dipolar magnetic field configurations assuming a weak coupling between the magnetic field and the disc matter. Results. We show that the presence of an external dipolar magnetic field leads to potentially observable modifications of these signals for both Kerr black holes and naked singularities, while an external uniform magnetic field has practically no...

  5. Mass loss from advective accretion disc around rotating black holes

    Aktar, Ramiz; Nandi, Anuj

    2015-01-01

    We examine the properties of the outflowing matter from an advective accretion disc around a spinning black hole. During accretion, rotating matter experiences centrifugal pressure supported shock transition that effectively produces a virtual barrier around the black hole in the form of post-shock corona (hereafter, PSC). Due to shock compression, PSC becomes hot and dense that eventually deflects a part of the inflowing matter as bipolar outflows because of the presence of extra thermal gradient force. In our approach, we study the outflow properties in terms of the inflow parameters, namely specific energy (${\\mathcal E}$) and specific angular momentum ($\\lambda$) considering the realistic outflow geometry around the rotating black holes. We find that spin of the black hole ($a_k$) plays an important role in deciding the outflow rate $R_{\\dot m}$ (ratio of mass flux of outflow and inflow), in particular, $R_{\\dot m}$ is directly correlated with $a_k$ for the same set of inflow parameters. It is found that ...

  6. Wind interactions above accretion discs: a model for broad-line regions and collimated outflow

    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. (author)

  7. Energy flows in thick accretion discs and their consequences for black hole feedback

    Sądowski, Aleksander; Lasota, Jean-Pierre; Abramowicz, Marek A.; Narayan, Ramesh

    2016-03-01

    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 per cent - 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. Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems

    Moss, D.; Sokoloff, D.; Suleimanov, V.

    2016-04-01

    Context. Magnetic fields are important for accretion disc structure. Magnetic fields in a disc system may be transported with the accreted matter. They can be associated with either the central body and/or jet, and be fossil or dynamo excited in situ. Aims: We consider dynamo excitation of magnetic fields in accretion discs of accreting binary systems in an attempt to clarify possible configurations of dynamo generated magnetic fields. We first model the entire disc with realistic radial extent and thickness using an alpha-quenching non-linearity. We then study the simultaneous effect of feedback from the Lorentz force from the dynamo-generated field. Methods: We perform numerical simulations in the framework of a relatively simple mean-field model which allows the generation of global magnetic configurations. Results: We explore a range of possibilities for the dynamo number, and find quadrupolar-type solutions with irregular temporal oscillations that might be compared to observed rapid luminosity fluctuations. The dipolar symmetry models with Rα< 0 have lobes of strong toroidal field adjacent to the rotation axis that could be relevant to jet launching phenomena. Conclusions: We have explored and extended the solutions known for thin accretion discs.

  9. Suppression of the accretion rate in thin discs around binary black holes

    Ragusa, Enrico; Lodato, Giuseppe; Price, Daniel J.

    2016-01-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with $H/R\\gtrsim 0.1$, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs, in this case the mass accretion ra...

  10. The influence of neutrinos on the nucleosynthesis of accretion disc outflows

    We present a short review of recent progress on the influence of neutrinos on the nucleosynthesis from black hole accretion disc outflows. Discs that form from the end of life of rapidly rotating massive stars as well as discs that form from mergers will produce large numbers of neutrinos. These neutrinos influence the nucleosynthesis that occurs in hot accretion disc outflows. A variety of nucleosynthesis products can be produced in these outflows. We review work on nickel, p-process and r-process elements. (paper)

  11. Accretion disc atmospheres and winds in low-mass X-ray binaries

    Díaz Trigo, M.; Boirin, L.

    2016-05-01

    In the last decade, X-ray spectroscopy has enabled a wealth of discoveries of photoionised absorbers in X-ray binaries. Studies of such accretion disc atmospheres and winds are of fundamental importance to understand accretion processes and possible feedback mechanisms to the environment. In this work, we review the current observational state and theoretical understanding of accretion disc atmospheres and winds in low-mass X-ray binaries, focusing on the wind launching mechanisms and on the dependence on accretion state. We conclude with issues that deserve particular attention.

  12. Accretion disc atmospheres and winds in low-mass X-ray binaries

    Trigo, M Díaz

    2015-01-01

    In the last decade, X-ray spectroscopy has enabled a wealth of discoveries of photoionised absorbers in X-ray binaries. Studies of such accretion disc atmospheres and winds are of fundamental importance to understand accretion processes and possible feedback mechanisms to the environment. In this work, we review the current observational state and theoretical understanding of accretion disc atmospheres and winds in low-mass X-ray binaries, focusing on the wind launching mechanisms and on the dependence on accretion state. We conclude with issues that deserve particular attention.

  13. Wind accretion: Theory and Observations

    Shakura, N I; Kochetkova, A Yu; Hjalmarsdotter, L; Sidoli, L; Paizis, A

    2014-01-01

    A review of wind accretion in HMXB is presented. We focus on different regimes of quasi-spherical accretion onto a NS: supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when the plasma remains hot until it meets the magnetospheric boundary. The two regimes are separated by a limit in X-ray luminosity at about 4 10^{36} erg/s. In subsonic accretion, which works a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto the NS is determined by the ability of the plasma to enter the magnetosphere due to the Rayleigh-Taylor instability. Two regimes of subsonic accretion are possible, depending on the plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity regime with Compton cooling to the low-luminosity (L_x < 3\\times 10^35 erg/s) regime with radiative cooling can be respon...

  14. The Eye of the Storm: Light from the Inner Plunging Region of Black Hole Accretion Discs

    Zhu, Yucong; Narayan, Ramesh; Kulkarni, Akshay K; Penna, Robert F; McClintock, Jeffrey E

    2012-01-01

    It is generally thought that the light coming from the inner plunging region of black hole accretion discs contributes negligibly to the disc's overall spectrum, i.e. the plunging fluid is swallowed by the black hole before it has time to radiate. In the standard disc model used to fit X-ray observations of accretion discs, the plunging region is assumed to be perfectly dark. However, numerical simulations that include the full physics of the magnetized flow predict that a small fraction of the disc's total luminosity emanates from this plunging region. In this work, we investigate the observational consequences of this neglected inner light. We compute radiative transfer based disc spectra that correspond to 3D general relativistic magnetohydrodynamic simulated discs (which produce light inside their plunging regions). In the context of black hole spin estimation, we find that this neglected inner light only has a modest effect (this bias is less than typical observational systematic errors). For rapidly spi...

  15. Magnetised accretion discs in Kerr spacetimes. II. Hot spots

    García, Federico; Ranea-Sandoval, Ignacio F.; Johannsen, Tim

    2016-03-01

    Context. Quasi-periodic variability has been observed in a number of X-ray binaries that harbor black hole candidates. In general relativity, black holes are uniquely described by the Kerr metric and, according to the cosmic censorship conjecture, curvature singularities always have to be clothed by an event horizon. Aims: In this paper, we study the observed light curves that arise from orbiting hotspots in thin accretion discs around Kerr black holes and naked singularities, and the effect introduced by the presence of an external magnetic field. Methods: We employ a ray-tracing algorithm to calculate the light curves and power spectra of these hot spots as seen by a distant observer for uniform and dipolar magnetic field configurations, assuming a weak coupling between the magnetic field and the disc matter. Results: We show that the presence of an external dipolar magnetic field leads to potentially observable modifications of these light curves for both Kerr black holes and naked singularities, while an external uniform magnetic field has practically no effect. In particular, we demonstrate that the emission from a hotspot, which is orbiting near the innermost stable circular orbit of a naked singularity in a dipolar magnetic field, can be significantly harder than the emission of the same hotspot in the absence of this type of magnetic field. Conclusions: The comparison of our model with observational data may allow us to study the geometry of magnetic fields around compact objects and to test the cosmic censorship conjecture in conjunction with other observables, such as thermal continuum spectra and iron line profiles.

  16. The growth of planets by pebble accretion in evolving protoplanetary discs

    Bitsch, Bertram; Lambrechts, Michiel; Johansen, Anders

    2015-10-01

    The formation of planets depends on the underlying protoplanetary disc structure, which in turn influences both the accretion and migration rates of embedded planets. The disc itself evolves on time scales of several Myr, during which both temperature and density profiles change as matter accretes onto the central star. Here we used a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model, the emergence of planetary embryos preferably should occur after approximately 2 Myr in order to not exclusively form gas giants, but also ice giants and smaller planets. The high pebble accretion rates ensure that critical core masses for gas accretion can be reached at all orbital distances. Gas giant planets nevertheless experience significant reduction in semi-major axes by migration. Considering instead planetesimal accretion for planetary growth, we show that formation time scales are too long to compete with the migration time scales and the dissipation time of the protoplanetary disc. All in all, we find that pebble accretion overcomes many of the challenges in the formation of ice and gas giants in evolving protoplanetary discs. Appendices are available in electronic form at http://www.aanda.org

  17. Foundations of Black Hole Accretion Disk Theory

    Abramowicz, Marek A.; P. Chris Fragile

    2011-01-01

    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 ...

  18. Accretion Disc Evolution in Single and Binary T Tauri Stars

    Armitage, Philip J.; C.J. Clarke; Tout, C.A.

    1998-01-01

    We present theoretical models for the evolution of T Tauri stars surrounded by circumstellar discs. The models include the effects of pre-main-sequence stellar and time dependent disc evolution, and incorporate the effects of stellar magnetic fields acting on the inner disc. For single stars, consistency with observations in Taurus-Auriga demands that disc dispersal occurs rapidly, on much less than the viscous timescale of the disc, at roughly the epoch when heating by stellar radiation firs...

  19. Foundations of Black Hole Accretion Disk Theory

    Marek A. Abramowicz

    2013-01-01

    Full Text Available 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.

  20. A laboratory plasma experiment for studying magnetic dynamics of accretion discs and jets

    Hsu, S. C.; Bellan, P. M.

    2002-01-01

    This work describes a laboratory plasma experiment and initial results which should give insight into the magnetic dynamics of accretion discs and jets. A high-speed multiple-frame CCD camera reveals images of the formation and helical instability of a collimated plasma, similar to MHD models of disc jets, and also plasma detachment associated with spheromak formation, which may have relevance to disc winds and flares. The plasmas are produced by a planar magnetized coaxial gun. The resulting...

  1. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    Chattopadhyay, Indranil; Kumar, Rajiv

    2016-01-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion - ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von - Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We foun...

  2. Braking down an accreting protostar: disc-locking, disc winds, stellar winds, X-winds and Magnetospheric Ejecta

    Ferreira, Jonathan

    2013-01-01

    Classical T Tauri stars are low mass young forming stars that are surrounded by a circumstellar accretion disc from which they gain mass. Despite this accretion and their own contraction that should both lead to their spin up, these stars seem to conserve instead an almost constant rotational period as long as the disc is maintained. Several scenarios have been proposed in the literature in order to explain this puzzling "disc-locking" situation: either deposition in the disc of the stellar angular momentum by the stellar magnetosphere or its ejection through winds, providing thereby an explanation of jets from Young Stellar Objects. In this lecture, these various mechanisms will be critically detailed, from the physics of the star-disc interaction to the launching of self-confined jets (disc winds, stellar winds, X-winds, conical winds). It will be shown that no simple model can account alone for the whole bulk of observational data and that "disc locking" requires a combination of some of them.

  3. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    Kennedy, Gareth F.; Meiron, Yohai; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-07-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterized by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical capability, we find that the stellar accretion rate converges for models with N ≥ 32k. The eccentricity distribution of accreted stars, however, does not converge. We find that there are two competing effects at work when improving the resolution: larger particle number leads to a smaller fraction of stars accreted on nearly circular orbits, while higher spatial resolution increases this fraction. We scale our simulations to some nearby galaxies and find that the expected boost in stellar accretion (or tidal disruption, which could be observed as X-ray flares) in the presence of a gas disc is about a factor of 10. Even with this boost, the accretion of mass from stars is still a factor of ∼100 slower than the accretion of gas from the disc. Thus, it seems accretion of stars is not a major contributor to black hole mass growth.

  4. Production of 56Ni in black hole-neutron star merger accretion disc outflows

    The likely outcome of a compact object merger event is a central black hole surrounded by a rapidly accreting torus of debris. This disc of debris is a rich source of element synthesis, the outcome of which is needed to predict electromagnetic counterparts of individual events and to understand the contribution of mergers to galactic chemical evolution. Here we study disc outflow nucleosynthesis in the context of a two-dimensional, time-dependent black hole-neutron star merger accretion disc model. We use two time snapshots from this model to examine the impact of the evolution of the neutrino fluxes from the disc on the element synthesis. While the neutrino fluxes from the early-time disc snapshot appear to favor neutron-rich outflows, by the late-time snapshot the situation is reversed. As a result we find copious production of 56Ni in the outflows. (paper)

  5. Accretion and magnetic field morphology around Class 0 stage protostellar discs

    Seifried, D; Pudritz, R E; Klessen, R S

    2014-01-01

    We analyse the results of existing and newly performed and simulations of turbulent, collapsing molecular cloud cores focussing on the formation of Class 0 stage protostellar discs and the physical conditions in their surroundings. We show that for a wide range of initial conditions rotationally supported discs are formed in the Class 0 stage already. In particular, we show that even mild, i.e. subsonic turbulent motions, reduce the magnetic braking efficiency sufficiently in order to allow Keplerian discs to form. Based on this result we suggest that already during the Class 0 stage the fraction of Keplerian discs is significantly higher than 50%. This is consistent with recent observational trends but significantly higher than predictions based on simulations with misaligned magnetic fields and disc angular momentum vectors, demonstrating the importance of turbulent motions for the formation of Keplerian discs. We show that the accretion of mass and angular momentum in the surroundings of protostellar discs...

  6. Iron line profiles and self-shadowing from relativistic thick accretion discs

    Wu, Sheng-Miao

    2007-01-01

    We present Fe Kalpha line profiles from and images of relativistic discs with finite thickness around a rotating black hole using a novel code. The line is thought to be produced by iron fluorescence of a relatively cold X-ray illuminated material in the innermost parts of the accretion disc and provides an excellent diagnostic of accretion flows in the vicinity of black holes. Previous studies have concentrated on the case of a thin, Keplerian accretion disc. This disc must become thicker and sub-Keplerian with increasing accretion rates. These can affect the line profiles and in turn can influence the estimation of the accretion disc and black hole parameters from the observed line profiles. We here embark on, for the first time, a fully relativistic computation which offers key insights into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. We include all relativistic effects such as frame-dragging, Doppler boost, time dilation, gravitational redshift and lig...

  7. Suppression of the accretion rate in thin discs around binary black holes

    Ragusa, Enrico; Price, Daniel J

    2016-01-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with $H/R\\gtrsim 0.1$, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs, in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on $H/R$) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of systems accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed...

  8. Suppression of the accretion rate in thin discs around binary black holes.

    Ragusa, Enrico; Lodato, Giuseppe; Price, Daniel J.

    2016-05-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H/R ≳ 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs, in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H/R) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of systems accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

  9. Suppression of the accretion rate in thin discs around binary black holes

    Ragusa, Enrico; Lodato, Giuseppe; Price, Daniel J.

    2016-08-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H/R ≳ 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs; in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H/R) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of system accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal-mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

  10. On the Bardeen-Petterson Effect in black hole accretion discs

    Nealon, Rebecca; Nixon, Chris

    2015-01-01

    We investigate the effect of black hole spin on warped or misaligned accretion discs - in particular i) whether or not the inner disc edge aligns with the black hole spin and ii) whether the disc can maintain a smooth transition between an aligned inner disc and a misaligned outer disc, known as the Bardeen-Petterson effect. We employ high resolution 3D smoothed particle hydrodynamics simulations of $\\alpha$-discs subject to Lense-Thirring precession, focussing on the bending wave regime where the disc viscosity is smaller than the aspect ratio $\\alpha \\lesssim H/R$. We first address the controversy in the literature regarding possible steady-state oscillations of the tilt close to the black hole. We successfully recover such oscillations in 3D at both small and moderate inclinations ($\\lesssim 15^{\\circ}$), provided both Lense-Thirring and Einstein precession are present, sufficient resolution is employed, and provided the disc is not so thick so as to simply accrete misaligned. Second, we find that discs in...

  11. Energetic Argument for Bimodal Black Hole Accretion discs

    林一清; 卢炬甫; 顾为民

    2002-01-01

    Based on simple energetic considerations, we show that two crucial ingredients of bimodal black hole accretiondiscs, namely the sonic point and the transition radius, can be determined from the disc constant parameters.Thus, we can further justify the model of bimodal discs containing thermal instability triggered transition.

  12. Problem of non-rotating black hole accretion disc main enegy releas region extension

    We consider the size of region of the main energy release of black hol acretion discs. This problem is deeply connected with accretion process at the innermost region of accretion disc and with boundary condition at its inner edge. Ou main result is demonstration that the region of the main energy release is strongly localized. It is shown that for accretion onto non-rotating black hol the main portion of the liberated energy is released within a rather narrow radial range 5N≤13M. This property can have profound effect on spectrum and variability of the radiation of the acretion disc as a whole. In particular, the asymmetry of the main energy release region is favourable for the mechanism of the millisecond variability proposed earlier by the authors. (author). 15 refs.; 5 figs

  13. Radiatively and thermally driven self-consistent bipolar outflows from accretion discs around compact objects

    Kumar, Rajiv; Mandal, Samir

    2013-01-01

    We investigate the role of radiative driving of shock ejected bipolar outflows from advective accretion discs in a self consistent manner. Radiations from the inner disc affects the subsonic part of the jet while those from the pre-shock disc affects the supersonic part, and there by constitutes a multi stage acceleration process. We show that the radiation from the inner disc not only accelerate but also increase the mass outflow rate, while the radiation from the pre-shock disc only increases the kinetic energy of the flow. With proper proportions of these two radiations, very high terminal speed is possible. We also estimated the post-shock luminosity from the pre-shock radiations, and showed that with the increase of viscosity parameter the disc becomes more luminous, and the resulting jet simultaneously becomes faster. This mimics the production of steady mildly relativistic but stronger jets as micro-quasars moves from low hard to intermediate hard spectral states.

  14. Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

    Sakurai, Y.; Vorobyov, E. I.; Hosokawa, T.; Yoshida, N.; Omukai, K.; Yorke, H. W.

    2016-06-01

    The formation of supermassive stars (SMSs) is a potential pathway to seed supermassive black holes in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper, we study the evolution of an accreting SMS and its UV emissivity with realistic variable accretion from a circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the protostellar accretion until the stellar mass exceeds 104 M⊙. The disc fragments by gravitational instability, creating many clumps that migrate inward to fall on to the star. The resulting accretion history is highly time-dependent: short episodic accretion bursts are followed by longer quiescent phases. We show that the disc for the direct collapse model is more unstable and generates greater variability than normal Pop III cases. Next, we conduct a stellar evolution calculation using the obtained accretion history. Our results show that, regardless of the variable accretion, the stellar radius monotonically increases with almost constant effective temperature at Teff ≃ 5000 K as the stellar mass increases. The resulting UV feedback is too weak to hinder accretion due to the low flux of stellar UV photons. The insensitivity of stellar evolution to variable accretion is attributed to the fact that time-scales of variability, ≲103 yr, are too short to affect the stellar structure. We argue that this evolution will continue until the SMS collapses to produce a black hole by the general relativistic instability after the mass reaches ≳105 M⊙.

  15. Revealing the accretion disc corona in Mrk 335 with multi-epoch X-ray spectroscopy

    Keek, L.; Ballantyne, D. R.

    2016-03-01

    Active galactic nuclei host an accretion disc with an X-ray producing corona around a supermassive black hole. In bright sources, such as the Seyfert 1 galaxy Mrk 335, reflection of the coronal emission off the accretion disc has been observed. Reflection produces spectral features such as an Fe Kα emission line, which allow for properties of the inner accretion disc and the corona to be constrained. We perform a multi-epoch spectral analysis of all XMM-Newton, Suzaku, and NuSTAR observations of Mrk 335, and we optimize our fitting procedure to unveil correlations between the Eddington ratio and the spectral parameters. We find that the disc's ionization parameter correlates strongly with the Eddington ratio: the inner disc is more strongly ionized at higher flux. The slope of the correlation is less steep than previously predicted. Furthermore, the cut-off of the power-law spectrum increases in energy with the Eddington ratio, whereas the reflection fraction exhibits a decrease. We interpret this behaviour as geometrical changes of the corona as a function of the accretion rate. Below ˜10 per cent of the Eddington limit, the compact and optically thick corona is located close to the inner disc, whereas at higher accretion rates the corona is likely optically thin and extends vertically further away from the disc surface. Furthermore, we find a soft excess that consists of two components. In addition to a contribution from reflection in low ionization states, a second component is present that traces the overall flux.

  16. Evolution of Accretion Discs around a Kerr Black Hole using Extended Magnetohydrodynamics

    Foucart, Francois; Gammie, Charles F; Quataert, Eliot

    2015-01-01

    Black holes accreting well below the Eddington rate are believed to have geometrically thick, optically thin, rotationally supported accretion discs in which the Coulomb mean free path is large compared to $GM/c^2$. In such an environment, the disc evolution may differ significantly from ideal magnetohydrodynamic predictions. We present non-ideal global axisymmetric simulations of geometrically thick discs around a rotating black hole. The simulations are carried out using a new code ${\\rm\\it grim}$, which evolves a covariant extended magnetohydrodynamics model derived by treating non-ideal effects as a perturbation of ideal magnetohydrodynamics. Non-ideal effects are modeled through heat conduction along magnetic field lines, and a difference between the pressure parallel and perpendicular to the field lines. The model relies on an effective collisionality in the disc from wave-particle scattering and velocity-space (mirror and firehose) instabilities. We find that the pressure anisotropy grows to match the ...

  17. Magnetic Coupling of a Rotating Black Hole with the SurroundingAccretion Disc

    汪定雄; 肖看; 雷卫华

    2001-01-01

    The evolution characteristics and energy extraction of a rotating black hole are investigated by considering the magnetic coupling with the surrounding accretion disc. It is found that both the mass and spin of the black hole might be reduced by the joint effects of disc accretion and magnetic coupling, provided that the latter is stronger than the former. The efficiencies of the two energy mechanisms are calculated and compared to a variety of parameters. In addition, the validity of the laws of black hole thermodynamics is discussed.

  18. The truncated and evolving inner accretion disc of the black hole GX 339-4

    Plant, D S; Ponti, G; Munoz-Darias, T; Coriat, M

    2013-01-01

    The nature of accretion onto stellar mass black holes in the low/hard state remains unresolved, with some evidence suggesting that the inner accretion disc is truncated and replaced by a hot flow. However the detection of relativistic broadened iron emission lines, even at relatively low luminosities, seems to require an accretion disc extending fully to its innermost stable circular orbit. Modelling such features is however highly susceptible to degeneracies, which could easily bias any interpretation. We present the first systematic study of the iron line region to track how the inner accretion disc evolves in the low/hard state of the black hole GX 339-4. Our four observations display increased broadening of the iron line over two magnitudes in luminosity, which we use to track any variation of the disc inner radius. We find that the disc extends closer to the black hole at higher luminosities, but is consistent with being truncated throughout the entire low/hard state, a result which renders black hole sp...

  19. Accretion discs as regulators of stellar angular momentum evolution in the ONC and Taurus-Auriga

    Davies, Claire L; Greaves, Jane S

    2014-01-01

    In light of recent substantial updates to spectral type estimations and newly established intrinsic colours, effective temperatures, and bolometric corrections for pre-main sequence (PMS) stars, we re-address the theory of accretion-disc regulated stellar angular momentum (AM) evolution. We report on the compilation of a consistent sample of fully convective stars within two of the most well-studied and youngest, nearby regions of star formation: the Orion Nebula Cluster (ONC) and Taurus-Auriga. We calculate the average specific stellar AM ($j_{\\star}$) assuming solid body rotation, using surface rotation periods gathered from the literature and new estimates of stellar radii and ages. We use published Spitzer IRAC fluxes to classify our stars as Class II or Class III and compare their $j_{\\star}$ evolution. Our results suggest that disc dispersal is a rapid process that occurs at a variety of ages. We find a consistent $j_{\\star}$ reduction rate between the Class II and Class III PMS stars which we interpret...

  20. Mass and spin coevolution during the alignment of a black hole in a warped accretion disc

    Perego, A; Colpi, M; Volonteri, M

    2009-01-01

    In this paper, we explore the gravitomagnetic interaction of a black hole (BH) with a misaligned accretion disc to study BH spin precession and alignment jointly with BH mass and spin parameter evolution, under the assumption that the disc is continually fed, in its outer region, by matter with angular momentum fixed on a given direction. We develop an iterative scheme based on the adiabatic approximation to study the BH-disc coevolution: in this approach, the accretion disc transits through a sequence of quasi-steady warped states (Bardeen-Petterson effect) and interacts with the BH until the BH spin aligns with the outer angular momentum direction. For a BH aligning with a co-rotating disc, the fractional increase in mass is typically less than a few percent, while the spin modulus can increase up to a few tens of percent. The alignment timescale is between ~ 100 thousands and ~ 1 millions years for a maximally rotating BH accreting at the Eddington rate. BH-disc alignment from an initially counter-rotating...

  1. Infrared accretion disc mapping of the dwarf nova V2051 Ophiuchi in outburst and in quiescence

    Wojcikiewicz, E.; Baptista, R.

    2014-10-01

    Dwarf novae are compact binaries where a late-type star (the secondary) fills its Roche lobe and transfers matter to a companion white dwarf (the primary) via an accretion disc. They show outbursts which recur on timescales of weeks to years, where the accretion disc brightens by factors 20 to 100 either due to a thermal-viscous instability in the disc (DI model) or to a burst of enhanced mass-transfer from the secondary (MTI model). We report time-series of fast photometry of the dwarf nova V2051 Oph in the J and H bands, obtained with the CAMIV at the 1.6 m telescope of Observatório Pico dos Dias/Brazil, during the decline of an outburst in 2005 June, and in 2008 when the object was in quiescence. We modeled the ellipsoidal variations caused by the secondary to infer its contribution to the J and H fluxes, and fitted stellar atmosphere models to find a photometric parallatic distance of d = (111± 14)pc. Front-back brightness asymmetries in J and H-band eclipse maps along the decline from the 2005 outburst suggest that the accretion disc had a non-negligible opening angle which decreased as the disc cooled down. The time evolution of the disc radial temperature distribution along the outburst decline shows a cooling wave which accelerates as is travels inwards - in contradiction to a basic prediction from the DI model.

  2. Supermassive star formation via episodic accretion: protostellar disc instability and radiative feedback efficiency

    Sakurai, Yuya; Hosokawa, Takashi; Yoshida, Naoki; Omukai, Kazuyuki; Yorke, Harold W

    2015-01-01

    The formation of SMSs is a potential pathway to seed SMBHs in the early universe. A critical issue for forming SMSs is stellar UV feedback, which may limit the stellar mass growth via accretion. In this paper we study the evolution of an accreting SMS and its UV emissivity under conditions of realistic variable accretion from a self-gravitating circumstellar disc. First we conduct a 2D hydrodynamical simulation to follow the long-term protostellar accretion until the stellar mass exceeds $10^4~M_\\odot$. The disc fragments due to gravitational instability, creating a number of small clumps that rapidly migrate inward to fall onto the star. The resulting accretion history is thus highly time-dependent: short episodic accretion bursts are followed by longer, relative quiescent phases. We show that the circumstellar disc for the so-called direct collapse model is more unstable and generates greater variability over shorter timescales than normal Pop III cases. We conduct a post-process stellar evolution calculati...

  3. Axisymmetric Waves in Isothermal Accretion Discs with Vertical Self-Gravity

    LIU Xiao-Ci; YANG Lan-Tian; WU Shao-Ping; DING Shi-Xue

    2001-01-01

    We extend the research of axisymmetric waves in accretion discs with three-dimensional structure to the case that vertical self-gravity of the discs is included. We derive and analyze the dispersion relation and solve the eigenfunctions numerically. The following results have been reached: vertical self-gravity expands the forbidden region of the wave propagation. As the influence of the vertical self-gravity increases, the group velocities of the waves get smaller and the vertical nodes of the wave shrink to the middle plane of the disc.

  4. A new way to measure supermassive black hole spin in accretion disc-dominated active galaxies

    Done, C.; Jin, C; Middleton, M; Ward, M.

    2013-01-01

    We show that disc continuum fitting can be used to constrain black hole spin in a subclass of narrow-line Seyfert 1 (NLS1) active galactic nuclei as their low mass and high mass accretion rate means that the disc peaks at energies just below the soft X-ray bandpass. We apply the technique to the NLS1 PG1244+026, where the optical/UV/X-ray spectrum is consistent with being dominated by a standard disc component. This gives a best estimate for black hole spin which is low, with a firm upper lim...

  5. A new way to measure supermassive black hole spin in accretion disc dominated Active Galaxies

    Done, C.; Jin, C; Middleton, M; Ward, M.

    2013-01-01

    We show that disc continuum fitting can be used to constrain black hole spin in a subclass of narrow-line Seyfert 1 (NLS1) active galactic nuclei as their low mass and high mass accretion rate means that the disc peaks at energies just below the soft X-ray bandpass. We apply the technique to the NLS1 PG1244+026, where the optical/UV/X-ray spectrum is consistent with being dominated by a standard disc component. This gives a best estimate for black hole spin which is low, with a firm upper lim...

  6. A Laboratory Plasma Experiment for Studying Magnetic Dynamics of Accretion Discs and Jets

    Hsu, S C

    2002-01-01

    This work describes a laboratory plasma experiment and initial results which should give insight into the magnetic dynamics of accretion discs and jets. A high-speed multiple-frame CCD camera reveals images of the formation and helical instability of a collimated plasma, similar to MHD models of disc jets, and also plasma detachment associated with spheromak formation, which may have relevance to disc winds and flares. The plasmas are produced by a planar magnetized coaxial gun. The resulting magnetic topology is dependent on the details of magnetic helicity injection, namely the force-free state eigenvalue alpha_gun imposed by the coaxial gun.

  7. Motivation and challenge to capture both large scale and local transport in next generation accretion theory

    Blackman, Eric G.; Nauman, Farrukh

    2015-01-01

    Accretion disc theory is less developed than stellar evolution theory although a similarly mature phenomenological picture is ultimately desired. While the interplay of theory and numerical simulations has amplified community awareness of the role of magnetic fields in angular momentum transport, there remains a long term challenge to incorporate insight gained from simulations back into improving practical models for comparison with observations. Here we emphasize the need to incorporate the...

  8. Relativistic ionized accretion disc models of MCG-6-30-15

    Ballantyne, D. R.; Fabian, A. C.

    2001-12-01

    We present results from fitting of ionized accretion disc models to three long ASCA observations of the Seyfert 1 galaxy MCG-6-30-15. All three data sets can be fitted by a model consisting of ionized reflection from the inner region of the accretion disc (with twice solar Fe abundance) and a separate disc-line component from farther out on the disc. The disc-line is required to fit the height of the observed Fe Kα line profile. However, we show that a much simpler model of reflection from a very weakly ionized constant-density disc also fits the data. In this case only a single cold Fe Kα line at 6.4keV is required to fit the observed line. The ionized disc models predict that OVIII Kα, CVI Kα, FeXVII Lα and FeXVIII Lα lines will appear in the soft X-ray region of the reflection spectrum, but are greatly blurred as a result of Compton scattering. The equivalent width (EW) of OVIII Kα is estimated to be about 10eV and seems to be as strong as the blend of the Fe L lines. This result creates difficulty for the claim of a strong relativistic OVIII line in the XMM-Newton grating spectrum of MCG-6-30-15, although we cannot strictly rule it out since MCG-6-30-15 was in an anomalously low state during that observation. We find that increasing the O abundance or breaking the continuum below 2keV will not significantly strengthen the line. The second Fe Kα line component in the ionized disc model may arise from neutral reflection from a flared disc, or from a second illumination event. The data cannot distinguish between the two cases, and we conclude that single-zone ionized disc models have difficulty fitting these hard X-ray data of MCG-6-30-15.

  9. Efficiency of gas cooling and accretion at the disc-corona interface

    Armillotta, L.; Fraternali, F.; Marinacci, F.

    2016-08-01

    In star-forming galaxies, stellar feedback can have a dual effect on the circumgalactic medium both suppressing and stimulating gas accretion. The trigger of gas accretion can be caused by disc material ejected into the halo in the form of fountain clouds and by its interaction with the surrounding hot corona. Indeed, at the disc-corona interface, the mixing between the cold/metal-rich disc gas (T ≲ 104 K) and the hot coronal gas (T ≳ 106 K) can dramatically reduce the cooling time of a portion of the corona and produce its condensation and accretion. We studied the interaction between fountain clouds and corona in different galactic environments through parsec-scale hydrodynamical simulations, including the presence of thermal conduction, a key mechanism that influences gas condensation. Our simulations showed that the coronal gas condensation strongly depends on the galactic environment, in particular it is less efficient for increasing virial temperature/mass of the haloes where galaxies reside and it is fully ineffective for objects with virial masses larger than 1013M⊙. This result implies that the coronal gas cools down quickly in haloes with low-intermediate virial mass (Mvir ≲ 3 × 1012M⊙) but the ability to cool the corona decreases going from late-type to early-type disc galaxies, potentially leading to the switching off of accretion and the quenching of star formation in massive systems.

  10. The signature of the magnetorotational instability in the Reynolds and Maxwell stress tensors in accretion discs

    Pessah, Martin Elias; Chan, Chi-kwan; Psaltis, Dimitrios

    2006-01-01

    The magnetorotational instability is thought to be responsible for the generation of magnetohydrodynamic turbulence that leads to enhanced outward angular momentum transport in accretion discs. Here, we present the first formal analytical proof showing that, during the exponential growth of the...

  11. Revealing the accretion disc corona in Mrk 335 with multi-epoch X-ray spectroscopy

    Keek, L

    2015-01-01

    Active galactic nuclei host an accretion disc with an X-ray producing corona around a supermassive black hole. In bright sources, such as the Seyfert 1 galaxy Mrk 335, reflection of the coronal emission off the accretion disc has been observed. Reflection produces spectral features such as an Fe K$\\alpha$ emission line, which allow for properties of the inner accretion disc and the corona to be constrained. We perform a multi-epoch spectral analysis of all XMM-Newton, Suzaku, and NuSTAR observations of Mrk 335, and we optimize our fitting procedure to unveil correlations between the Eddington ratio and the spectral parameters. We find that the disc's ionization parameter correlates strongly with the Eddington ratio: the inner disc is more strongly ionized at higher flux. The slope of the correlation is less steep than previously predicted. Furthermore, the cut-off of the power-law spectrum increases in energy with the Eddington ratio, whereas the reflection fraction exhibits a decrease. We interpret this beha...

  12. Hydrodynamical stability of thin accretion discs: transient growth of global axisymmetric perturbations

    Umurhan, O M; Regev, O; Shaviv, G

    2005-01-01

    The purpose of this paper is to explore how accretion discs manifest the phenomenon of transient growth on a global scale. We investigate analytically the time response of a thin accretion disc to particular axisymmetric perturbations. To facilitate an analytical treatment we replace the energy equation with a general polytropic assumption. The asymptotic expansion of Klu\\'zniak & Kita (2000), which extended the method of Regev (1983) to a full steady polytropic disc (with $n=3/2$), is further developed and implemented for both the steady (for any polytropic index) and time-dependent problems. The spatial form and temporal behaviour of selected dynamical disturbances are studied in detail. We identify the perturbation space which leads to transient growth and provide analytical solutions which manifest this expected transient growth behaviour. Three terms (physical causes) responsible for the appearance of transient growth are identified. Two depend explicitly on the viscosity while the third one is relev...

  13. X-ray reflection in oxygen-rich accretion discs of ultracompact X-ray binaries

    Madej, O. K.; Garcia, Jeronimo; Jonker, P. G.;

    2014-01-01

    donor star in these sources is a carbon-oxygen or oxygen-neon-magnesium white dwarf. Hence, the accretion disc is enriched with oxygen which makes the O viii Ly alpha line particularly strong. Modelling the X-ray reflection off a carbon- and oxygen-enriched, hydrogen- and helium-poor disc with models...... assuming solar composition likely biases several of the best-fitting parameters. In order to describe the X-ray reflection spectra self-consistently, we modify the currently available xillver reflection model. We present initial grids that can be used to model X-ray reflection spectra in UCXBs with carbon-oxygen......-rich (and hydrogen- and helium-poor) accretion disc. We find that the new reflection model provides a better overall description of the reflection spectra of 4U 0614+091 and 4U 1543-624 than the reflection models that assume solar abundances....

  14. 3D Finite Volume Simulation of Accretion Discs with Spiral Shocks

    Makita, M; Makita, Makoto; Matsuda, Takuya

    1998-01-01

    We perform 2D and 3D numerical simulations of an accretion disc in a close binary system using the Simplified Flux vector Splitting (SFS) finite volume method. In our calculations, gas is assumed to be the ideal one, and we calculate the cases with gamma=1.01, 1.05, 1.1 and 1.2. The mass ratio of the mass losing star to the mass accreting star is unity. Our results show that spiral shocks are formed on the accretion disc in all cases. In 2D calculations we find that the smaller gamma is, the more tightly the spiral winds. We observe this trend in 3D calculations as well in somewhat weaker sense.

  15. Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

    Sutton, Andrew D.; Done, Chris; Roberts, Timothy P.

    2014-11-01

    Although attempts have been made to constrain the stellar types of optical counterparts to ultraluminous X-ray sources (ULXs), the detection of optical variability instead suggests that they may be dominated by reprocessed emission from X-rays which irradiate the outer accretion disc. Here, we report results from a combined X-ray and optical spectral study of a sample of ULXs, which were selected for having broadened disc-like X-ray spectra and known optical counterparts. We simultaneously fit optical and X-ray data from ULXs with a new spectral model of emission from an irradiated, colour-temperature-corrected accretion disc around a black hole, with a central Comptonizing corona. We find that the ULXs require reprocessing fractions of ˜10-3, which is similar to sub-Eddington thermal dominant state black hole binaries (BHBs), but less than has been reported for ULXs with soft ultraluminous X-ray spectra. We suggest that the reprocessing fraction may be due to the opposing effects of self-shielding in a geometrically thick supercritical accretion disc and reflection from far above the central black hole by optically thin material ejected in a natal super-Eddington wind. Then, the higher reprocessing fractions reported for ULXs with wind-dominated X-ray spectra may be due to enhanced scattering on to the outer disc via the stronger wind in these objects. Alternatively, the accretion discs in these ULXs may not be particularly geometrically thick, rather they may be similar in this regard to the thermal dominant state BHBs.

  16. Three-dimensional simulations of supercritical black hole accretion discs - luminosities, photon trapping and variability

    Sądowski, Aleksander; Narayan, Ramesh

    2016-03-01

    We present a set of four three-dimensional, general relativistic, radiation magnetohydrodynamical simulations of black hole accretion at supercritical mass accretion rates, dot{M} > dot{M}_Edd. We use these simulations to study how disc properties are modified when we vary the black hole mass, the black hole spin, or the mass accretion rate. In the case of a non-rotating black hole, we find that the total efficiency is of the order of 3 per cent dot{M} c^2, approximately a factor of 2 less than the efficiency of a standard thin accretion disc. The radiation flux in the funnel along the axis is highly super-Eddington, but only a small fraction of the energy released by accretion escapes in this region. The bulk of the 3 per cent dot{M} c^2 of energy emerges farther out in the disc, either in the form of photospheric emission or as a wind. In the case of a black hole with a spin parameter of 0.7, we find a larger efficiency of about 8 per cent dot{M} c^2. By comparing the relative importance of advective and diffusive radiation transport, we show that photon trapping is effective near the equatorial plane. However, near the disc surface, vertical transport of radiation by diffusion dominates. We compare the properties of our fiducial three-dimensional run with those of an equivalent two-dimensional axisymmetric model with a mean-field dynamo. The latter simulation runs nearly 100 times faster than the three-dimensional simulation, and gives very similar results for time-averaged properties of the accretion flow, but does not reproduce the time-variability.

  17. Spiral-driven accretion in protoplanetary discs - II Self-similar solutions

    Hennebelle, Patrick; Fromang, Sébastien

    2016-01-01

    Accretion discs are ubiquitous in the universe and it is a crucial issue to understand how angular momentum and mass are being radially transported in these objects. Here, we study the role played by non-linear spiral patterns within hydrodynamical and non self-gravitating accretion disc assuming that external disturbances such as infall onto the disc may trigger them. To do so, we computed self-similar solutions that describe discs in which a spiral wave propagates. Such solutions present both shocks and critical sonic points that we carefully analyze. For all allowed temperatures and for several spiral shocks, we calculated the wave structure. In particular we inferred the angle of the spiral patern, the stress it exerts on the disc as well as the associated flux of mass and angular momentum as a function of temperature. We quantified the rate of angular momentum transport by means of the dimensionless $\\alpha$ parameter. For the thickest disc we considered (corresponding to $h/r$ values of about 1/3), we f...

  18. Migration of massive black hole binaries in self--gravitating accretion discs: Retrograde versus prograde

    Roedig, Constanze

    2013-01-01

    We study the interplay between mass transfer, accretion and gravitational torques onto a black hole binary migrating in a self-gravitating, retrograde circumbinary disc. A direct comparison with an identical prograde disc shows that: (i) because of the absence of resonances, the cavity size is a factor a(1+e) smaller for retrograde discs; (ii) nonetheless the shrinkage of a circular binary semi--major axis, a, is identical in both cases; (iii) a circular binary in a retrograde disc remains circular while eccentric binaries grow more eccentric. For non-circular binaries, we measure the orbital decay rates and the eccentricity growth rates to be exponential as long as the binary orbits in the plane of its disc. Additionally, for these co-planar systems, we find that interaction (~ non--zero torque) stems only from the cavity edge plus a(1+e) in the disc, i.e. for dynamical purposes, the disc can be treated as a annulus of small radial extent. We find that simple 'dust' models in which the binary- disc interacti...

  19. Magneto centrifugal winds from accretion discs around black hole binaries

    Chakravorty, S; Ferreira, J; Henri, G; Belmont, R; Clavel, M; Corbel, S; Rodriguez, J; Coriat, M; Drappeau, S; Malzac, J

    2016-01-01

    We want to test if self-similar magneto-hydrodynamic (MHD) accretion-ejection models can explain the observational results for accretion disk winds in BHBs. In our models, the density at the base of the outflow, from the accretion disk, is not a free parameter, but is determined by solving the full set of dynamical MHD equations without neglecting any physical term. Different MHD solutions were generated for different values of (a) the disk aspect ratio ($\\varepsilon$) and (b) the ejection efficiency ($p$). We generated two kinds of MHD solutions depending on the absence (cold solution) or presence (warm solution) of heating at the disk surface. The cold MHD solutions are found to be inadequate to account for winds due to their low ejection efficiency. The warm solutions can have sufficiently high values of $p (\\gtrsim 0.1)$ which is required to explain the observed physical quantities in the wind. The heating (required at the disk surface for the warm solutions) could be due to the illumination which would b...

  20. Magneto centrifugal winds from accretion discs around black hole binaries

    Chakravorty, S.; Petrucci, P.-O.; Ferreira, J.; Henri, G.; Belmont, R.; Clavel, M.; Corbel, S.; Rodriguez, J.; Coriat, M.; Drappeau, S.; Malzac, J.

    2016-05-01

    We want to test if self-similar magneto-hydrodynamic (MHD) accretion-ejection models can explain the observational results for accretion disk winds in BHBs. In our models, the density at the base of the outflow from the accretion disk is not a free parameter but is determined by solving the full set of dynamical MHD equations without neglecting any physical term. Different MHD solutions were generated for different values of (a) the disk aspect ratio (ǎrepsilon) and (b) the ejection efficiency (p). We generated two kinds of MHD solutions depending on the absence (cold solution) or presence (warm solution) of heating at the disk surface. The cold MHD solutions are found to be inadequate to account for winds due to their low ejection efficiency. The warm solutions can have sufficiently high values of p (\\gtrsim 0.1) which is required to explain the observed physical quantities in the wind. The heating (required at the disk surface for the warm solutions) could be due to the illumination which would be more efficient in the Soft state. We found that in the Hard state a range of ionisation parameter is thermodynamically unstable, which makes it impossible to have any wind at all, in the Hard state. Our results would suggest that a thermo-magnetic process is required to explain winds in BHBs.

  1. Viscous Driving of Global Oscillations in Accretion Discs Around Black Holes

    Miranda, Ryan; Lai, Dong

    2014-01-01

    We examine the role played by viscosity in the excitation of non-axisymmetric global oscillation modes in accretion discs around black holes using two-dimensional hydrodynamic simulations. The turbulent viscosity is modeled by the $\\alpha$-ansatz, with different equations of state. We consider both discs with transonic radial inflows across the innermost stable circular orbit, and stationary discs truncated by a reflecting wall at their inner edge, representing a magnetosphere. In transonic discs, viscosity can excite several types of global oscillation modes. These modes are either axisymmetric with frequencies close to multiples of the maximum radial epicyclic frequency $\\kappa_\\mathrm{max}$, non-axisymmetric with frequencies close to multiples of of the innermost stable orbit frequency $\\Omega_\\mathrm{ISCO}$, or hybrid modes whose frequencies are linear combinations of these two frequencies. Small values of the viscosity parameter $\\alpha$ primarily produce non-axisymmetric modes, while axisymmetric modes ...

  2. Accretion disc time lag distributions: applying CREAM to simulated AGN light curves

    Starkey, D. A.; Horne, Keith; Villforth, C.

    2016-02-01

    Active galactic nuclei (AGN) vary in their brightness across all wavelengths. Moreover, longer wavelength ultraviolet-optical continuum light curves appear to be delayed with respect to shorter wavelength light curves. A simple way to model these delays is by assuming thermal reprocessing of a variable point source (a lamp post) by a blackbody accretion disc. We introduce a new method, CREAM (Continuum REprocessed AGN Markov Chain Monte Carlo), that models continuum variations using this lamp post model. The disc light curves lag the lamp post emission with a time delay distribution sensitive to the disc temperature-radius profile and inclination. We test CREAM's ability to recover both inclination and product of black hole mass and accretion rate {Mdot{M}}, and show that the code is also able to infer the shape of the driving light curve. CREAM is applied to synthetic light curves expected from 1000 s exposures of a 17th magnitude AGN with a 2-m telescope in Sloan g and i bands with Signal-to-Noise Ratio (SNR) of 500-900 depending on the filter and lunar phase. We also test CREAM on poorer quality g and i light curves with SNR = 100. We find in the high-SNR case that CREAM can recover the accretion disc inclination to within an uncertainty of 5° and an {Mdot{M}} to within 0.04 dex.

  3. The structure and radiation spectra of illuminated accretion discs in AGN. I. Moderate illumination

    Rózanska, A R; Czerny, B; Collin, S

    2002-01-01

    We present detailed computations of the vertical structure of an accretion disc illuminated by hard X-ray radiation with the code {\\sc titan-noar} suitable for Compton thick media. The energy generated via accretion is dissipated partially in the cold disc as well as in the X-ray source. We study the differences between the case where the X-ray source is in the form of a lamp post above the accretion disc and the case of a heavy corona. We consider radiative heating via Comptonization together with heating via photo-absorption on numerous heavy elements as carbon, oxygen, silicon, iron. The transfer in lines is precisely calculated. A better description of the heating/cooling through the inclusion of line transfer, a correct description of the temperature in the deeper layers, a correct description of the entire disc vertical structure, as well as the study of the possible coronal pressure effect, constitute an improvement in comparison to previous works. We show that exact calculations of hydrostatic equilib...

  4. An irradiated accretion disc in the narrow-line Seyfert 1 RE J1034+396?

    Soria, R.; Puchnarewicz, E. M.

    2002-01-01

    We model the optical to X-ray continuum spectrum of the narrow-line Seyfert 1 galaxy RE J1034+396. We show that the flat optical spectrum is consistent with emission from an irradiated accretion disc. The X-ray emission can be modelled with a disc blackbody and a Comptonized component. The temperature at the inner edge of the disc Tin=(0.12+/-0.02)keV. Using this constraint, we show that the flat optical spectrum is consistent with emission from the irradiatively heated outer part of the accretion disc. We constrain the outer radius of the optically thick disc (R>~5×1016cm) and the inner radius of the irradiation-dominated region (R>~5×1012cm). Our optical and X-ray spectral fits indicate a mass 0.6<~M<~3×106Msolar, and do not rule out a low (i.e. face-on) inclination angle for the system.

  5. Integrated accretion disc angular momentum removal and astrophysical jet acceleration mechanism

    Bellan, P. M.

    2016-06-01

    Ions and neutrals in the weakly ionized plasma of an accretion disc are tightly bound because of the high ion-neutral collision frequency. A cluster of a statistically large number of ions and neutrals behaves as a fluid element having the charge of the ions and the mass of the neutrals. This fluid element is effectively a metaparticle having such an extremely small charge-to-mass ratio that its cyclotron frequency can be of the order of the Kepler angular frequency. In this case, metaparticles with a critical charge-to-mass ratio can have zero canonical angular momentum. Zero canonical angular momentum metaparticles experience no centrifugal force and spiral inwards towards the central body. Accumulation of these inward spiralling metaparticles near the central body produces radially and axially outward electric fields. The axially outward electric field drives an out-of-plane poloidal electric current along arched poloidal flux surfaces in the highly ionized volume outside the disc. This out-of-plane current and its associated magnetic field produce forces that drive bidirectional astrophysical jets flowing normal to and away from the disc. The poloidal electric current circuit removes angular momentum from the accreting mass and deposits this removed angular momentum at near infinite radius in the disc plane. The disc region is an electric power source (E\\cdot J 0).

  6. Retrograde binaries of massive black holes in circum-binary accretion discs

    Amaro-Seoane, Pau; Dotti, Massimo; Colpi, Monica

    2016-01-01

    We explore the hardening of a massive black hole binary embedded in a circum-binary gas disc when the binary and the gas are coplanar and the gas is counter-rotating. The secondary black hole, revolving in the direction opposite to the gas, experiences a drag from gas-dynamical friction and from direct accretion of part of it. Using two-dimensional (2D) hydrodynamical grid simulations we investigate the effect of changing the accretion prescriptions on the dynamics of the secondary black hole which in turn affect the binary hardening and eccentricity evolution. We find that realistic accretion prescriptions lead to results that differ from those inferred assuming accretion of all the gas within the Roche Lobe of the secondary black hole. Different accretion prescriptions result in different disc's surface densities which alter the black hole's dynamics back. Full 3D SPH realizations of a number of representative cases, run over a shorter interval of time, validate the general trends observed in the less compu...

  7. Retrograde binaries of massive black holes in circum-binary accretion discs

    Amaro-Seoane, Pau; Maureira-Fredes, Cristián; Dotti, Massimo; Colpi, Monica

    2016-01-01

    We explore the hardening of a massive black hole binary embedded in a circum-binary gas disc when the binary and the gas are coplanar and the gas is counter-rotating. The secondary black hole, revolving in the direction opposite to the gas, experiences a drag from gas-dynamical friction and from direct accretion of part of it. Using two-dimensional (2D) hydrodynamical grid simulations we investigate the effect of changing the accretion prescriptions on the dynamics of the secondary black hole...

  8. Tilted black hole accretion disc models of Sagittarius A*: time-variable millimetre to near-infrared emission

    Dexter, Jason

    2012-01-01

    High-resolution, multi-wavelength, and time-domain observations of the Galactic centre black hole candidate, Sgr A*, allow for a direct test of contemporary accretion theory. To date, all models have assumed alignment between the accretion disc and black hole angular momentum axes, but this is unjustified for geometrically thick accretion flows like that onto Sgr A*. Instead, we calculate images and spectra from a set of simulations of accretion flows misaligned ('tilted') by 15 degrees from the black hole spin axis and compare them with millimetre (mm) to near-infrared (NIR) observations. Non-axisymmetric standing shocks from eccentric fluid orbits dominate the emission, leading to a wide range of possible image morphologies. These effects invalidate previous parameter estimates from model fitting, including estimates of the dimensionless black hole spin, except possibly at low values of spin or tilt. At 1.3mm, the images have crescent morphologies, and the black hole shadow may still be accessible to future...

  9. Active galactic nuclei at z ˜ 1.5: III. Accretion discs and Black Hole Spin

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

    2016-04-01

    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 9 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.

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

    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.

  11. Energy dissipation and angular momentum transfer within a magnetically torqued accretion disc

    2010-01-01

    We discuss transportation and redistribution of energy and angular momentum in the magnetic connection(MC) process and Blandford-Payne(BP) process.MC results in readjusting the interior viscous torque,and its effects are operative not only in but also beyond the MC region.The BP process is invoked to transfer the "excessive" angular momentum from an accretion disc.In addition,we derive a criterion for the interior viscous torque to resolve the puzzle of the overall equilibrium of angular momentum in disc accretion.It turns out that the efficiency of BP at extracting angular momentum and the intensity of the outflow are required to be greater than some critical values.

  12. Modelling steep radial emissivity in relativistic iron lines from black-hole accretion discs

    Svoboda, Jiří; Dovčiak, Michal; Karas, Vladimír; Goosmann, R. W.

    Opava: Silesian University, 2014 - (Stuchlík, Z.), s. 209-221. (Publications of the Institute of Physics. 6). ISBN 9788075101242. ISSN 2336-5668. [RAGtime /10.-13./. Opava (CZ), 15.09.2008-17.09.2008] R&D Projects: GA ČR(CZ) GP14-20970P Institutional support: RVO:67985815 Keywords : black hole s * accretion discs * relativistic iron lines Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  13. Viscous driving of global oscillations in accretion discs around black holes

    Miranda, R.; Horák, Jiří; Lai, D.

    2015-01-01

    Roč. 446, č. 1 (2015), s. 240-253. ISSN 0035-8711 R&D Projects: GA MŠk(CZ) LH14049 Grant ostatní: Rada Programu interní podpory projektů mezinárodní spolupráce AV ČR(CZ) M100031202 Institutional support: RVO:67985815 Keywords : accretion discs * hydrodynamics * waves Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.107, year: 2014

  14. Efficiency of gas cooling and accretion at the disc-corona interface

    Armillotta, L; Marinacci, F

    2016-01-01

    In star-forming galaxies, stellar feedback can have a dual effect on the circumgalactic medium both suppressing and stimulating gas accretion. The trigger of gas accretion can be caused by disc material ejected into the halo in the form of fountain clouds and by its interaction with the surrounding hot corona. Indeed, at the disc-corona interface, the mixing between the cold/metal-rich disc gas (T ~ 10^6 K) can dramatically reduce the cooling time of a portion of the corona and produce its condensation and accretion. We studied the interaction between fountain clouds and corona in different galactic environments through parsec-scale hydrodynamical simulations, including the presence of thermal conduction, a key mechanism that influences gas condensation. Our simulations showed that the coronal gas condensation strongly depends on the galactic environment, in particular it is less efficient for increasing virial temperature/mass of the haloes where galaxies reside and it is fully ineffective for objects with v...

  15. Evidence of an irradiated accretion disc in XTE J1818-245*

    The X-ray transient source XTE J1818-245 went through an outburst in 2005 that was observed during a multiwavelength campaign from radio to soft γ-rays. Observations in V and R optical bands with the 1-m Swope telescope allowed the discovery of a new bright source. As we aimed to reveal the nature of the companion star, we performed new optical observations with the ESO/NTT telescope at La Silla, both in photometry and spectroscopy. We confirm the optical counterpart found by the Swope telescope, but the spectral type of the secondary star could not be identified. The spectrum showed a blue-dominated shape and an Hα emission line was detected, indicating that the optical emission was dominated by the presence of an accretion disc. The broad-band spectral energy distribution revealed that the outer parts of the accretion disc had to be irradiated by its inner parts to explain the optical emission. New observations of XTE J1818-245 in quiescence are needed to find the nature of the companion star. Moreover, radio-to-X-ray strictly simultaneous observations of transient black holes are needed to disentangle the importance of jets and irradiated accretion discs in the infrared-optical-ultraviolet domain. (authors)

  16. Irradiated, colour-temperature-corrected accretion discs in ultraluminous X-ray sources

    Sutton, Andrew D; Roberts, Timothy P

    2014-01-01

    Although attempts have been made to constrain the stellar types of optical counterparts to ULXs, the detection of optical variability instead suggests that they may be dominated by reprocessed emission from X-rays which irradiate the outer accretion disc. Here, we report results from a combined X-ray and optical spectral study of a sample of ULXs, which were selected for having broadened disc-like X-ray spectra, and known optical counterparts. We simultaneously fit optical and X-ray data from ULXs with a new spectral model of emission from an irradiated, colour-temperature-corrected accretion disc around a black hole, with a central Comptonising corona. We find that the ULXs require reprocessing fractions of $\\sim 10^{-3}$, which is similar to sub-Eddington thermal dominant state BHBs, but less than has been reported for ULXs with soft ultraluminous X-ray spectra. We suggest that the reprocessing fraction may be due to the opposing effects of self-shielding in a geometrically thick super-critical accretion di...

  17. An accretion disc instability induced by a temperature sensitive {\\alpha} parameter

    Potter, William J

    2014-01-01

    In the standard thin disc formalism the dimensionless {\\alpha} parameter is usually assumed to be constant. However, there are good theoretical reasons for believing, as well as evidence from simulations, that {\\alpha} is dependent on intrinsic disc properties. In this paper we analyse the conditions for the stability of a thin accretion disc in which {\\alpha} is a function of the magnetic Prandtl number, the ratio of collisional viscosity to resistivity. In the inner disc, where the free electron opacity and radiation viscosity dominate, the disc is unstable if {\\alpha} is proportional to the magnetic Prandtl number with an exponent > 0.5. This is within the range of values for the power-law index found in MHD simulations with simple energetics. We calculate the evolution of the unstable disc within the {\\alpha} formalism and show that the physically accessible solutions form a limit cycle, analogous to the behaviour seen in recurrent dwarf novae. It is noteworthy that the time-dependent global behaviour of ...

  18. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    Chattopadhyay, Indranil

    2016-01-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion - ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von - Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist {for $\\alpha \\gsim0.06$} in the general relativistic prescription, but is lower if mass - loss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock {location}. The jet terminal speed increases with stronger shocks, quantitatively speaking, the terminal speed of ...

  19. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    Kennedy, Gareth F; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-01-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterised by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical cap...

  20. Tilted black hole accretion disc models of Sagittarius A*: time-variable millimetre to near-infrared emission

    Dexter, Jason; Fragile, P. Chris

    2013-07-01

    High-resolution, multiwavelength and time-domain observations of the Galactic Centre black hole candidate, Sgr A*, allow for a direct test of contemporary accretion theory. Most models assume alignment between the accretion disc and black hole angular momentum axes, but this is not necessarily the case for geometrically thick accretion flows like that on to Sgr A*. Instead, we calculate images and spectra from a set of numerical simulations of accretion flows misaligned (`tilted') by 15° from the black hole spin axis and compare them with millimetre (mm) to near-infrared (NIR) observations. Non-axisymmetric standing shocks from eccentric fluid orbits dominate the emission, leading to a wide range of possible image morphologies. The strong effects of disc tilt lead to poorly constrained model parameters. These results suggest that previous parameter estimates from fitting aligned models, including estimates of the dimensionless black hole spin, likely only apply for small values of spin or tilt (upper limits of a 1012 K). These electrons can naturally produce the observed NIR flux, spectral index and rapid variability (`flaring'). This NIR emission is uncorrelated with that in the mm, which also agrees with observations. These are the first numerical models to explain the time-variable mm to NIR emission of Sgr A*. Predictions of the model include significant structural changes observable with mm-VLBI on both the dynamical (hour) and Lense-Thirring precession (day-year) time-scales, and ≃ 30-50 μas changes in centroid position from extreme gravitational lensing events during NIR flares, detectable with the future VLT instrument GRAVITY. We further predict that multiwavelength monitoring should find no significant correlations between mm and NIR/X-ray light curves. The weak correlations reported to date are shown to be consistent with our model, where they are artefacts of the short light-curve durations. If the observed NIR emission is caused by shock heating in

  1. Evolution of accretion discs around a kerr black hole using extended magnetohydrodynamics

    Foucart, Francois; Chandra, Mani; Gammie, Charles F.; Quataert, Eliot

    2016-02-01

    Black holes accreting well below the Eddington rate are believed to have geometrically thick, optically thin, rotationally supported accretion discs in which the Coulomb mean free path is large compared to GM/c2. In such an environment, the disc evolution may differ significantly from ideal magnetohydrodynamic (MHD) predictions. We present non-ideal global axisymmetric simulations of geometrically thick discs around a rotating black hole. The simulations are carried out using a new code GRIM, which evolves a covariant extended magnetohydrodynamics model derived by treating non-ideal effects as a perturbation of ideal MHD. Non-ideal effects are modelled through heat conduction along magnetic field lines, and a difference between the pressure parallel and perpendicular to the field lines. The model relies on an effective collisionality in the disc from wave-particle scattering and velocity-space (mirror and firehose) instabilities. We find that the pressure anisotropy grows to match the magnetic pressure, at which point it saturates due to the mirror instability. The pressure anisotropy produces outward angular momentum transport with a magnitude comparable to that of MHD turbulence in the disc, and a significant increase in the temperature in the wall of the jet. We also find that, at least in our axisymmetric simulations, conduction has a small effect on the disc evolution because (1) the heat flux is constrained to be parallel to the field and the field is close to perpendicular to temperature gradients, and (2) the heat flux is choked by an increase in effective collisionality associated with the mirror instability.

  2. Thunderclouds and accretion discs: a model for the spectral and temporal variability of Seyfert 1 galaxies

    Merloni, A.; Fabian, A. C.

    2001-12-01

    X-ray observations of Seyfert 1 galaxies offer the unique possibility of observing spectral variability on time-scales comparable to the dynamical time of the inner accretion flow. They typically show highly variable light curves, on a wide range of time-scales, with power density spectra characterized by `red noise' and a break at low frequencies. On the other hand, time-resolved spectral analyses have established that spectral variability on the shortest time-scales is important in all these sources, with the spectra getting softer at high fluxes (in the 2-10keV band, typically), while the reflection component and the iron line often exhibit a complex behaviour. Here we present a model that is able to explain a number of the above mentioned properties, in terms of magnetic flares shining above a standard accretion disc producing the X-ray spectrum via inverse Compton scattering of soft photons (both intrinsic and reprocessed thermal emission from the accretion disc and locally produced synchrotron radiation). We show that the fundamental heating event, likely caused by magnetic reconnection, must be compact, with typical size comparable to the accretion disc thickness, and must be triggered at a height at least an order of magnitude larger than its size. The fundamental property of our `thundercloud' model is that the spatial and temporal distributions of flares are not random: the heating of the corona proceeds in correlated trains of events in an avalanche fashion. The amplitude of the avalanches obeys a power-law distribution and determines the size of the active regions where the spectrum is produced. As a result of the feedback effect of the X-ray radiation reprocessed in the disc, larger active regions produce softer spectra. With our model we simulate X-ray light curves that reproduce the main observational properties of the power density spectra and of the X-ray continuum short-term variability of Seyfert 1 galaxies. By comparing them with observations of

  3. Modelling accretion disc and stellar wind interactions: the case of Sgr A*

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2016-07-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ˜108 cm s-1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s-1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of dot{M}_w= 10^{-7} M_{⊙} yr^{-1}, nd = 105 cm-3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ˜3000 gravitational radii from the supermassive black hole.

  4. Response of a circumbinary accretion disc to black hole mass loss

    Rosotti, Giovanni P; Price, Daniel J

    2012-01-01

    We investigate the evolution of the surface density of a circumbinary accretion disc after the mass loss induced by the merger of two supermassive black holes. We first introduce an ana- lytical model, under the assumption of a disc composed of test particles, to derive the surface density evolution of the disc following the mass loss. The model predicts the formation of sharp density peaks in the disc; the model also allows us to compute the typical timescale for the formation of these peaks. To test and validate the model, we run numerical simulations of the process using the Smoothed Particle Hydrodynamics (SPH) code PHANTOM, taking fluid effects into account. We find good agreement in the shape and position of the peaks between the model and the simulations. In a fluid disc, however, the epicyclic oscillations induced by the mass loss can dissipate, and only some of the predicted peaks form in the simulation. To quantify how fast this dissipation proceeds, we introduce an appropriate parameter, and we sho...

  5. A transition in circumbinary accretion discs at a binary mass ratio of 1:25

    D'Orazio, Daniel J; Duffell, Paul; MacFadyen, Andrew I; Farris, Brian D

    2015-01-01

    We study circumbinary accretion discs in the framework of the restricted three-body problem (R3Bp) and via numerically solving the height-integrated equations of viscous hydrodynamics. Varying the mass ratio of the binary, we find a pronounced change in the behaviour of the disc near mass ratio $q \\equiv M_s/M_p \\sim 0.04$. For mass ratios above $q=0.04$, solutions for the hydrodynamic flow transition from steady, to strongly-fluctuating; a narrow annular gap in the surface density around the secondary's orbit changes to a hollow central cavity; and a spatial symmetry is lost, resulting in a lopsided disc. This phase transition is coincident with the mass ratio above which stable orbits do not exist around the L4 and L5 equilibrium points of the R3B problem. Using the DISCO code, we find that for thin discs, for which a gap or cavity can remain open, the mass ratio of the transition is relatively insensitive to disc viscosity and pressure. The $q=0.04$ transition has relevance for the evolution of massive bla...

  6. Accretion Disc Time Lag Distributions: Applying CREAM to Simulated AGN Light Curves

    Starkey, David; Villforth, Carolin

    2015-01-01

    Active Galactic Nuclei (AGN) vary in their brightness across all wavelengths. Moreover, longer wavelength ultraviolet - optical continuum light curves appear to be delayed with respect to shorter wavelength light curves. A simple way to model these delays is by assuming thermal reprocessing of a variable point source (a lamp post) by a blackbody accretion disc. We introduce a new method, CREAM (\\textbf{C}ontinuum \\textbf{RE}processed \\textbf{A}GN \\textbf{M}arkov Chain Monte Carlo), that models continuum variations using this lamp post model. The disc light curves lag the lamp post emission with a time delay distribution sensitive to the disc temperature-radius profile and inclination. We test CREAM's ability to recover both inclination and product of black hole mass and accretion rate $\\mmdot$, and show that the code is also able to infer the shape of the driving light curve. CREAM is applied to synthetic light curves expected from 1000 second exposures of a 17th magnitude AGN with a 2m telescope in Sloan g a...

  7. X-Ray Evidence for the Accretion Disc-Outflow Connection in 3C 111

    Tombesi, Frank; Sambruna, R. M.; Reeves, J. N.; Reynolds, C. S.; Braito, V.

    2011-01-01

    We present the spectral analysis of three Suzaku X-ray Imaging Spectrometer observations of 3C III requested to monitor the predicted variability of its ultrafast outflow on approximately 7 d time-scales. We detect an ionized iron emission line in the first observation and a blueshifted absorption line in the second, when the flux is approximately 30 per cent higher. The location of the material is constrained at less than 0.006 pc from the variability. Detailed modelling supports an identification with ionized reflection off the accretion disc at approximately 20-100rg from the black hole and a highly ionized and massive ultrafast outflow with velocity approximately 0.1c, respectively. The outflow is most probably accelerated by radiation pressure, but additional magnetic thrust cannot be excluded. The measured high outflow rate and mechanical energy support the claims that disc outflows may have a significant feedback role. This work provides the first direct evidence for an accretion disc-outflow connection in a radio-loud active galactic nucleus, possibly linked also to the jet activity.

  8. Accretion discs, coronae and jets in black hole binaries: prospects for Simbol-X

    Malzac, Julien

    2008-01-01

    The phenomenology of accretion disc, coronnae and jets in X-ray binaries is rather well established. However the structure of the accretion flow in the various spectral states is still debated and the connection between the hot flow and compact jet is far from being understood. Simbol-X should address these two important questions in several ways. First, it will provide us with the capability of producing high sensivity, broad band spectra and therefore constrain simultaneously the shape and luminosity of all spectral components (iron line, reflection bump, thermal disc and comptonised emission) which in turn provides information on the geometry of the accretion flow. It will also determine the exact contribution of jets to the X-ray band both in bright and quiescent states. Finally it will shed new lights on the underlying mechanisms triggering spectral state transitions by allowing us to follow in exquisite details the rapid spectral evolution and its correlation with the radio jet emission during those tra...

  9. Wave-like warp propagation in circumbinary discs - I. Analytic theory and numerical simulations

    Facchini, Stefano; Lodato, Giuseppe; Price, Daniel J.

    2013-08-01

    In this paper we analyse the propagation of warps in protostellar circumbinary discs. We use these systems as a test environment in which to study warp propagation in the bending-wave regime, with the addition of an external torque due to the binary gravitational potential. In particular, we want to test the linear regime, for which an analytic theory has been developed. In order to do so, we first compute analytically the steady-state shape of an inviscid disc subject to the binary torques. The steady-state tilt is a monotonically increasing function of radius, but misalignment is found at the disc inner edge. In the absence of viscosity, the disc does not present any twist. Then, we compare the time-dependent evolution of the warped disc calculated via the known linearized equations both with the analytic solutions and with full 3D numerical simulations. The simulations have been performed with the PHANTOM smoothed particle hydrodynamics (SPH) code using two million particles. We find a good agreement both in the tilt and in the phase evolution for small inclinations, even at very low viscosities. Moreover, we have verified that the linearized equations are able to reproduce the diffusive behaviour when α > H/R, where α is the disc viscosity parameter. Finally, we have used the 3D simulations to explore the non-linear regime. We observe a strongly non-linear behaviour, which leads to the breaking of the disc. Then, the inner disc starts precessing with its own precessional frequency. This behaviour has already been observed with numerical simulations in accretion discs around spinning black holes. The evolution of circumstellar accretion discs strongly depends on the warp evolution. Therefore, the issue explored in this paper could be of fundamental importance in order to understand the evolution of accretion discs in crowded environments, when the gravitational interaction with other stars is highly likely, and in multiple systems. Moreover, the evolution of

  10. Theory of Disk Accretion onto Magnetic Stars

    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. Viscous driving of global oscillations in accretion discs around black holes

    Miranda, Ryan; Horák, Jiří; Lai, Dong

    2015-01-01

    We examine the role played by viscosity in the excitation of global oscillation modes (both axisymmetric and non-axisymmetric) in accretion discs around black holes using two-dimensional hydrodynamic simulations. The turbulent viscosity is modelled by the α-ansatz, with different equations of state. We consider both discs with transonic radial inflows across the innermost stable circular orbit, and stationary discs truncated by a reflecting wall at their inner edge, representing a magnetosphere. In transonic discs, viscosity can excite several types of global oscillation modes. These modes are either axisymmetric with frequencies close to multiples of the maximum radial epicyclic frequency κmax, non-axisymmetric with frequencies close to multiples of the innermost stable orbit frequency ΩISCO, or hybrid modes whose frequencies are linear combinations of these two frequencies. Small values of the viscosity parameter α primarily produce non-axisymmetric modes, while axisymmetric modes become dominant for large α. The excitation of these modes may be related to an instability of the sonic point, at which the radial infall speed is equal to the sound speed of the gas. In discs with a reflective inner boundary, we explore the effect of viscosity on trapped p modes which are intrinsically overstable due to the corotation resonance effect. The effect of viscosity is either to reduce the growth rates of these modes, or to completely suppress them and excite a new class of higher frequency modes. The latter requires that the dynamic viscosity scales positively with the disc surface density, indicating that it is a result of the classic viscous overstability effect.

  12. Microphysical dissipation, turbulence and magnetic fields in hyper-accreting discs

    Rossi, Elena M; Menou, Kristen

    2008-01-01

    Hyper-accreting discs occur in compact-object mergers and collapsars, and may power gamma-ray bursts (GRBs). We calculate the microscopic viscosity and resistivity of plasma in these discs, and discuss the implications for their global structure and evolution. In the neutrino-cooled innermost regions, the viscosity is provided mainly by mildly degenerate electrons, while the resistivity is modified from the Spitzer value due to the effects of both relativity and degeneracy. The plasma behaves as an almost ideal MHD fluid. Among the non-ideal MHD effects the Hall term is relatively the most important, while the magnetic Prandtl number, Pr (the ratio of viscosity to resistivity), is typically larger than unity: 10 < Pr < 6000. The outer radiatively inefficient regions also display high Pr. Numerical simulations of the magneto-rotational instability indicate that the saturation level and angular momentum transport efficiency may be greatly enhanced at high Pr. If this behaviour persists in the presence of ...

  13. Effect of the flow composition on outflow rates from accretion discs around black holes

    Kumar, Rajiv; Chattopadhyay, Indranil; Chakrabarti, Sandip K

    2013-01-01

    We studied the outflow behaviour from accretion discs around black holes taking into account the vertical equilibrium accretion flow model. The outflow rate is found to depend crucially on flow composition. Our approach is to study the outflow behaviour as function of inflow around black holes with an equation of state which allows flow to be thermally relativistic close to black holes and non relativistic far away from black holes. We studied shock ejection model. A pure electron positron pair flow never undergoes shock transition while presence of some baryons (common in outflows and jets) makes it possible to have standing shock waves in the flow. It can be concluded that the presence of protons is necessary for the flow to show the outflow behaviour. The outflow rate is maximum when the flow contains the proton number density which is 27% of the electron number density. We conclude that a pure electron-positron jet is unlikely to form.

  14. Linear and nonlinear evolution of the vertical shear instability in accretion discs

    Nelson, Richard P; Umurhan, Orkan M

    2012-01-01

    (Abridged) We analyse the stability and evolution of power-law accretion disc models. These have midplane densities that follow radial power-laws, and have either temperature or entropy distributions that are power-law functions of cylindrical radius. We employ two different hydrodynamic codes to perform 2D-axisymmetric and 3D simulations that examine the long-term evolution of the disc models as a function of the power-law indices of the temperature or entropy, the thermal relaxation time of the fluid, and the viscosity. We present a stability analysis of the problem that we use to interpret the simulation results. We find that disc models whose temperature or entropy profiles cause the equilibrium angular velocity to vary with height are unstable to the growth of modes with wavenumber ratios |k_R/k_Z| >> 1 when the thermodynamic response of the fluid is isothermal, or the thermal evolution time is comparable to or shorter than the local dynamical time scale. These discs are subject to the Goldreich-Schubert...

  15. Comparison of ejection events in the jet and accretion disc outflows in 3C 111

    Tombesi, F.; Sambruna, R. M.; Marscher, A. P.; Jorstad, S. G.; Reynolds, C. S.; A. Markowitz(University of California, San Diego, United States)

    2012-01-01

    We present a comparison of the parameters of accretion disc outflows and the jet of the broad-line radio galaxy 3C 111 on sub-pc scales. We make use of published X-ray observations of ultra-fast outflows (UFOs) and new 43GHz VLBA images to track the jet knots ejection. We find that the superluminal jet coexists with the mildly relativistic outflows on sub-pc scales, possibly indicating a transverse stratification of a global flow. The two are roughly in pressure equilibrium, with the UFOs pot...

  16. Corotation resonance and overstable oscillations in black hole accretion discs: general relativistic calculations

    Horák, Jiří; Lai, D.

    2013-01-01

    Roč. 434, č. 4 (2013), s. 2761-2771. ISSN 0035-8711 R&D Projects: GA MŠk ME09036; GA ČR(CZ) GAP209/11/2004 Grant ostatní: NASA(US) NNX12AF85G; NSF(US) AST-1008245; NSF(US) AST-1211061; Rada Programu interní podpory projektů mezinárodní spolupráce AV ČR(CZ) M100031202 Institutional support: RVO:67985815 Keywords : accretion discs * hydrodynamics * X-ray binaries Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.226, year: 2013

  17. Polarization in lamp-post model of black-hole accretion discs

    Dovčiak, Michal; Muleri, F.; Goosmann, R. W.; Karas, Vladimír; Matt, G.

    Bristol: Institute of Physics Publishing, 2012, 012056/1-012056/6. (Journal of Physics Conference Series. 372). ISSN 1742-6588. [Astronomy at High Angular Resolution - The Central Kiloparsec in Galactic Nuclei. Bad Honnef (DE), 28.08.2011-02.09.2011] R&D Projects: GA ČR GA202/09/0772 Grant ostatní: ESA(XE) ESA-PECS project No. 98040 Institutional research plan: CEZ:AV0Z10030501 Keywords : black - hole accretion disc Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  18. Flickering of accreting white dwarfs: the remarkable amplitude - flux relation and disc viscocity

    Zamanov, R K; Latev, G; Sokoloski, J L; Stoyanov, K A; Genkov, V; Tsvetkova, S V; Tomov, T; Antov, A; Bode, M F

    2015-01-01

    We analyze optical photometric data of short term variability (flickering) of accreting white dwarfs in cataclysmic variables (KR Aur, MV Lyr, V794 Aql, TT Ari, V425 Cas), recurrent novae (RS Oph and T CrB) and jet-ejecting symbiotic stars (CH Cyg and MWC 560). We find that the amplitude-flux relationship is visible over four orders of magnitude, in the range of fluxes from $10^{29}$ to $10^{33}$ erg s$^{-1}$ \\AA$^{-1}$, as a "statistically perfect" correlation with correlation coefficient 0.96 and p-value $ \\sim 10^{-28}$. In the above range, the amplitude of variability for any of our 9 objects is proportional to the flux level with (almost) one and the same factor of proportionality for all 9 accreting white dwarfs with $\\Delta F = 0.36 (\\pm 0.05) F_{av}$, $\\sigma_{rms} = 0.086(\\pm 0.011) F_{av}$, and $\\sigma_{rms} / \\Delta F = 0.24 \\pm 0.02$. Over all, our results indicate that the viscosity in the accretion discs is practically the same for all 9 objects in our sample, in the mass accretion rate range $2...

  19. Flickering of accreting white dwarfs: the remarkable amplitude-flux relation and disc viscosity

    Zamanov, R. K.; Boeva, S.; Latev, G.; Sokoloski, J. L.; Stoyanov, K. A.; Genkov, V.; Tsvetkova, S. V.; Tomov, T.; Antov, A.; Bode, M. F.

    2016-03-01

    We analyse optical photometric data of short term variability (flickering) of accreting white dwarfs in cataclysmic variables (KR Aur, MV Lyr, V794 Aql, TT Ari, V425 Cas), recurrent novae (RS Oph and T CrB) and jet-ejecting symbiotic stars (CH Cyg and MWC 560). We find that the amplitude-flux relationship is visible over four orders of magnitude, in the range of fluxes from 1029 to 1033 erg s-1 Å-1, as a `statistically perfect correlation with correlation coefficient 0.96 and p-value ˜10-28. In the above range, the amplitude of variability for any of our 9 objects is proportional to the flux level with (almost) one and the same factor of proportionality for all nine accreting white dwarfs with ΔF = 0.36(±0.05)Fav, σrms = 0.086(±0.011)Fav, and σrms/ΔF = 0.24 ± 0.02. Overall, our results indicate that the viscosity in the accretion discs is practically the same for all nine objects in our sample, in the mass accretion rate range 2 × 10-11 - 2 × 10-7 M⊙ yr-1.

  20. X-ray variability of SS 433: effects of the supercritical accretion disc

    Atapin, Kirill; Fabrika, Sergei; Medvedev, Aleksei; Vinokurov, Alexander

    2015-01-01

    We study a stochastic variability of SS 433 in the 10-4-5 × 10-2 Hz frequency range based on RXTE data, and on simultaneous observations with RXTE and optical telescopes. We find that the cross-correlation functions and power spectra depend drastically on the precession phase of the supercritical accretion disc. When the wind funnel of the disc is maximally open to the observer, a flat part emerges in the power spectrum; a break is observed at the frequency 1.7 × 10-3 Hz, with a power-law index β ≈ 1.67 at higher frequencies. The soft emission forming mostly in the jets lags behind the hard and optical emission. When the observer does not see the funnel and jets (the `edge-on' disc), the power spectrum is described by a single power-law with β ≈ 1.34 and no correlations between X-ray ranges are detected. We investigated two mechanisms to explain the observed variability at the open disc phase: (1) reflection of radiation at the funnel wall (X-rays and optical) and (2) the gas cooling in the jets (X-rays only). The X-ray variability is determined by the contribution of both mechanisms; however, the contribution of the jets is much higher. We found that the funnel size is (2-2.5) × 1012 cm, and the opening angle is ϑf ˜ 50°. X-ray jets may consist of three fractions with different densities: 8 × 1013, 3 × 1013 and 5 × 1011 cm-3, with most of the jet's mass falling within the latter fraction. We suppose that revealed flat part in the power spectrum may be related to an abrupt change in the disc structure and viscous time-scale at the spherization radius, because the accretion disc becomes thick at this radius, h/r ˜ 1. The extent of the flat spectrum depends on the variation of viscosity at the spherization radius.

  1. Maximum Accretion Efficiency in General Theory of Relativity

    Mitra, Abhas

    1998-01-01

    We derive here the expression for the accretion luminosity, $L(\\infty)$, as seen by a distant inertial observer $S_\\infty$, for the case of spherical accretion onto a static compact object having a surface gravitational red-shift $z_x$. It is found that the ``efficiency'' for conversion of mass energy into accretion energy is given by $\\epsilon = z_x/(1+z_x)$. And since the maximum value of $z_x$ permitted by General Theory of Relativity (GTR) is 2, the maximum theoretical value of the accret...

  2. Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs

    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...

  3. Modelling accretion disc and stellar wind interactions: the case of Sgr A*

    Christie, I. M.; Petropoulou, M.; Mimica, P.; Giannios, D.

    2016-01-01

    Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin–Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ∼108 cm s−1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s−1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{upgreek} \\usepackage{mathrsfs} \\setlength{\\oddsidemargin}{-69pt} \\begin{document} }{}$\\dot{M}_{\\rm w}= 10^{-7} \\,\\mathrm{M}_{\\odot }\\, {\\rm yr}^{-1}$\\end{document}, nd = 105 cm−3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole. PMID:27279781

  4. Convergence of SPH simulations of self-gravitating accretion discs: Sensitivity to the implementation of radiative cooling

    Rice, W. K. M.; Forgan, D. H.; Armitage, P. J.

    2011-01-01

    Recent simulations of self-gravitating accretion discs, carried out using a three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate, have been interpreted as implying that three-dimensional global discs fragment much more easily than would be expected from a two-dimensional local model. Subsequently, global and local two-dimensional models have been shown to display similar fragmentation properties, leaving it unclear whether the three-dimensional results reflect a physi...

  5. HERO: A 3D General Relativistic Radiative Postprocessor for Accretion Discs around Black Holes

    Zhu, Yucong; Sadowski, Aleksander; Psaltis, Dimitrios

    2015-01-01

    HERO (Hybrid Evaluator for Radiative Objects) is a 3D general relativistic radiative transfer code which has been tailored to the problem of analyzing radiation from simulations of relativistic accretion discs around black holes. HERO is designed to be used as a postprocessor. Given some fixed fluid structure for the disc (i.e. density and velocity as a function of position from a hydrodynamics or magnetohydrodynamics simulation), the code obtains a self-consistent solution for the radiation field and for the gas temperatures using the condition of radiative equilibrium. The novel aspect of HERO is that it combines two techniques: 1) a short characteristics (SC) solver that quickly converges to a self consistent disc temperature and radiation field, with 2) a long characteristics (LC) solver that provides a more accurate solution for the radiation near the photosphere and in the optically thin regions. By combining these two techniques, we gain both the computational speed of SC and the high accuracy of LC. W...

  6. A Constraint of Black Hole Mass and the Inner Edge Radius of Relativistic Accretion Disc

    HE Liang; HUANG Chang-Yin; WANG Ding-Xiong

    2011-01-01

    A constraint to black hole(BH)accretion has previously been derived for the inner edge fixed at the innermost stable circular orbit(ISCO) and the innermost bound circular orbit(IBCO). This constraint is referred to as the mass-radius(MR) relation in this study, and the validity of the MR relation is discussed for different cases. It is shown that the product of the BH mass and the inner edge radius decreases monotonically in the accretion process for the inner edge located between IBCO and ISCO. In addition, we discuss the validity of the MR relation by considering the magnetic coupling(MC) effects of a Kerr BH with its surrounding disc. Although theoretically the product of the BH mass and the radius of ISCO increases(decreases) with time for a BH spin greater(less)than some critical value in the MC process, this relation is approximately valid for an Eddington accretion rate persisting for a rather long time, such as more than 10(6) years. Finally ,we discuss the possible application of the MR relation to astrophysics.

  7. Lithium and oxygen in globular cluster dwarfs and the early disc accretion scenario

    Salaris, M

    2014-01-01

    A new scenario --early disc accretion-- has been recently proposed to explain the discovery of multiple stellar populations in Galactic globular clusters. According to this model, the existence of well defined (anti)-correlations amongst light element abundances (i.e. C, N, O, Na) in the photospheres of stars belonging to the same cluster (and the associated helium enrichment), is caused by accretion of the ejecta of short lived interacting massive binary systems (and single fast rotating massive stars) on fully convective pre-main sequence low- and very low-mass stars, during the early stages of the cluster evolution. We investigated the constraints provided by considering simultaneously the observed spread of lithium and oxygen (and when possible also sodium) abundances for samples of turn-off stars in NGC6752, NGC6121 (M4), and NGC104 (47Tuc), and the helium abundance of their multiple main sequences. These observations provide a very powerful test for the accretion scenario, because the observed O, Li and...

  8. The main sequences of NGC2808: constraints on the early disc accretion scenario

    Cassisi, Santi

    2013-01-01

    [Abridged] A new scenario --early disc accretion-- has been proposed very recently to explain the origin of the multiple population phenomenon in Galactic globular clusters. It envisages the possibility that a fraction of low- and very low-mass cluster stars may accrete the ejecta of interacting massive binary (and possibly also fast rotating massive) stars during the fully convective, pre-main sequence stage, to reproduce the CN and ONa anticorrelations observed among stars in individual clusters. This scenario is assumed to be able to explain the presence (and properties) of the multiple populations in the majority of globular clusters in the Milky Way. Here we have considered the well studied cluster NGC 2808, which displays a triple main sequence with well defined and separate He abundances. Knowledge of these abundances allowed us to put strong constraints on the He mass fraction and amount of matter to be accreted by low-mass pre-main sequence stars. We find that the minimum He mass fraction in the accr...

  9. A Constraint of Black Hole Mass and the Inner Edge Radius of Relativistic Accretion Disc

    A constraint to black hole (BH) accretion has previously been derived for the inner edge fixed at the innermost stable circular orbit (ISCO) and the innermost bound circular orbit (IBCO). This constraint is referred to as the mass-radius (MR) relation in this study, and the validity of the MR relation is discussed for different cases. It is shown that the product of the BH mass and the inner edge radius decreases monotonically in the accretion process for the inner edge located between IBCO and ISCO. In addition, we discuss the validity of the MR relation by considering the magnetic coupling (MC) effects of a Kerr BH with its surrounding disc. Although theoretically the product of the BH mass and the radius of ISCO increases (decreases) with time for a BH spin greater (less) than some critical value in the MC process, this relation is approximately valid for an Eddington accretion rate persisting for a rather long time, such as more than 106 years. Finally, we discuss the possible application of the MR relation to astrophysics. (geophysics, astronomy, and astrophysics)

  10. The central parsecs of M87: jet emission and an elusive accretion disc

    Prieto, M. A.; Fernández-Ontiveros, J. A.; Markoff, S.; Espada, D.; González-Martín, O.

    2016-04-01

    We present the first simultaneous spectral energy distribution (SED) of M87 core at a scale of 0.4 arcsec ( ˜ 32 pc) across the electromagnetic spectrum. Two separate, quiescent, and active states are sampled that are characterized by a similar featureless SED of power-law form, and that are thus remarkably different from that of a canonical active galactic nuclei or a radiatively inefficient accretion source. We show that the emission from a jet gives an excellent representation of the core of M87 core covering ten orders of magnitude in frequency for both the active and the quiescent phases. The inferred total jet power is, however, one to two orders of magnitude lower than the jet mechanical power reported in the literature. The maximum luminosity of a thin accretion disc allowed by the data yields an accretion rate of < 6 × 10- 5 M⊙ yr- 1, assuming 10 per cent efficiency. This power suffices to explain M87 radiative luminosity at the jet frame, it is however two to three order of magnitude below that required to account for the jet's kinetic power. The simplest explanation is variability, which requires the core power of M87 to have been two to three orders of magnitude higher in the last 200 yr. Alternatively, an extra source of power may derive from black hole spin. Based on the strict upper limit on the accretion rate, such spin power extraction requires an efficiency an order of magnitude higher than predicted from magnetohydrodynamic simulations, currently in the few hundred per cent range.

  11. General Overview of Black Hole Accretion Theory

    Blaes, Omer

    2013-01-01

    I provide a broad overview of the basic theoretical paradigms of black hole accretion flows. Models that make contact with observations continue to be mostly based on the four decade old alpha stress prescription of Shakura & Sunyaev (1973), and I discuss the properties of both radiatively efficient and inefficient models, including their local properties, their expected stability to secular perturbations, and how they might be tied together in global flow geometries. The alpha stress is a prescription for turbulence, for which the only existing plausible candidate is that which develops from the magnetorotational instability (MRI). I therefore also review what is currently known about the local properties of such turbulence, and the physical issues that have been elucidated and that remain uncertain that are relevant for the various alpha-based black hole accretion flow models.

  12. Non-linear evolution of thermally unstable slim accretion discs with a diffusive form of viscosity

    Szuszkiewicz, E; Szuszkiewicz, Ewa; Miller, John C.

    2001-01-01

    We are carrying out a programme of non-linear time-dependent numerical calculations to study the evolution of the thermal instability driven by radiation pressure in transonic accretion discs around black holes. In our previous studies we first investigated the original version of the slim-disc model with low viscosity (parameter alpha = 0.001) for a stellar-mass (10 solar masses) black hole, comparing the behaviour seen with results from local stability analysis (which were broadly confirmed). In some of the unstable models, we saw a violently evolving shock-like feature appearing near to the sonic point. Next, we retained the original model simplifications but considered a higher value of alpha = 0.1 and demonstrated the existence of limit-cycle behaviour under suitable circumstances. The present paper describes more elaborate calculations with a more physical viscosity prescription and including a vertically integrated treatment of acceleration in the vertical direction. Limit-cycle behaviour is still foun...

  13. Line driven winds and the UV turnover in AGN accretion discs

    Laor, Ari

    2013-01-01

    AGN SEDs generally show a turnover at lambda 1000A, implying a maximal accretion disc (AD) temperature of T_max~50,000K. Massive O stars display a similar T_max, associated with a sharp rise in a line driven mass loss Mdot_wind with increasing surface temperature. AGN AD are also characterized by similar surface gravity to massive O stars. The Mdot_wind of O stars reaches ~10^-5 Msun/year. Since the surface area of AGN AD can be 10^6 larger, the implied Mdot_wind in AGN AD can reach the accretion rate Mdot. A rise to Mdot_wind Mdot towards the AD center may therefore set a similar cap of T_max~50,000K. To explore this idea, we solve the radial structure of an AD with a mass loss term, and calculate the implied AD emission using the mass loss term derived from observations of O stars. We find that Mdot_wind becomes comparable to Mdot typically at a few 10s of GM/c^2. Thus, the standard thin AD solution is effectively truncated well outside the innermost stable orbit. The calculated AD SED shows the observed tu...

  14. AcDc - A new code for the NLTE spectral analysis of accretion discs: application to the helium CV AM CVn

    Nagel, T.; Dreizler, S; Rauch, T.; Werner, K.

    2004-01-01

    We present a recently developed code for detailed NLTE calculations of accretion disc spectra of cataclysmic variables and compact X-ray binaries. Assuming a radial structure of a standard alpha-disc, the disc is divided into concentric rings. For each disc ring the solution of the radiation transfer equation and the structure equations, comprising the hydrostatic and radiative equilibrium, the population of the atomic levels as well as charge and particle conservation, is done self-consisten...

  15. Simultaneous independent measurements of a truncated inner accretion disc in the low/hard state of GX 399$-$4

    Plant, D S; Fender, R P

    2014-01-01

    We present results from three recent XMM-Newton observations of GX 339$-$4 in the low/hard state, taken during the decay of a bright (peak $\\sim 0.05$ L$_{\\rm Edd}$) failed outburst. Uniquely, these are the first XMM-Newton EPIC-pn observations of this source using an imaging mode, which significantly enhances the quality of the data at hand. In particular, thanks to the larger available bandpass, this allows an unprecedented constraint of the thermal accretion disc component, and the level of photoelectric absorption. We simultaneously measured the inner radius of the accretion disc via the broadened Fe K$\\alpha$ line and the disc component. The two methods agree, and the measured radii show good consistency over the three epochs. We find that the inner radius is at 20-30 $r_{\\rm g}$, adding to the growing direct evidence for truncation of the inner accretion disc in the low/hard state.

  16. Long-term quasi-periodicity of 4U 1636-536 resulting from accretion disc instability

    Wisniewicz, Mateusz; Gondek-Rosinska, Dorota; Zdziarski, Andrzej A; Janiuk, Agnieszka

    2015-01-01

    We present the results of a study of the low-mass X-ray binary 4U 1636-536. We have performed temporal analysis of all available RXTE/ASM, Swift/BAT and MAXI data. We have confirmed the previously discovered quasi-periodicity of ~45 d present during ~2004, however we found it continued to 2006. At other epochs, the quasi-periodicity is only transient, and the quasi-period, if present, drifts. We have then applied a time-dependent accretion disc model to the interval with the significant X-ray quasi-periodicity. For our best model, the period and the amplitude of the theoretical light curve agree well with that observed. The modelled quasi-periodicity is due to the hydrogen thermal-ionization instability occurring in outer regions of the accretion disc. The model parameters are the average mass accretion rate (estimated from the light curves), and the accretion disc viscosity parameters, for the hot and cold phases. Our best model gives relatively low values of viscosity parameter for cold phase 0.01 and for h...

  17. Comparison of ejection events in the jet and accretion disc outflows in 3C 111

    Tombesi, F.; Sambruna, R. M.; Marscher, A. P.; Jorstad, S. G.; Reynolds, C. S.; Markowitz, A.

    2012-07-01

    We present a comparison of the parameters of accretion disc outflows and the jet of the broad-line radio galaxy 3C 111 on subparsec (sub-pc) scales. We make use of published X-ray observations of ultra-fast outflows (UFOs) and new 43-GHz Very Long Baseline Array images to track the jet knot ejection. We find that the superluminal jet coexists with the mildly relativistic outflows on sub-pc scales, possibly indicating a transverse stratification of a global flow. The two are roughly in pressure equilibrium, with the UFOs potentially providing additional support for the initial jet collimation. The UFOs are much more massive than the jet, but their kinetic power is probably about an order of magnitude lower, at least for the observations considered here. However, their momentum flux is equivalent and both of them are powerful enough to exert a concurrent feedback impact on the surrounding environment. A link between these components is naturally predicted in the context of magnetohydrodynamic models for jet/outflow formation. However, given the high radiation throughput of active galactic nuclei, radiation pressure should also be taken into account. From the comparison with the long-term 2-10 keV Rossi X-ray Timing Explorer light curve, we find that the UFOs are preferentially detected during periods of increasing flux. We also find the possibility to place the UFOs within the known X-ray dips-jet ejection cycles, which has been shown to be a strong proof of the disc-jet connection, in analogue with stellar mass black holes. However, given the limited number of observations presently available, these relations are only tentative and additional spectral monitoring is needed to test them conclusively.

  18. Comparison of Ejection Events in the Jet and Accretion Disc Outflows in 3C 111

    Tombesi, F.; Sambruna, R. M.; Marscher, A. P.; Jorstad, S. G.; Reynolds, C. S.; Markowtiz, A.

    2012-01-01

    We present a comparison of the parameters of accretion disc outflows and the jet of the broad-line radio galaxy 3C 111 on sub-pc scales. We make use of published X-ray observations of ultra-fast outflows (UFOs) and new 43 GHz VLBA images to track the jet knots ejection. We find that the superluminal jet coexists with the mildly relativistic outflows on sub-pc scales, possibly indicating a transverse stratification of a global flow. The two are roughly in pressure equilibrium, with the UFOs potentially providing additional support for the initial jet collimation. The UFOs are much more massive than the jet, but their kinetic power is probably about an order of magnitude lower, at least for the observations considered here. However, their momentum flux is equivalent and both of them are powerful enough to exert a concurrent feedback impact on the surrounding environment. A link between these components is naturally predicted in the context of MHD models for jet/outflow formation. However, given the high radiation throughput of AGNs, radiation pressure should also be taken into account. From the comparison with the long-term 2-10 keV RXTE light curve we find that the UFOs are preferentially detected during periods of increasing flux. We also find the possibility to place the UFOs within the known X-ray dips-jet ejection cycles, which has been shown to be a strong proof of the disc-jet connection, in analogue with stellar-mass black holes. However, given the limited number of observations presently available, these relations are only tentative and additional spectral monitoring is needed to test them conclusively.

  19. Super- and sub-Eddington accreting massive black holes: A comparison of slim and thin accretion discs through study of the spectral energy distribution

    Castello-Mor, Nuria; Kaspi, Shai

    2016-01-01

    We employ optical and UV observations to present SEDs for two reverberation-mapped samples of super-Eddington and sub-Eddington AGN with similar luminosity distributions. The samples are fitted with accretion disc models in order to look for SED differences that depend on the Eddington ratio. The fitting takes into account measured BH mass and accretion rates, BH spin and intrinsic reddening of the sources. All objects in both groups can be fitted by thin AD models over the range 0.2-1$\\,\\mu$m with reddening as a free parameter. The intrinsic reddening required to fit the data are relatively small, $E(B-V)\\leq0.2$~mag, except for one source. Super-Eddington AGN seem to require more reddening. The distribution of $E(B-V)$ is similar to what is observed in larger AGN samples. The best fit disc models for the two groups are very different, with super-Eddington sources require much more luminous far-UV continuum. The exact amount depends on the possible saturation of the UV radiation in slim discs. In particular,...

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

    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...

  1. Comparison of ejection events in the jet and accretion disc outflows in 3C 111

    Tombesi, F; Marscher, A P; Jorstad, S G; Reynolds, C S; Markowitz, A

    2012-01-01

    We present a comparison of the parameters of accretion disc outflows and the jet of the broad-line radio galaxy 3C 111 on sub-pc scales. We make use of published X-ray observations of ultra-fast outflows (UFOs) and new 43GHz VLBA images to track the jet knots ejection. We find that the superluminal jet coexists with the mildly relativistic outflows on sub-pc scales, possibly indicating a transverse stratification of a global flow. The two are roughly in pressure equilibrium, with the UFOs potentially providing additional support for the initial jet collimation. The UFOs are much more massive than the jet, but their kinetic power is probably about an order of magnitude lower, at least for the observations considered here. However, their momentum flux is equivalent and both of them are powerful enough to exert a concurrent feedback impact on the surrounding environment. A link between these components is naturally predicted in the context of MHD models for jet/outflow formation. However, given the high radiatio...

  2. Active galactic nuclei at z ~ 1.5: I. Spectral energy distribution and accretion discs

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

    2014-01-01

    The physics of active super massive black holes (BHs) is governed by their mass (M_BH), spin (a*) and accretion rate ($\\dot{M}$). This work is the first in a series of papers with the aim of testing how these parameters determine the observable attributes of active galactic nuclei (AGN). We have selected a sample in a narrow redshift range, centered on z~1.55, that covers a wide range in M_BH and $\\dot{M}$, and are observing them with X-shooter, covering rest wavelengths ~1200-9800 \\AA. The current work covers 30 such objects and focuses on the origin of the AGN spectral energy distribution (SED). After estimating M_BH and $\\dot{M}$ based on each observed SED, we use thin AD models and a Bayesian analysis to fit the observed SEDs in our sample. We are able to fit 22/30 of the SEDs. Out of the remaining 8 SEDs, 3 can be fit by the thin AD model by correcting the observed SED for reddening within the host galaxy and 4 can be fit by adding a disc wind to the model. In four of these 8 sources, Milky Way-type exti...

  3. Fundamental parameters of the close interacting binary HD170582 and its luminous accretion disc

    Mennickent, R E; Cabezas, M; Cséki, A; G., J Rosales; Niemczura, E; Araya, I; Curé, M

    2015-01-01

    We present a spectroscopic and photometric study of the Double Period Variable HD170582. Based on the study of the ASAS V-band light curve we determine an improved orbital period of 16.87177 $\\pm$ 0.02084 days and a long period of 587 days. We disentangled the light curve into an orbital part, determining ephemerides and revealing orbital ellipsoidal variability with unequal maxima, and a long cycle, showing quasi-sinusoidal changes with amplitude $\\Delta V$= 0.1 mag. Assuming synchronous rotation for the cool stellar component and semi-detached configuration we find a cool evolved star of $M_{2}$ = 1.9 $\\pm$ 0.1 $M_{\\odot}$, $T_{2}$ = 8000 $\\pm$ 100 $K$ and $R_{2}$ = 15.6 $\\pm$ 0.2 $R_{\\odot}$, and an early B-type dwarf of $M_{1}$ = 9.0 $\\pm$ 0.2 $M_{\\odot}$. The B-type star is surrounded by a geometrically and optically thick accretion disc of radial extension 20.8 $\\pm$ 0.3 $R_{\\odot}$ contributing about 35% to the system luminosity at the $V$ band. Two extended regions located at opposite sides of the dis...

  4. An Accretion Disc-Irradiation Hybrid Model for The Optical/UV Variability in Radio-Quiet Quasars

    Liu, Hui; Gu, Minfeng

    2016-01-01

    The optical/ultraviolet (UV) variability of quasars has been discovered to be correlated with other quasar properties, such as luminosity, black hole mass and rest-frame wavelength. However,the origin of variability has been a puzzle so far. In this work, we upgrade the accretion disc model (Li & Cao 2008), which assumed the variability is caused by the change of global mass accretion rate, by constraining the disc size to match the viscous timescale of accretion disc to the variability timescale observed and by including the irradiation/X-ray reprocessing to make the emitted spectrum become steeper. We find this hybrid model can reproduce the observed bluer-when-brighter trend quite well, which is used to validate the theoretical model by several works recently. The traditional correlation between the variability amplitude and rest-frame wavelength can also be well fitted by our model. In addition, a weak positive correlation between variability amplitude and black hole mass is present, qualitatively con...

  5. Accretion disks and dynamos: toward a unified mean field theory

    Conversion of gravitational energy into radiation near stars and compact objects in accretion disks and the origin of large-scale magnetic fields in astrophysical rotators have often 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 disk theory have exemplified such ongoing pursuits. Twenty-first century MFD theory has more nonlinear predictive power compared to 20th century MFD theory, whereas alpha-viscosity accretion theory is still in a 20th century state. In fact, insights from MFD theory are applicable to accretion theory and the two are really artificially separated pieces of what should ultimately be a single coupled theory. I discuss pieces of progress that provide clues toward a unified theory. A key concept is that large-scale magnetic fields can be sustained via local or global magnetic helicity fluxes or via relaxation of small-scale magnetic fluctuations, without appealing to the traditional kinetic helicity driver of 20th century textbooks. These concepts may help explain the formation of large-scale fields that supply non-local angular momentum transport via coronae and jets in a unified theory of accretion and dynamos. In diagnosing the role of helicities and helicity fluxes in disk simulations, it is important to study each disk hemisphere separately to avoid being potentially misled by the cancelation that occurs as a result of reflection asymmetry. The fraction of helical field energy in disks is expected to be small compared to the total field in

  6. The Accretion Disc Particle Method for Simulations of Black Hole Feeding and Feedback

    Power, Chris; Nayakshin, Sergei; King, Andrew

    2010-01-01

    Black holes grow by accreting matter from their surroundings. However, angular momentum provides an efficient natural barrier to accretion and so only the lowest angular momentum material will be available to feed the black holes. The standard sub-grid model for black hole accretion in galaxy formation simulations - based on the Bondi-Hoyle method - does not account for the angular momentum of accreting material, and so it is unclear how representative the black hole accretion rate estimated ...

  7. On the variation of black hole accretion disc radii as a function of state and accretion rate

    Cabanac, C; Dunn, R J H; Koerding, E G

    2009-01-01

    Black hole binary transients undergo dramatic evolution in their X-ray timing and spectral behaviour during outbursts. In recent years a paradigm has arisen in which "soft" X-ray states are associated with an inner disc radius at, or very close to, the innermost stable circular orbit (ISCO) around the black hole, while in "hard" X-ray states the inner edge of the disc is further from the black hole. Models of advective flows suggest that as the X-ray luminosity drops in hard states, the inner disc progressively recedes, from a few to hundreds gravitational radii. Recent observations which show broad iron line detections and estimates of the disc component strength suggest that a non-recessed disc could still be present in bright hard states. In this study we present a comprehensive analysis of the spectral components associated with the inner disc, utilising data from instruments with sensitive low-energy responses and including reanalyses of previously published results. A key component of the study is to fu...

  8. Super- and sub-Eddington accreting massive black holes: a comparison of slim and thin accretion discs through study of the spectral energy distribution

    Castelló-Mor, N.; Netzer, H.; Kaspi, S.

    2016-05-01

    We employ optical and ultraviolet (UV) observations to present spectral energy distributions (SEDs) for two reverberation-mapped samples of super-Eddington and sub-Eddington active galactic nuclei (AGN) with similar luminosity distributions. The samples are fitted with accretion disc (AD) models in order to look for SED differences that depend on the Eddington ratio. The fitting takes into account measured black hole (BH) mass and accretion rates, BH spin and intrinsic reddening of the sources. All objects in both groups can be fitted by thin AD models over the range 0.2-1 μm with reddening as a free parameter. The intrinsic reddening required to fit the data are relatively small, E(B - V) ≤ 0.2 mag, except for one source. Super-Eddington AGN seems to require more reddening. The distribution of E(B - V) is similar to what is observed in larger AGN samples. The best-fitting disc models recover very well the BH mass and accretion for the two groups. However, the SEDs are very different, with super-Eddington sources requiring much more luminous far-UV continuum. The exact amount depends on the possible saturation of the UV radiation in slim discs. In particular, we derive for the super-Eddington sources a typical bolometric correction at 5100 Å of 60-150 compared with a median of ˜20 for the sub-Eddington AGN. The measured torus luminosity relative to λLλ(5100 Å) are similar in both groups. The αOX distribution is similar too. However, we find extremely small torus covering factors for super-Eddington sources, an order of magnitude smaller than those of sub-Eddington AGN. The small differences between the groups regarding the spectral range 0.2-22 μm, and the significant differences related to the part of the SED that we cannot observe may be consistent with some slim disc models. An alternative explanation is that present day slim-disc models overestimate the far-UV luminosity of such objects by a large amount.

  9. Dynamic processes during accretion into a black hole

    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.

  10. Erratum: Thunderclouds and accretion discs: a model for the spectral and temporal variability of Seyfert 1 galaxies

    Merloni, A.; Fabian, A. C.

    2002-03-01

    X-ray observations of Seyfert 1 galaxies offer the unique possibility of observing spectral variability on timescales comparable to the dynamical time of the inner accretion flow. They typically show highly variable lightcurves, with Power Density Spectra characterized by `red noise' and a break at low frequencies. Time resolved spectral analysis have established that spectral variability on the shortest timescales is important in all these sources, with the spectra getting softer at high fluxes. Here we present a model that is able to explain a number of the above mentioned properties in terms of magnetic flares shining above a standard accretion disc and producing the X-ray spectrum via inverse Compton scattering soft photons (both intrinsic and reprocessed thermal emission from the accretion disc and locally produced synchrotron radiation). We show that the fundamental heating event, likely caused by magnetic reconnection, must be compact, with typical size comparable to the accretion disc thickness and must be triggered at a height at least an order of magnitude larger than its size; the spatial and temporal distribution of flares are not random: the heating of the corona proceeds in correlated trains of events in an avalanche fashion. The amplitude of the avalanches obeys a power-law distribution and determines the size of the active regions where the spectrum is produced. With our model we simulate X-ray lightcurves that reproduce the main observational properties of the Power Density Spectra and of the X-ray continuum short-term variability of Seyfert 1 galaxies. By comparing them with observations of MGC--6-30-15, we are able to infer that the corona in this source must have a large optical depth (tau >1.5) and small average covering fraction.

  11. Accretion disc dynamo activity in local simulations spanning weak-to-strong net vertical magnetic flux regimes

    Salvesen, Greg; Armitage, Philip J; Begelman, Mitchell C

    2015-01-01

    Strongly magnetized accretion discs around black holes have attractive features that may explain enigmatic aspects of X-ray binary behaviour. The structure and evolution of these discs are governed by a dynamo-like mechanism, which channels part of the accretion power liberated by the magnetorotational instability (MRI) into an ordered toroidal magnetic field. To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations. The strength of the self-sustained toroidal magnetic field depends on the net vertical magnetic flux, which we vary across almost the entire range over which the MRI is linearly unstable. We quantify disc structure and dynamo properties as a function of the initial ratio of mid-plane gas pressure to vertical magnetic field pressure, $\\beta_0^{\\rm mid} = p_{\\rm gas} / p_B$. For $10^5 \\geq \\beta_0^{\\rm mid} \\geq 10$ the effective $\\alpha$-viscosity parameter scales as a power-law. Dynamo activity persists up to and includin...

  12. Bending instability in galactic discs. Advocacy of the linear theory

    Rodionov, S A

    2013-01-01

    We demonstrate that in N-body simulations of isolated disc galaxies there is numerical vertical heating which slowly increases the vertical velocity dispersion and the disc thickness. Even for models with over a million particles in a disc, this heating can be significant. Such an effect is just the same as in numerical experiments by Sellwood (2013). We also show that in a stellar disc, outside a boxy/peanut bulge, if it presents, the saturation level of the bending instability is rather close to the value predicted by the linear theory. We pay attention to the fact that the bending instability develops and decays very fast, so it couldn't play any role in secular vertical heating. However the bending instability defines the minimal value of the ratio between the vertical and radial velocity dispersions $\\sigma_z / \\sigma_R \\approx 0.3$ (so indirectly the minimal thickness) which could have stellar discs in real galaxies. We demonstrate that observations confirm last statement.

  13. Phantom energy accretion and primordial black holes evolution in Brans-Dicke theory

    Nayak, B; Singh, L. P.

    2011-01-01

    In this work, we study the evolution of primordial black holes within the context of Brans-Dicke theory by considering the presence of a dark energy component with a super-negative equation of state called phantom energy as a background. Besides Hawking evaporation, here we consider two type of accretions - radiation accretion and phantom energy accretion. We found that radiation accretion increases the lifetime of primordial black holes whereas phantom accretion decreases the lifespan of pri...

  14. Motivation and challenge to capture both large-scale and local transport in next generation accretion theory

    Blackman, Eric G.; Nauman, Farrukh

    2015-10-01

    > Accretion disc theory is less developed than stellar evolution theory although a similarly mature phenomenological picture is ultimately desired. While the interplay of theory and numerical simulations has amplified community awareness of the role of magnetic fields in angular momentum transport, there remains a long term challenge to incorporate the insights gained from simulations into improving practical models for comparison with observations. What has been learned from simulations that can lead to improvements beyond SS73 in practical models? Here, we emphasize the need to incorporate the role of non-local transport more precisely. To show where large-scale transport would fit into the theoretical framework and how it is currently missing, we review why the wonderfully practical approach of Shakura & Sunyaev (Astron. Astrophys., vol. 24, 1973, pp. 337-355, SS73) is necessarily a mean field theory, and one which does not include large-scale transport. Observations of coronae and jets, combined with the interpretation of results from shearing box simulations, of the magnetorotational instability (MRI) suggest that a significant fraction of disc transport is indeed non-local. We show that the Maxwell stresses in saturation are dominated by large-scale contributions and that the physics of MRI transport is not fully captured by a viscosity. We also clarify the standard physical interpretation of the MRI as it applies to shearing boxes. Computational limitations have so far focused most attention toward local simulations, but the next generation of global simulations should help to inform improved mean field theories. Mean field accretion theory and mean field dynamo theory should in fact be unified into a single theory that predicts the time evolution of spectra and luminosity from separate disc, corona and outflow contributions. Finally, we note that any mean field theory, including that of SS73, has a finite predictive precision that needs to be quantified

  15. Multidimensional modelling of X-ray spectra for AGN accretion disc outflows - III. Application to a hydrodynamical simulation

    Sim, S. A.; Proga, D.; Miller, L.; Long, K. S.; Turner, T. J.

    2010-11-01

    We perform multidimensional radiative transfer simulations to compute spectra for a hydrodynamical simulation of a line-driven accretion disc wind from an active galactic nucleus. The synthetic spectra confirm expectations from parametrized models that a disc wind can imprint a wide variety of spectroscopic signatures including narrow absorption lines, broad emission lines and a Compton hump. The formation of these features is complex with contributions originating from many of the different structures present in the hydrodynamical simulation. In particular, spectral features are shaped both by gas in a successfully launched outflow and in complex flows where material is lifted out of the disc plane but ultimately falls back. We also confirm that the strong Fe Kα line can develop a weak, red-skewed line wing as a result of Compton scattering in the outflow. In addition, we demonstrate that X-ray radiation scattered and reprocessed in the flow has a pivotal part in both the spectrum formation and determining the ionization conditions in the wind. We find that scattered radiation is rather effective in ionizing gas which is shielded from direct irradiation from the central source. This effect likely makes the successful launching of a massive disc wind somewhat more challenging and should be considered in future wind simulations.

  16. Convergence of SPH simulations of self-gravitating accretion discs: Sensitivity to the implementation of radiative cooling

    Rice, W K M; Armitage, P J

    2011-01-01

    Recent simulations of self-gravitating accretion discs, carried out using a three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate, have been interpreted as implying that three-dimensional global discs fragment much more easily than would be expected from a two-dimensional local model. Subsequently, global and local two-dimensional models have been shown to display similar fragmentation properties, leaving it unclear whether the three-dimensional results reflect a physical effect or a numerical problem associated with the treatment of cooling or artificial viscosity in SPH. Here, we study how fragmentation of self-gravitating disc flows in SPH depends upon the implementation of cooling. We run disc simulations that compare a simple cooling scheme, in which each particle loses energy based upon its internal energy per unit mass, with a method in which the cooling is derived from a smoothed internal energy density field. For the simple per particle cooling scheme, we find a significant in...

  17. Accretion disc/corona emission from a radio-loud narrow-line Seyfert 1 galaxy PKS 0558-504

    Ghosh, R.; Dewangan, G. C.; Raychaudhuri, B.

    2016-02-01

    Approximately 10-20 per cent of active galactic nuclei (AGN) are known to eject powerful jets from the innermost regions. There is very little observational evidence if the jets are powered by spinning black holes and if the accretion discs extend to the innermost regions in radio-loud AGN. Here, we study the soft X-ray excess, the hard X-ray spectrum and the optical/UV emission from the radio-loud narrow-line Seyfert 1 galaxy PKS 0558-504 using Suzaku and Swift observations. The broad-band X-ray continuum of PKS 0558-504 consists of a soft X-ray excess emission below 2 keV that is well described by a blackbody (kT ˜ 0.13 keV) and high-energy emission that is well described by a thermal Comptonization (compps) model with kTe ˜ 250 keV, optical depth τ ˜ 0.05 (spherical corona) or kTe ˜ 90 keV, τ ˜ 0.5 (slab corona). The Comptonizing corona in PKS 0558-504 is likely hotter than in radio-quiet Seyferts such as IC 4329A and Swift J2127.4+5654. The observed soft X-ray excess can be modelled as blurred reflection from an ionized accretion disc or optically thick thermal Comptonization in a low-temperature plasma. Both the soft X-ray excess emission when interpreted as the blurred reflection and the optical/UV to soft X-ray emission interpreted as intrinsic disc Comptonized emission implies spinning (a > 0.6) black hole. These results suggest that disc truncation at large radii and retrograde black hole spin both are unlikely to be the necessary conditions for launching the jets.

  18. SOAR observations of the high-viscosity accretion disc of the dwarf nova V4140 Sagitarii in quiescence and in outburst

    Baptista, Raymundo; Oliveira, Alexandre S

    2016-01-01

    We report the analysis of 22 B-band light curves of the dwarf nova V4140 Sgr obtained with SOI/SOAR during two nights along the decline of a superoutburst in 2006 Sep 12-24 and in quiescence over 50 days following the superoutburst. Three-dimensional eclipse mapping of the outburst light curves indicates that the accretion disc is elliptical (eccentricity e=0.13) and that superhump maximum occurs when the mass donor star is aligned with the bulge of the elliptical disc. The accretion disc is geometrically thin both in outburst and in quiescence; it fills the primary Roche lobe in outburst and shrinks to about half this size in quiescence. The stability of the eclipse shape, width and depth along quiescence and the derived disc surface brightness distribution indicate that the quiescent accretion disc is in a high-viscosity, steady-state. Flickering mapping of the quiescent data reveal that the low-frequency flickering arises from an azimuthally-extended stream-disc impact region at disc rim and from the inner...

  19. Free-floating planets from core accretion theory: microlensing predictions

    Ma, Sizheng; Ida, Shigeru; Zhu, Wei; Lin, Douglas N C

    2016-01-01

    We calculate the microlensing event rate and typical time-scales for the free-floating planet (FFP) population that is predicted by the core accretion theory of planet formation. The event rate is found to be ~$1.8\\times 10^{-3}$ of that for the stellar population. While the stellar microlensing event time-scale peaks at around 20 days, the median time-scale for FFP events (~0.1 day) is much shorter. Our values for the event rate and the median time-scale are significantly smaller than those required to explain the \\cite{Sum+11} result, by factors of ~13 and ~16, respectively. The inclusion of planets at wide separations does not change the results significantly. This discrepancy may be too significant for standard versions of both the core accretion theory and the gravitational instability model to explain satisfactorily. Therefore, either a modification to the planet formation theory is required, or other explanations to the excess of short-time-scale microlensing events are needed. Our predictions can be t...

  20. Convergence of smoothed particle hydrodynamics simulations of self-gravitating accretion discs: sensitivity to the implementation of radiative cooling

    Rice, W. K. M.; Forgan, D. H.; Armitage, P. J.

    2012-02-01

    Recent simulations of self-gravitating accretion discs, carried out using a three-dimensional smoothed particle hydrodynamics (SPH) code by Meru & Bate, have been interpreted as implying that three-dimensional global discs fragment much more easily than would be expected from a two-dimensional local model. Subsequently, global and local two-dimensional models have been shown to display similar fragmentation properties, leaving it unclear whether the three-dimensional results reflect a physical effect or a numerical problem associated with the treatment of cooling or artificial viscosity in SPH. Here, we study how fragmentation of self-gravitating disc flows in SPH depends upon the implementation of cooling. We run disc simulations that compare a simple cooling scheme, in which each particle loses energy based upon its internal energy per unit mass, with a method in which the cooling is derived from a smoothed internal energy density field. For the simple per particle cooling scheme, we find a significant increase in the minimum cooling time-scale for fragmentation with increasing resolution, matching previous results. Switching to smoothed cooling, however, results in lower critical cooling time-scales, and tentative evidence for convergence at the highest spatial resolution tested. We conclude that precision studies of fragmentation using SPH require careful consideration of how cooling (and, probably, artificial viscosity) is implemented, and that the apparent non-convergence of the fragmentation boundary seen in prior simulations is likely a numerical effect. In real discs, where cooling is physically smoothed by radiative transfer effects, the fragmentation boundary is probably displaced from the two-dimensional value by a factor that is only of the order of unity.

  1. Corotational Instability of Inertial-Acoustic Modes in Black Hole Accretion Discs and Quasi-Periodic Oscillations

    Lai, Dong

    2008-01-01

    We study the global stability of non-axisymmetric p-modes (also called inertial-acoustic modes) trapped in the inner-most regions of accretion discs around black holes. We show that the lowest-order (highest-frequency) p-modes, with frequencies $\\omega=(0.5-0.7) m\\Omega_{\\rm ISCO}$, can be overstable due to general relativistic effects, according to which the radial epicyclic frequency is a non-monotonic function of radius near the black hole. The mode is trapped inside the corotation resonance radius and carries a negative energy. The mode growth arises primarily from wave absorption at the corotation resonance, and the sign of the wave absorption depends on the gradient of the disc vortensity. When the mode frequency is sufficiently high, such that the slope of the vortensity is positive at corotation positive wave energy is absorbed at the resonance, leading to the growth of mode amplitude. We also study how the rapid radial inflow at the inner edge of the disc affects the mode trapping and growth. Our ana...

  2. The response of an accretion disc to an inclined dipole with application to $AA \\Tau$

    Terquem, C; Terquem, Caroline; Papaloizou, John C. B.

    2000-01-01

    We compute the warping of a disc induced by an inclined dipole. We consider a magnetised star surrounded by a thin Keplerian diamagnetic disc with an inner edge that corotates with the star. We suppose the stellar field is a dipole with an axis that is slightly misaligned with the stellar rotation axis. The rotation axes of the disc material orbiting at large distances from the star and that of the star are supposed to coincide. The misalignment of the magnetic and rotation axes results in the magnetic pressure not being the same on the upper and lower surfaces of the disc. The resultant net vertical force produces a warp which appears stationary in a frame corotating with the star. We find that, if viscosity is large enough (alpha on the order of 0.01-0.1) to damp bending waves as they propagate away, a smoothly varying warp of the inner region of the disc is produced. The amplitude of the warp can easily be on the order of ten percent of the disc inner radius for reasonably small misalignment angles (less t...

  3. The imprint of satellite accretion on the chemical and dynamical properties of disc galaxies

    Ruiz-Lara, T; Gibson, B K; Pérez, I; Florido, E; Minchev, I; Sánchez-Blázquez, P

    2015-01-01

    Aims: We study the effects of the cosmological assembly history on the chemical and dynamical properties of the discs of spiral galaxies as a function of radius. Methods: We make use of the simulated Milky-Way mass, fully-cosmological discs, from {\\tt RaDES} (Ramses Disc Environment Study). We analyse their assembly history by examining the proximity of satellites to the galactic disc, instead of their merger trees, to better gauge which satellites impact the disc. We present stellar age and metallicity profiles, Age-Metallicity Relation (AMR), Age-Velocity dispersion Relation (AVR), and Stellar Age Distribution (SAD) in several radial bins for the simulated galaxies. Results: Assembly histories can be divided into three different stages: i) a merger dominated phase, when a large number of mergers with mass ratios of $\\sim$1:1 take place (lasting $\\sim$3.2$\\pm$0.4 Gyr on average); ii) a quieter phase, when $\\sim$1:10 mergers take place (lasting $\\sim$4.4$\\pm$2.0 Gyr) - these two phases are able to kinematical...

  4. Wind-driving protostellar accretion discs. II. Numerical method and illustrative solutions

    Salmeron, R; Wardle, M

    2010-01-01

    (abridged) We continue our study of weakly ionized protostellar discs that are threaded by a large-scale magnetic field and power a centrifugally driven wind. It has been argued that in several protostellar systems such a wind transports a significant fraction of the angular momentum from at least some part of the disc. We model this case by considering a radially localized disc model in which the matter is well coupled to the field and the wind is the main repository of excess angular momentum. We consider stationary solutions in which magnetic diffusion counters the shearing and advection of the field lines. In Wardle & K\\"onigl we analysed the disc structure in the hydrostatic approximation and presented disc/wind solutions for the ambipolar diffusivity regime. In K\\"onigl, Salmeron & Wardle (Paper I) we generalized the hydrostatic analysis to the Hall and Ohm diffusivity domains and identified the parameter sub-regimes in which viable solutions occur. In this paper we test these results by derivin...

  5. The excitation of inertial-acoustic waves through turbulent fluctuations in accretion discs II: Numerical Simulations with MRI driven turbulence

    Heinemann, T

    2008-01-01

    We present fully three-dimensional local simulations of compressible MRI turbulence with the object of studying and elucidating the excitation of the non-axisymmetric inertial acoustic waves that are observed to always be present. They are potentially important for affecting protoplanetary migration through the action of associated stochastic gravitational forces and producing residual transport in MHD inactive regions into which they may propagate. The simulations we perform are with zero net flux and produce mean activity levels corresponding to the Shakura & Sunyaev alpha ~ 0.005, being at the lower end of the range usually considered in accretion disc modelling. We reveal the nature of the mechanism responsible for the excitation of these waves by determining the time dependent evolution of the Fourier transforms of the participating state variables. The dominant waves are found to have no vertical structure and to be excited during periodically repeating swings in which they change from leading to tr...

  6. On the Interplay Between Radial and Angular Reflection Emissivity from the Black Hole Accretion Disc

    Svoboda, Jiří; Dovčiak, Michal; Goosmann, René; Karas, Vladimír

    Cham: Springer International Publishing, 2014 - (Bičák, J.; Ledvinka, T.), s. 415-422. (Springer Proceedings in Physics. 157). ISBN 978-3-319-06760-5. ISSN 0930-8989. [100 Years after Einstein in Prague. Prague (CZ), 25.06.2012-29.06.2012] Institutional support: RVO:67985815 Keywords : black holes * accretion disks Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  7. Nonlinear hydrodynamical evolution of eccentric Keplerian discs in two dimensions: validation of secular theory

    Barker, Adrian J

    2016-01-01

    We perform global two-dimensional hydrodynamical simulations of Keplerian discs with free eccentricity over thousands of orbital periods. Our aim is to determine the validity of secular theory in describing the evolution of eccentric discs, and to explore their nonlinear evolution for moderate eccentricities. Linear secular theory is found to correctly predict the structure and precession rates of discs with small eccentricities. However, discs with larger eccentricities (and eccentricity gradients) are observed to precess faster (retrograde relative to the orbital motion), at a rate that depends on their eccentricities (and eccentricity gradients). We derive analytically a nonlinear secular theory for eccentric gas discs, which explains this result as a modification of the pressure forces whenever eccentric orbits in a disc nearly intersect. This effect could be particularly important for highly eccentric discs produced in tidal disruption events, or for narrow gaseous rings; it might also play a role in cau...

  8. Theory of disk accretion onto supermassive black holes

    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.

  9. Transient accretion disc-like envelope in the symbiotic binary BF Cygni during its 2006 - 2015 optical outburst

    Tomov, N A; Bisikalo, D V

    2015-01-01

    The optical light of the symbiotic binary BF Cygni during its last eruption after 2006 shows orbital variations because of an eclipse of the outbursting compact object. The first orbital minimum is deeper than the following ones. Moreover, the Balmer profiles of this system acquired additional satellite components indicating bipolar collimated outflow at one time between the first and second orbital minima. This behaviour is interpreted in the framework of the model of collimated stellar wind from the outbursting object. It is supposed that one extended disc-like envelope covering the accretion disc of the compact object and collimating its stellar wind forms in the period between the first and second minima. The uneclipsed part of this envelope is responsible for the decrease of the depth of the orbital minimum. The calculated $UBVR_{C}I_{C}$ fluxes of this uneclipsed part are in agreement with the observed residual of the depths of the first and second orbital minima. The parameters of the envelope require ...

  10. X-ray reflection in oxygen-rich accretion discs of ultra-compact X-ray binaries

    Madej, O K; Jonker, P G; Parker, M L; Ross, R; Fabian, A C; Chenevez, J

    2014-01-01

    We present spectroscopic X-ray data of two candidate ultra-compact X-ray binaries: 4U~0614+091 and 4U~1543$-$624. We confirm the presence of a broad O VIII Ly$\\alpha$ reflection line (at $\\approx18\\ \\AA$) using {\\it XMM-Newton} and {\\it Chandra} observations obtained in 2012 and 2013. The donor star in these sources is carbon-oxygen or oxygen-neon-magnesium white dwarf. Hence, the accretion disc is enriched with oxygen which makes the O VIII Ly$\\alpha$ line particularly strong. We also confirm the presence of a strong absorption edge at $\\approx14$ \\AA\\ so far interpreted in the literature as due to absorption by neutral neon in the circumstellar and interstellar medium. However, the abundance required to obtain a good fit to this edge is $\\approx3-4$ times solar, posing a problem for this interpretation. Furthermore, modeling the X-ray reflection off a carbon and oxygen enriched, hydrogen and helium poor disc with models assuming solar composition likely biases several of the best-fit parameters. In order to...

  11. Time-dependent models of accretion discs with nuclear burning following the tidal disruption of a white dwarf by a neutron star

    Margalit, Ben; Metzger, Brian D.

    2016-09-01

    We construct time-dependent one-dimensional (vertically averaged) models of accretion discs produced by the tidal disruption of a white dwarf (WD) by a binary neutron star (NS) companion. Nuclear reactions in the disc mid-plane burn the WD matter to increasingly heavier elements at sequentially smaller radii, releasing substantial energy which can impact the disc dynamics. A model for disc outflows is employed, by which cooling from the outflow balances other sources of heating (viscous, nuclear) in regulating the Bernoulli parameter of the mid-plane to a fixed value ≲0. We perform a comprehensive parameter study of the compositional yields and velocity distributions of the disc outflows for WDs of different initial compositions. For C/O WDs, the radial composition profile of the disc evolves self-similarly in a quasi-steady-state manner, and is remarkably robust to model parameters. The nucleosynthesis in helium WD discs does not exhibit this behaviour, which instead depends sensitively on factors controlling the disc mid-plane density (e.g. the strength of the viscosity, α). By the end of the simulation, a substantial fraction of the WD mass is unbound in outflows at characteristic velocities of ˜109 cm s-1. The outflows from WD-NS merger discs contain 10-4-3 × 10-3 M⊙ of radioactive 56Ni, resulting in fast (˜ week long) dim (˜1040 erg s-1) optical transients; shock heating of the ejecta by late-time outflows may increase the peak luminosity to ˜1043 erg s-1. The accreted mass on to the NS is probably not sufficient to induce gravitational collapse, but may be capable of spinning up the NS to periods of ˜10 ms, illustrating the feasibility of this channel in forming isolated millisecond pulsars.

  12. Accretion disc-corona and jet emission from the radio-loud narrow-line Seyfert 1 galaxy RX J1633.3+4719

    Mallick, Labani; Dewangan, G. C.; Gandhi, P.; Misra, R.; Kembhavi, A. K.

    2016-08-01

    We perform X-ray/ultraviolet (UV) spectral and X-ray variability studies of the radio-loud narrow-line Seyfert 1 (NLS1) galaxy RX J1633.3+4719 using XMM-Newton and Suzaku observations from 2011 and 2012. The 0.3-10 keV spectra consist of an ultrasoft component described by an accretion disc blackbody (kT_in = 39.6^{+11.2}_{-5.5} eV) and a power law due to the thermal Comptonization (Γ = 1.96^{+0.24}_{-0.31}) of the disc emission. The disc temperature inferred from the soft excess is at least a factor of 2 lower than that found for the canonical soft excess emission from radio-quiet NLS1s. The UV spectrum is described by a power law with photon index 3.05^{+0.56}_{-0.33}. The observed UV emission is too strong to arise from the accretion disc or the host galaxy, but can be attributed to a jet. The X-ray emission from RX J1633.3+4719 is variable with fractional variability amplitude Fvar = 13.5 ± 1.0 per cent. In contrast to radio-quiet active galactic nuclei (AGN), X-ray emission from the source becomes harder with increasing flux. The fractional rms variability increases with energy and the rms spectrum is well described by a constant disc component and a variable power-law continuum with the normalization and photon index being anticorrelated. Such spectral variability cannot be caused by variations in the absorption and must be intrinsic to the hot corona. Our finding of possible evidence for emission from the inner accretion disc, jet and hot corona from RX J1633.3+4719 in the optical to X-ray bands makes this object an ideal target to probe the disc-jet connection in AGN.

  13. Accretion disc-corona and jet emission from the radio-loud Narrow Line Seyfert 1 galaxy RXJ1633.3+4719

    Mallick, Labani; Dewangan, G. C.; Gandhi, P.; Misra, R.; Kembhavi, A. K.

    2016-05-01

    We perform X-ray/UV spectral and X-ray variability studies of the radio-loud Narrow Line Seyfert 1 (NLS1) galaxy RXJ1633.3+4719 using XMM-Newton and Suzaku observations from 2011 and 2012. The 0.3-10 keV spectra consist of an ultra-soft component described by an accretion disc blackbody (kT_in = 39.6^{+11.2}_{-5.5} eV) and a power-law due to the thermal Comptonization (Γ =1.96^{+0.24}_{-0.31}) of the disc emission. The disc temperature inferred from the soft excess is at least a factor of two lower than that found for the canonical soft excess emission from radio-quiet NLS1s. The UV spectrum is described by a power-law with photon index 3.05^{+0.56}_{-0.33}. The observed UV emission is too strong to arise from the accretion disc or the host galaxy, but can be attributed to a jet. The X-ray emission from RXJ1633.3+4719 is variable with fractional variability amplitude Fvar=13.5±1.0%. In contrast to radio-quiet AGN, X-ray emission from the source becomes harder with increasing flux. The fractional RMS variability increases with energy and the RMS spectrum is well described by a constant disc component and a variable power-law continuum with the normalization and photon index being anti-correlated. Such spectral variability cannot be caused by variations in the absorption and must be intrinsic to the hot corona. Our finding of possible evidence for emission from the inner accretion disc, jet and hot corona from RXJ1633.3+4719 in the optical to X-ray bands makes this object an ideal target to probe the disc-jet connection in AGN.

  14. Testing black hole neutrino-dominated accretion discs for long-duration gamma-ray bursts

    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.

  15. Testing black hole neutrino-dominated accretion discs for long-duration gamma-ray bursts

    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.

  16. Theory of disk accretion onto supermassive black holes

    Armitage, Philip 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 ...

  17. On the mechanism of self gravitating Rossby interfacial waves in proto-stellar accretion discs

    Yellin-Bergovoy, Ron; Heifetz, Eyal; Umurhan, Orkan M.

    2015-01-01

    The dynamical response of edge waves under the influence of self-gravity is examined in an idealized two-dimensional model of a proto-stellar disc, characterized in steady state as a rotating vertically infinite cylinder of fluid with constant density except for a single density interface at some radius r0. The fluid in basic state is prescribed to rotate with a Keplerian profile $\\Omega_k(r)\\sim r^{-3/2}$ modified by some additional azimuthal sheared flow. A linear analysis shows that there ...

  18. A study of accretion discs around rapidly rotating neutron stars in general relativity and its applications to four low mass X-ray binaries

    Bhattacharyya, Sudip

    2002-02-01

    We calculate the accretion disc temperature profiles, disc luminosities and boundary layer luminosities for rapidly rotating neutron stars considering the full effect of general relativity. We compare the theoretical values of these quantities with their values inferred from EXOSAT data for four low mass X-ray binary sources: XB 1820-30, GX 17+2, GX 9+1 and GX 349+2 and constrain the values of several properties of these sources. According to our calculations, the neutron stars in GX 9+1 and GX 349+2 are rapidly rotating and stiffer equations of state are unfavoured.

  19. Accretion theory for 'spontaneous' rotation of toroidal plasmas

    The accretion theory of spontaneous toroidal rotation connects this phenomenon to the energy and particle transport properties of the plasma column and to the relevant collective modes. The consequent prediction that an inversion of the velocity direction in the transition from the H to the L regime should occur has been verified by the experiments. The theory is consistent also with the observation that the velocity is depressed when a peaked density profile appears as a result of a transport barrier. The fact that the rotation velocity increases with the total energy content is explained by the fact that the inflow of angular momentum, whose source is at the edge of the plasma column, results from the excitation of modes driven by the plasma pressure gradient. A quasi-linear derivation of the angular momentum transport produced by these modes, whose novel feature is the inflow, is given and a model of the relevant (transport) equation is solved. Fluctuations at the edge of the plasma column are considered responsible for the scattering, out of confinement, of particles that transfer, to the surrounding material wall, angular momentum in the same direction as that of the phase velocity of the prevalent modes. Thus the fact that the plasma rotates in the direction of the ion diamagnetic velocity in the H-regime, when the prevalent modes are expected to have phase velocity in the direction of the electron diamagnetic velocity, can be explained and the rate of rotation decrease, as the plasma current is increased, can be justified. (author)

  20. Accretion theory of 'spontaneous' rotation in toroidal plasmas

    The accretion theory of 'spontaneous' toroidal rotation connects this phenomenon to the energy and particle transport properties of the plasma column and to the relevant collective modes. The consequent prediction that an inversion of the velocity direction in the transition from the H to the L regime should occur has been verified by the experiments. The theory is consistent also with the observation that the velocity is depressed when a peaked density profile appears as a result of a transport barrier. The fact that the rotation velocity increases with the total energy content is explained by the fact that the inflow of angular momentum, whose source is at the edge of the plasma column, results from the excitation of modes driven by the plasma pressure gradient. A quasi-linear derivation of the angular momentum transport produced by these modes, whose novel feature is the inflow, is given. A model of the relevant (transport) equation that can be easily solved is discussed. Fluctuations at the edge of the plasma column are considered responsible for the scattering out of confinement of particles that transfer to the surrounding material wall angular momentum in the same direction as that of the phase velocity of the prevalent modes. Thus the fact that the plasma rotates in the direction of the ion diamagnetic velocity in the H regime, when the prevalent modes are expected to have phase velocity in the direction of the electron diamagnetic velocity, can be explained. The rate of rotation decrease observed when the plasma current is increased is also justified. (author)

  1. On the warping of Be star discs

    Porter, John M.

    1998-01-01

    The theory of radiatively-induced warps in accretion discs is applied to the discs of Be stars. It is found that these discs may develop warps in their inner regions, although once the warp amplitude is large enough then the interaction between the disc and fast radiatively-driven wind will determine its evolution. The warping is shown to be more important for later than earlier B stars. Although the interaction of the fast-wind with the disc will limit the amplitude of the warp, it cannot dr...

  2. Infalling clouds onto super-massive black hole binaries - I. Formation of discs, accretion and gas dynamics

    Goicovic, F G; Sesana, A; Stasyszyn, F; Amaro-Seoane, P; Tanaka, T L

    2015-01-01

    There is compelling evidence that most -if not all- galaxies harbour a super-massive black hole (SMBH) at their nucleus, hence binaries of these massive objects are an inevitable product of the hierarchical evolution of structures in the universe, and represent an important but thus-far elusive phase of galaxy evolution. Gas accretion via a circumbinary disc is thought to be important for the dynamical evolution of SMBH binaries, as well as in producing luminous emission that can be used to infer their properties. One plausible source of the gaseous fuel is clumps of gas formed due to turbulence and gravitational instabilities in the interstellar medium, that later fall toward and interact with the binary. In this context, we model numerically the evolution of turbulent clouds in near-radial infall onto equal-mass SMBH binaries, using a modified version of the SPH code GADGET-3. We present a total of 12 simulations that explore different possible pericentre distances and relative inclinations, and show that t...

  3. Irradiation of an Accretion Disc by a Jet: General Properties and Implications for Spin Measurements of Black Holes

    T.Dauser; Garcia, J.; Wilms, J.; Boeck, M.; Brenneman, L. W.; Falanga, M.; Fukumura, Keigo; Reynolds, C. S.

    2013-01-01

    X-ray irradiation of the accretion disc leads to strong reflection features, which are then broadened and distorted by relativistic effects. We present a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source. These geometries include the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating. Incorporating this code in the RELLINE model for relativistic line emission, the line shape for any configuration can be predicted. We study how different irradiation geometries affect the determination of the spin of the black hole. Broad emission lines are produced only for compact irradiating sources situated close to the black hole. This is the only case where the black hole spin can be unambiguously determined. In all other cases the line shape is narrower, which could either be explained by a low spin or an elongated source. We conclude that for those cases and independent of the quality of the data, no unique solution for the spin exists and therefore only a lower limit of the spin value can be given

  4. On the mechanism of self gravitating Rossby interfacial waves in proto-stellar accretion discs

    Yellin-Bergovoy, Ron; Umurhan, Orkan M

    2015-01-01

    The dynamical response of edge waves under the influence of self-gravity is examined in an idealized two-dimensional model of a proto-stellar disc, characterized in steady state as a rotating vertically infinite cylinder of fluid with constant density except for a single density interface at some radius r0. The fluid in basic state is prescribed to rotate with a Keplerian profile $\\Omega_k(r)\\sim r^{-3/2}$ modified by some additional azimuthal sheared flow. A linear analysis shows that there are two azimuthally propagating edge waves, kin to the familiar Rossby waves and surface gravity waves in terrestrial studies, which move opposite to one another with respect to the local basic state rotation rate at the interface. Instability only occurs if the radial pressure gradient is opposite to that of the density jump (unstably stratified) where self-gravity acts as a wave stabilizer irrespective of the stratification of the system. The propagation properties of the waves are discussed in detail in the language of...

  5. A Toy Model for Magnetic Extraction of Energy from Black Hole Accretion Disc

    Wang, D X; Ma, R Y; Wang, Ding-Xiong; Ye, Yong-Cun; Ma, Ren-Yi

    2004-01-01

    A toy model for magnetic extraction of energy from black hole (BH) accretion disk is discussed by considering the restriction of the screw instability to the magnetic field configuration. Three mechanisms of extracting energy magnetically are involved. (1) The Blandford-Znajek (BZ) process is related to the open magnetic field lines connecting the BH with the astrophysical load; (2) the magnetic coupling (MC) process is related to the closed magnetic field lines connecting the BH with its surrounding disk; and (3) a new scenario (henceforth the DL process) for extracting rotational energy from the disk is related to the open field lines connecting the disk with the astrophysical load. The expressions for the electromagnetic powers and torques are derived by using the equivalent circuits corresponding to the above energy mechanisms. It turns out that the DL power is comparable with the BZ and MC powers as the BH spin approaches unity. The radiation from a quasi-steady thin disk is discussed in detail by applyi...

  6. The central parsecs of M87: jet emission and an elusive accretion disc

    Prieto, M A; Markoff, S; Espada, D; Gonzalez-Martin, O

    2015-01-01

    We present the first time-simultaneous high angular resolution spectral energy distribution (SED) of the core of M87 at a scale of 0.4 arcsecs across the electromagnetic spectrum. Two activity periods of the core of M87 are sampled: a quiescent mode, representative of the most common state of M87, and an active one, represented by the outburst occurring in 2005. The main difference between both SEDs is a shift in flux in the active SED by a factor of about two, their shapes remaining similar across the entire spectrum. The shape of the compiled SEDs is remarkably different from those of active galactic nuclei (AGN). It lacks three major AGN features: the IR bump, the inflection point at about 1 micron and the blue bump. The SEDs also differ from the spectrum of a radiatively inefficient accretion flow. Down to the scales of ~12 pc from the centre, we find that the emission from a jet gives an excellent representation of the spectrum over ten orders of magnitude in frequency for both the active and the quiesce...

  7. Accretion disc-corona and jet emission from the radio-loud Narrow Line Seyfert 1 galaxy RXJ1633.3+4719

    Mallick, Labani; Gandhi, P; Misra, R; Kembhavi, A K

    2016-01-01

    We perform X-ray/UV spectral and X-ray variability study on the radio-loud Narrow Line Seyfert 1 (NLS1) galaxy RXJ1633.3+4719 using XMM-Newton and Suzaku observations performed in 2011 and 2012. The 0.3$-$10 keV spectra consist of an ultra-soft component below $0.5$ keV, described by an accretion disc blackbody ($kT_{in} = 39.6^{+11.2}_{-5.5}$ eV) and a power-law due to thermal Comptonization ($\\Gamma=1.96^{+0.24}_{-0.31}$) of the disc emission. The disc temperature inferred from the soft excess is at least a factor of two lower than that found for the canonical soft excess emission from radio-quiet NLS1s. The UV spectrum is described by a power-law with photon index $3.05^{+0.56}_{-0.33}$. The observed UV emission is too strong to arise from the accretion disc or the host galaxy but can be attributed to a jet. The optical to X-ray spectral index of the source is consistent with radio-loud AGN. The X-ray emission from RXJ1633.3+4719 is variable with fractional variability amplitude $F_{var}$=13.5$\\pm1.0\\%$. I...

  8. On the mechanism of self gravitating Rossby interfacial waves in proto-stellar accretion discs

    Yellin-Bergovoy, Ron; Heifetz, Eyal; Umurhan, Orkan M.

    2016-05-01

    The dynamical response of edge waves under the influence of self-gravity is examined in an idealized two-dimensional model of a proto-stellar disc, characterized in steady state as a rotating vertically infinite cylinder of fluid with constant density except for a single density interface at some radius r0. The fluid in basic state is prescribed to rotate with a Keplerian profile $\\Omega_k(r)\\sim r^{-3/2}$ modified by some additional azimuthal sheared flow. A linear analysis shows that there are two azimuthally propagating edge waves, kin to the familiar Rossby waves and surface gravity waves in terrestrial studies, which move opposite to one another with respect to the local basic state rotation rate at the interface. Instability only occurs if the radial pressure gradient is opposite to that of the density jump (unstably stratified) where self-gravity acts as a wave stabilizer irrespective of the stratification of the system. The propagation properties of the waves are discussed in detail in the language of vorticity edge waves. The roles of both Boussinesq and non-Boussinesq effects upon the stability and propagation of these waves with and without the inclusion of self-gravity are then quantified. The dynamics involved with self-gravity non- Boussinesq effect is shown to be a source of vorticity production where there is a jump in the basic state density, in addition, self-gravity also alters the dynamics via the radial main pressure gradient, which is a Boussinesq effect . Further applications of these mechanical insights are presented in the conclusion including the ways in which multiple density jumps or gaps may or may not be stable.

  9. A correspondence between H3+ WZW and Liouville theories on discs

    We discuss how the disc correlators of H3+ WZW model are determined in terms of those of Liouville theory. (A presentation based on a collaboration with S. Ribault [K. Hosomichi and S. Ribault, (hep-th/0610117)].)

  10. A correspondence between H3+ WZW and Liouville theories on discs

    Hosomichi, Kazuo

    2007-09-01

    We discuss how the disc correlators of H3+ WZW model are determined in terms of those of Liouville theory. (A presentation based on a collaboration with S. Ribault [K. Hosomichi and S. Ribault, hep-th/0610117].)

  11. Interpreting the large amplitude X-ray variation of GRS 1915+105 and IGR J17091-3624 as modulations of an accretion disc

    Pahari, Mayukh; Mukherjee, Arunava; Yadav, J S; Pandey, S K

    2013-01-01

    Using the flux resolved spectroscopy for the first time, we analyse the RXTE/PCA data of the black hole X-ray binaries GRS 1915+105 and IGR J17091-3624, when both sources show large amplitude, quasi-regular oscillations in 2.0-60.0 keV X-ray light curves. For different observations, we extract spectra during the peak (spectrally soft) and dip (spectrally hard) intervals of the oscillation, and find that their spectra are phenomenologically complex, requiring at least two distinct spectral components. Besides a thermal Comptonization component, we find that the disc emission is better modelled by an index-free multicolour disc blackbody component (p-free disc model) rather than that from a standard accretion disc. While the peak and dip spectra are complex, remarkably, their difference spectra constructed by treating dip spectra as the background spectra of the peak spectra, can be modelled as a single p-free disc component. Moreover, the variability at different time-scales and energy bands of the peak flux l...

  12. Note on nonstationarity and accretion of Primordial Black Holes in Brans-Dicke theory

    Nayak, Bibekananda; Singh, Lambodar Prasad

    2010-01-01

    We consider the evolution of primordial black holes by including non-stationarity in their formation process and accretion of radiation in Brans-Dicke theory. Specifically, we focus on how $\\eta$, the fraction of the horizon mass the black hole comprises capturing nonstationarity, affects the lifetimes of these primordial black holes. Our calculation reveals that the primordial black hole dynamics is controlled by the product $f\\eta$ where $f$ is the accretion efficiency. We also estimate the...

  13. Predictions for Microlensing Planetary Events from Core Accretion Theory

    Wei ZHU; Penny, Matthew; Mao, Shude; Gould, Andrew; Gendron, Rieul

    2014-01-01

    We conduct the first microlensing simulation in the context of planet formation model. The planet population is taken from the Ida & Lin core accretion model for $0.3M_\\odot$ stars. With $6690$ microlensing events, we find for a simplified Korea Microlensing Telescopes Network (KMTNet) the fraction of planetary events is $2.9\\%$ , out of which $5.5\\%$ show multiple-planet signatures. The number of super-Earths, super-Neptunes and super-Jupiters detected are expected to be almost equal. Our si...

  14. Large-scale magnetic field in the accretion discs of young stars: the influence of magnetic diffusion, buoyancy and Hall effect

    Khaibrakhmanov, Sergey A; Parfenov, Sergey Yu; Sobolev, Andrey M

    2016-01-01

    We investigate the fossil magnetic field in the accretion and protoplanetary discs using the Shakura and Sunyaev approach. The distinguishing feature of this study is the accurate solution of the ionization balance equations and the induction equation with Ohmic diffusion, magnetic ambipolar diffusion, buoyancy and the Hall effect. We consider the ionization by cosmic rays, X-rays and radionuclides, radiative recombinations, recombinations onto dust grains, and also thermal ionization. The buoyancy appears as the additional mechanism of magnetic flux escape in the steady-state solution of the induction equation. Calculations show that Ohmic diffusion and magnetic ambipolar diffusion constraint the generation of the magnetic field inside the `dead' zones. The magnetic field in these regions is quasi-vertical. The buoyancy constraints the toroidal magnetic field strength close to the disc inner edge. As a result, the toroidal and vertical magnetic fields become comparable. The Hall effect is important in the re...

  15. Appearance of Keplerian discs orbiting Kerr superspinars

    Stuchlik, Zdenek; 10.1088/0264-9381/27/21/215017

    2011-01-01

    We study optical phenomena related to appearance of Keplerian accretion discs orbiting Kerr superspinars predicted by the string theory. The superspinar exterior is described by the standard Kerr naked singularity geometry breaking the black hole limit on the internal angular momentum (spin). We construct local photon escape cones for a variety of orbiting sources that enable to determine the superspinars silhouette in the case of distant observers. We show that the superspinar silhouette depends strongly on the assumed edge where the external Kerr spacetime is joined to the internal spacetime governed by the string theory and significantly differs from the black hole silhouette. The appearance of the accretion disc is strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile. Apparent extension of the disc grows significantly with growing spin, while the frequency shift grows with descending spin. This behavior differs substantially from appearance of discs orbit...

  16. Interpreting the large amplitude X-ray variation of GRS 1915+105 and IGR J17091-3624 as modulations of an accretion disc

    Pahari, Mayukh; Misra, Ranjeev; Mukherjee, Arunava; Yadav, J. S.; Pandey, S. K.

    2013-12-01

    Using the flux resolved spectroscopy for the first time, we analyse the RXTE/PCA data of the black hole X-ray binaries GRS 1915+105 and IGR J17091-3624, when both sources show large-amplitude, quasi-regular oscillations in 2.0-60.0 keV X-ray light curves (similar to the κ and λ classes in GRS 1915+105). For different observations, we extract spectra during the peak (spectrally soft) and dip (spectrally hard) intervals of the oscillation, and find that their spectra are phenomenologically complex, requiring at least two distinct spectral components. Besides a thermal Comptonization component, we find that the disc emission is better modelled by an index-free multicolour disc blackbody component (p-free disc model) rather than that from a standard accretion disc. While the peak and dip spectra are complex, remarkably, their difference spectra, constructed by treating dip spectra as the background spectra of the peak spectra, can be modelled as a single p-free disc component. Moreover, the variability at different time-scales and energy bands of the peak flux level is always greater than or equal to the variability of the dip flux level, which strengthens the possibility that the peak flux level may be due to an independent spectral component added to the dip one. Using joint spectral analysis of peak and dip spectra with a variable emission component, we verify that the variable component is consistent with p-free disc blackbody and its spectral parameters are similar to that found from the difference spectral analysis. In contrast, we show that for oscillations in the θ class where soft dips are observed, the difference spectra cannot be similarly fitted. Our result substantiates the standard hypothesis that the oscillations are due to the limit cycle behaviour of an unstable radiation pressure dominated inner disc. However, in this interpretation, the flux variation of the unstable disc can be several order of magnitudes as expected from some theoretical

  17. Observations vs theory: from metallicity correlations of exoplanets and debris discs to HL Tau

    Nayakshin, Sergei V.

    2015-12-01

    Boley et al (2010) and Nayakshin (2010) proposed Tidal Downsizing (TD), a new hypothesis for forming all types of planets. Gas fragments born by gravitational disc instability at ~ 100 AU migrate inwards rapidly, with some becoming hot Jupiters. Grain sedimentation inside the fragments makes rocky cores. These cores are future Earths and Super Earths, leaved behind when most of the migrating fragments are tidally disrupted.TD can now be tested against data in detail thanks to a numerical population synthesis model (Nayakshin and Fletcher 2015). TD scenario is fundamentally different from Core Accretion (CA), with sub-Saturn planets and debris discs born in gas fragment disruptions, and not vice versa. I therefore find robust observational differences between CA and TD despite uncertainties inherent in any population synthesis. Here I use metallicity correlations of all sorts to test the model. In TD, the only population that correlates with metallicity (Z) of the host strongly is that of moderately massive gas giants interior to a few AU from the host. Super-Earths and debris discs correlate in mass but not in numbers with Z; very massive gas giants, brown dwarfs and directly imaged gas giants are neutral to Z. Fragment self-destruction by core feedback explains simultaneously the core mass function roll-over at ~20 Earth masses, the rapid formation of suspected planets in HL Tau, and the paucity of directly imaged gas giants. Debris discs and gas giants do not correlate in TD, as observed.I argue that TD does a better job in accounting for many of the observed properties of exoplanets and planetary debris than CA. I finish with observational predictions that can distinguish TD from Core Accretion in the near future.

  18. Predictions for Microlensing Planetary Events from Core Accretion Theory

    Zhu, Wei; Mao, Shude; Gould, Andrew; Gendron, Rieul

    2014-01-01

    We conduct the first microlensing simulation in the context of planet formation model. The planet population is taken from the Ida & Lin core accretion model for $0.3M_\\odot$ stars. With $6690$ microlensing events, we find for a simplified Korea Microlensing Telescopes Network (KMTNet) the fraction of planetary events is $2.9\\%$ , out of which $5.8\\%$ show multiple-planet signatures. The number of super-Earths, super-Neptunes and super-Jupiters detected are expected to be almost equal. Our simulation shows that high-magnification events and massive planets are favored by planet detections, which is consistent with previous expectation. However, we notice that extremely high-magnification events are less sensitive to planets, which is possibly because the 10 min sampling of KMTNet is not intensive enough to capture the subtle anomalies that occur near the peak. This suggests that while KMTNet observations can be systematically analyzed without reference to any follow-up data, follow-up observations will be...

  19. Predictions for microlensing planetary events from core accretion theory

    Zhu, Wei; Mao, Shude [National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012 (China); Penny, Matthew; Gould, Andrew [Department of Astronomy, The Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210 (United States); Gendron, Rieul, E-mail: weizhu@astronomy.ohio-state.edu [Jodrell Bank Centre for Astrophysics, University of Manchester, Alan Turing Building, Manchester M13 9PL (United Kingdom)

    2014-06-10

    We conduct the first microlensing simulation in the context of a planet formation model. The planet population is taken from the Ida and Lin core accretion model for 0.3 M {sub ☉} stars. With 6690 microlensing events, we find that for a simplified Korea Microlensing Telescopes Network (KMTNet), the fraction of planetary events is 2.9%, out of which 5.5% show multiple-planet signatures. The numbers of super-Earths, super-Neptunes, and super-Jupiters detected are expected to be almost equal. Our simulation shows that high-magnification events and massive planets are favored by planet detections, which is consistent with previous expectation. However, we notice that extremely high-magnification events are less sensitive to planets, which is possibly because the 10 minute sampling of KMTNet is not intensive enough to capture the subtle anomalies that occur near the peak. This suggests that while KMTNet observations can be systematically analyzed without reference to any follow-up data, follow-up observations will be essential in extracting the full science potential of very high magnification events. The uniformly high-cadence observations expected for KMTNet also result in ∼55% of all detected planets not being caustic crossing, and more low-mass planets even down to Mars mass being detected via planetary caustics. We also find that the distributions of orbital inclinations and planet mass ratios in multiple-planet events agree with the intrinsic distributions.

  20. Predictions for microlensing planetary events from core accretion theory

    We conduct the first microlensing simulation in the context of a planet formation model. The planet population is taken from the Ida and Lin core accretion model for 0.3 M ☉ stars. With 6690 microlensing events, we find that for a simplified Korea Microlensing Telescopes Network (KMTNet), the fraction of planetary events is 2.9%, out of which 5.5% show multiple-planet signatures. The numbers of super-Earths, super-Neptunes, and super-Jupiters detected are expected to be almost equal. Our simulation shows that high-magnification events and massive planets are favored by planet detections, which is consistent with previous expectation. However, we notice that extremely high-magnification events are less sensitive to planets, which is possibly because the 10 minute sampling of KMTNet is not intensive enough to capture the subtle anomalies that occur near the peak. This suggests that while KMTNet observations can be systematically analyzed without reference to any follow-up data, follow-up observations will be essential in extracting the full science potential of very high magnification events. The uniformly high-cadence observations expected for KMTNet also result in ∼55% of all detected planets not being caustic crossing, and more low-mass planets even down to Mars mass being detected via planetary caustics. We also find that the distributions of orbital inclinations and planet mass ratios in multiple-planet events agree with the intrinsic distributions.

  1. Proto-planetary disc evolution and dispersal

    Rosotti, Giovanni Pietro

    2015-05-01

    Planets form from gas and dust discs in orbit around young stars. The timescale for planet formation is constrained by the lifetime of these discs. The properties of the formed planetary systems depend thus on the evolution and final dispersal of the discs, which is the main topic of this thesis. Observations reveal the existence of a class of discs called "transitional", which lack dust in their inner regions. They are thought to be the last stage before the complete disc dispersal, and hence they may provide the key to understanding the mechanisms behind disc evolution. X-ray photoevaporation and planet formation have been studied as possible physical mechanisms responsible for the final dispersal of discs. However up to now, these two phenomena have been studied separately, neglecting any possible feedback or interaction. In this thesis we have investigated what is the interplay between these two processes. We show that the presence of a giant planet in a photo-evaporating disc can significantly shorten its lifetime, by cutting the inner regions from the mass reservoir in the exterior of the disc. This mechanism produces transition discs that for a given mass accretion rate have larger holes than in models considering only X-ray photo-evaporation, constituting a possible route to the formation of accreting transition discs with large holes. These discs are found in observations and still constitute a puzzle for the theory. Inclusion of the phenomenon called "thermal sweeping", a violent instability that can destroy a whole disc in as little as 10 4 years, shows that the outer disc left can be very short-lived (depending on the X-ray luminosity of the star), possibly explaining why very few non accreting transition discs are observed. However the mechanism does not seem to be efficient enough to reconcile with observations. In this thesis we also show that X-ray photo-evaporation naturally explains the observed correlation between stellar masses and accretion

  2. A correspondence between H{sub 3}{sup +} WZW and Liouville theories on discs

    Hosomichi, Kazuo [Service de Physique Theorique, CEA Saclay, F-91191 Gif-sur-Yvette Cedex (France)

    2007-09-15

    We discuss how the disc correlators of H{sub 3}{sup +} WZW model are determined in terms of those of Liouville theory. (A presentation based on a collaboration with S. Ribault [K. Hosomichi and S. Ribault, (hep-th/0610117)].)

  3. Wave-like warp propagation in circumbinary discs I. Analytic theory and numerical simulations

    Facchini, Stefano; Lodato, Giuseppe; Price, Daniel J.

    2013-01-01

    In this paper we analyse the propagation of warps in protostellar circumbinary discs. We use these systems as a test environment in which to study warp propagation in the bending-wave regime, with the addition of an external torque due to the binary gravitational potential. In particular, we want to test the linear regime, for which an analytic theory has been developed. In order to do so, we first compute analytically the steady state shape of an inviscid disc subject to the binary torques. ...

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

    Pecháček, Tomáš; Goosmann, R. W.; Karas, Vladimír; Czerny, B.; Dovčiak, Michal

    2013-01-01

    Roč. 556, August (2013), A77/1-A77/12. ISSN 0004-6361 R&D Projects: GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : accretion * black holes Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.479, year: 2013

  5. Accretion rates and accretion efficiency in AGNs

    Weihao, Bian; Yongheng, Zhao

    2003-01-01

    We used the standard geometrical thin accretion theory to obtain the accretion rates in Seyfert 1 galaxies and quasars. Combining accretion rates with the bolometric luminosity, we obtained the accretion efficiency. We found most of Seyfert 1 galaxies and radio quiet quasars have lower accretion efficiencies while most of the radio loud quasars possess higher accretion efficiencies. This finding further implies most of radio loud quasars possess Kerr black holes while Seyfert 1 galaxies and r...

  6. Particle acceleration from an inner accretion disc into compact corona and further out: case of an organised magnetic field near a supermassive black hole

    Karas, Vladimir; Kopacek, Ondrej; Kunneriath, Devaky; Kovar, Jiri; Slany, Petr

    2016-04-01

    Upcoming observational techniques in X-rays and millimeter spectral bands will allow to probe the inner corona of accretion discs near supermassive black holes. Size of this region only a few gravitational radii has been inferred from various circumstantial evidence. To populate ithe region with particles, pair-creation in ergosphere and transport of particles via accretion have been invoked.Electromagnetic fields are a likely agent of acceleration in strong gravity of a rotating black hole. We put forward a scenario with an organised component of the magnetic field near a supermassive black hole. An emergent flow of particles may be induced in a preferentially bi-polar direction. Our mechanism does not seem to be capable of producing ultra-high energy cosmic rays but it does expel particles along unbound trajectories.The mentioned concept is relevant also from a purely theoretical viewpoint of dynamical properties of particle motion in General Relativity, namely, the onset of chaos near a black hole. We conclude that the role of black-hole spin in setting the chaos is more complicated than initially thought (based on http://arxiv.org/abs/1408.2452).

  7. Planet-Disc Interactions in Fully Radiative Discs

    Bitsch, Bertram

    2011-01-01

    In this dissertation the movement of planets in accretion discs surrounding young stars is investigated. The accretion disc surrounds a protostar, which in turn is created due to the gravitational collapse of an interstellar gas cloud. In this accretion disc, small dust particles can grow to larger objects with a size of up to a few kilometers. These objects are called planetesimals. Through collisions, these planetesimals can grow further until they reach the size of a protoplanet. The proto...

  8. Can the viscosity in astrophysical black hole accretion disks be close to its string theory bound?

    String theory and gauge/gravity duality suggest the lower bound of shear viscosity (η) to entropy density (s) for any matter to be ∼μℏ/4πkB, when ℏ and kB are reduced Planck and Boltzmann constants respectively and μ⩽1. Motivated by this, we explore η/s in black hole accretion flows, in order to understand if such exotic flows could be a natural site for the lowest η/s. Accretion flow plays an important role in black hole physics in identifying the existence of the underlying black hole. This is a rotating shear flow with insignificant molecular viscosity, which could however have a significant turbulent viscosity, generating transport, heat and hence entropy in the flow. However, in presence of strong magnetic field, magnetic stresses can help in transporting matter independent of viscosity, via celebrated Blandford–Payne mechanism. In such cases, energy and then entropy produces via Ohmic dissipation. In addition, certain optically thin, hot, accretion flows, of temperature ≳109 K, may be favourable for nuclear burning which could generate/absorb huge energy, much higher than that in a star. We find that η/s in accretion flows appears to be close to the lower bound suggested by theory, if they are embedded by strong magnetic field or producing nuclear energy, when the source of energy is not viscous effects. A lower bound on η/s also leads to an upper bound on the Reynolds number of the flow

  9. Wave-like warp propagation in circumbinary discs I. Analytic theory and numerical simulations

    Facchini, Stefano; Price, Daniel J

    2013-01-01

    In this paper we analyse the propagation of warps in protostellar circumbinary discs. We use these systems as a test environment in which to study warp propagation in the bending-wave regime, with the addition of an external torque due to the binary gravitational potential. In particular, we want to test the linear regime, for which an analytic theory has been developed. In order to do so, we first compute analytically the steady state shape of an inviscid disc subject to the binary torques. The steady state tilt is a monotonically increasing function of radius. In the absence of viscosity, the disc does not present any twist. Then, we compare the time-dependent evolution of the warped disc calculated via the known linearised equations both with the analytic solutions and with full 3D numerical simulations, which have been performed with the PHANTOM SPH code using 2 million particles. We find a good agreement both in the tilt and in the phase evolution for small inclinations, even at very low viscosities. Mor...

  10. Theory of magnetohydrodynamic accretion of matter with an ultrahard equation of state onto a black hole

    We consider the magnetohydrodynamic theory of spherically symmetric accretion of a perfect fluid onto a Schwarzschild black hole with an ultrahard equation of state, p = μ ∼ ρ2, where p is the pressure, μ is the total energy density, and ρ is the fluid density. An approximate analytical solution is written out. We show that one critical sonic surface that coincides with the black hole event horizon is formed instead of two critical surfaces (fast and slow magnetosonic surfaces) for a degenerate ultrahard equation of state of matter

  11. Theory of magnetohydrodynamic accretion of matter with an ultrahard equation of state onto a black hole

    Chernov, S. V., E-mail: chernov@lpi.ru [Russian Academy of Sciences, Astrospace Center, Lebedev Physical Institute (Russian Federation)

    2015-06-15

    We consider the magnetohydrodynamic theory of spherically symmetric accretion of a perfect fluid onto a Schwarzschild black hole with an ultrahard equation of state, p = μ ∼ ρ{sup 2}, where p is the pressure, μ is the total energy density, and ρ is the fluid density. An approximate analytical solution is written out. We show that one critical sonic surface that coincides with the black hole event horizon is formed instead of two critical surfaces (fast and slow magnetosonic surfaces) for a degenerate ultrahard equation of state of matter.

  12. The broad emission-line region: the confluence of the outer accretion disc with the inner edge of the dusty torus

    Goad, M R; Ruff, A J

    2012-01-01

    (Abridged) We investigate the observational characteristics of BLR geometries in which the BLR clouds bridge the gap, both in distance and scale height, between the outer accretion disc and the hot dust, forming an effective surface of a "bowl". The gas dynamics are dominated by gravity, and we include the effects of transverse Doppler shift, gravitational redshift and scale-height dependent macro-turbulence. Our simple model reproduces many of the phenomena observed in broad emission-line variability studies, including (i) the absence of response in the core of the optical recombination lines on short timescales, (ii) the enhanced red-wing response on short timescales, (iii) differences between the measured delays for the HILs and LILs, and (iv) identifies turbulence as a means of producing Lorentzian profiles (esp. for LILs) in low inclination systems, and for suppressing significant continuum--emission-line delays between the line wings and line core (esp. in LILs). A key motivation of this work was to rev...

  13. Giant outburst from the supergiant fast X-ray transient IGR J17544-2619: accretion from a transient disc?

    Romano, P; Mangano, V; Esposito, P; Israel, G; Tiengo, A; Campana, S; Ducci, L; Ferrigno, C; Kennea, J A

    2015-01-01

    Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterised by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on timescales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and explain their unusually low average X-ray luminosity. We present the Swift observations of an exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014 October 10 when the source achieved a peak luminosity of $3\\times10^{38}$ erg s$^{-1}$. This extends the total source dynamic range to $\\gtrsim$10$^6$, the largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence for pulsations at a period of 11.6 s is also reported. We show that these observations challenge, for the first time, the maximum theoretical luminosity achievable by an SFXT and propose that this giant outburst was due to the formation of a transient ac...

  14. Stationary Axisymmetric Configuration of the Resistive Thick Accretion Tori around a Schwarzschild Black Hole

    Shaghaghian, M

    2016-01-01

    We examine a thick accretion disc in the presence of external gravity and intrinsic dipolar magnetic field due to a non-rotating central object. In this paper, we generalize the Newtonian theory of stationary axisymmetric resistive tori of Tripathy, Prasanna $\\&$ Das (1990) by including the fully general relativistic features. If we are to obtain the steady state configuration, we have to take into account the finite resistivity for the magnetofluid in order to avoid the piling up of the field lines anywhere in the accretion discs. The efficient value of conductivity must be much smaller than the classical conductivity to be astrophysically interesting. The accreting plasma in the presence of an external dipole magnetic field gives rise to a current in the azimuthal direction. The azimuthal current produced due to the motion of the magnetofluid modifies the magnetic field structure inside the disc and generates a poloidal magnetic field for the disc. The solutions we have found show that the radial inflow...

  15. Testing theories for longterm accretion variability in black hole X-ray binaries

    Cambier, Hal J.

    Many X-ray sources are now understood to be "black hole X-ray binaries'' in which a stellar remnant black hole either tidally "squeezes'' gas off a companion star, or pulls in some fraction the companion's wind. This gas can drain inward through a dense, thin disk characterized by thermalized radiation, or a sparse and radiatively-inefficient flow, or some combination of the two. Observations at other energies often provide crucial information, but our primary tools to study accretion, especially closest to the black hole, are X-ray spectra and their time evolution. This evolution includes numerous behaviors spanning orders of magnitude in timescale and luminosity, and also hints at spatial structure since draining is generally faster at smaller radii. This includes variability at time-scales of weeks to months which remains difficult to explain despite an abundance of possible variability mechanisms since direct simulations covering the full spatial and temporal range remain impractical. After reviewing general aspects of accretion, I present both more and less familiar forms of longterm variability. Based on these, I argue the problem involves finding a physical process (or combination) that can generate repeatable yet adjustable cycles in luminosity and evolution of low and high energy spectral components, while letting the ionization instability dominate conventional outbursts. Specific models examined include: disks embedded in, and interacting with, hot, sparse flows, and another instability that quenches viscous-draining of the disk at more fundamental level. Testing these theories, alone and in combination, motivates building a very general and simplified numerical model presented here. I find that two-phase flow models still predict excessive recondensation in LMC X-3 among other problems, while the viscosity-quenching instability may account for rapid drops and slow recoveries in disk accretion rate but also likely requires diffusivity orders of magnitude

  16. Notes on Collective Field Theory of Large N Vector Models as Classical Mechanics on the Siegel Disc

    Agarwal, A

    2004-01-01

    We use deformation quantization to construct the large N limits of Bosonic vector models as classical dynamical systems on the Siegel disc and study the relation of this formulation to standard results of collective field theory. Special emphasis is paid to relating the collective potential of the large N theory to a particular cocycle of the symplectic group.

  17. Propeller outflows from an MRI disc

    Lii, Patrick S; Ustyugova, Galina V; Koldoba, Alexander V; Lovelace, Richard V E

    2013-01-01

    We present the results of axisymmetric simulations of MRI-driven accretion onto a rapidly rotating, magnetized star accreting in the propeller regime. The stellar magnetosphere corotates with the star, forming a centrifugal barrier at the disc-magnetosphere boundary which inhibits matter accretion onto the star. Instead, the disc matter accumulates at the disc-magnetosphere interface and slowly diffuses into the inner magnetosphere where it picks up angular momentum and is quickly ejected from the system as an outflow. Due to the interaction of the matter with the magnetosphere, this wind is discontinuous and is launched as discrete plasmoids. If the ejection rate is lower than the disc accretion rate, the matter accumulates at the disc-magnetosphere boundary faster than it can be ejected. In this case, accretion onto the star proceeds through the episodic accretion instability in which episodes of matter accumulation are followed by simultaneous accretion and ejection. During the accretion phase of this inst...

  18. The broad emission-line region: the confluence of the outer accretion disc with the inner edge of the dusty torus

    Goad, M. R.; Korista, K. T.; Ruff, A. J.

    2012-11-01

    We have investigated the observational characteristics of a class of broad emission line region (BLR) geometries that connect the outer accretion disc with the inner edge of the dusty toroidal obscuring region (TOR). We suggest that the BLR consists of photoionized gas of densities which allow for efficient cooling by ultraviolet (UV)/optical emission lines and of incident continuum fluxes which discourage the formation of grains, and that such gas occupies the range of distance and scale height between the continuum-emitting accretion disc and the dusty TOR. As a first approximation, we assume a population of clouds illuminated by ionizing photons from the central source, with the scale height of the illuminated clouds growing with increasing radial distance, forming an effective surface of a 'bowl'. Observer lines of sight which peer into the bowl lead to a Type 1 active galactic nuclei (AGN) spectrum. We assume that the gas dynamics are dominated by gravity, and we include in this model the effects of transverse Doppler shift (TDS), gravitational redshift (GR) and scale-height-dependent macroturbulence. Our simple model reproduces many of the commonly observed phenomena associated with the central regions of AGN, including (i) the shorter than expected continuum-dust delays (geometry), (ii) the absence of response in the core of the optical recombination lines on short time-scales (geometry/photoionization), (iii) an enhanced redwing response on short time-scales (GR and TDS), (iv) the observed differences between the delays for high- and low-ionization lines (photoionization), (v) identifying one of the possible primary contributors to the observed line widths for near face-on systems even for purely transverse motion (GR and TDS), (vi) a mechanism responsible for producing Lorentzian profiles (especially in the Balmer and Mg II emission lines) in low-inclination systems (turbulence), (vii) the absence of significant continuum-emission-line delays between the

  19. Identifying Deficiencies of Standard Accretion Disk Theory: Lessons from a Mean-Field Approach

    Hubbard, Alexander

    2008-01-01

    Turbulent viscosity is frequently used in accretion disk theory to replace the microphysical viscosity in order to accomodate the observational need for in- stabilities in disks that lead to enhanced transport. However, simply replacing the microphysical transport coefficient by a single turbulent transport coeffi- cient hides the fact that the procedure should formally arise as part of a closure in which the hydrodynamic or magnetohydrodynamic equations are averaged, and correlations of turbulent fluctuations are replaced by transport coefficients. Here we show how a mean field approach leads quite naturally two transport coefficients, not one, that govern mass and angular momentum transport. In particular, we highlight that the conventional approach suffers from a seemingly inconsistent neglect of turbulent diffusion in the surface density equation. We constrain these new transport coefficients for specific cases of inward, outward, and zero net mass transport. In addition, we find that one of the new trans...

  20. On the formation of a quasi-stationary twisted disc after a tidal disruption event

    Xiang-Gruess, M; Papaloizou, J C B

    2016-01-01

    We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the $\\alpha$ model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from $\\sim 1$ to $\\s...

  1. Theory of quasi-spherical accretion in X-ray pulsars

    Shakura, N.; Postnov, K.; Kochetkova, A.; Hjalmarsdotter, L.

    2011-01-01

    A theoretical model for quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is constructed. In this model the accreting matter subsonically settles down onto the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter...

  2. Propeller outflows from an MRI disc

    Lii, Patrick S.; Romanova, Marina M.; Ustyugova, Galina V.; Koldoba, Alexander V.; Lovelace, Richard V. E.

    2013-01-01

    We present the results of axisymmetric simulations of MRI-driven accretion onto a rapidly rotating, magnetized star accreting in the propeller regime. The stellar magnetosphere corotates with the star, forming a centrifugal barrier at the disc-magnetosphere boundary which inhibits matter accretion onto the star. Instead, the disc matter accumulates at the disc-magnetosphere interface and slowly diffuses into the inner magnetosphere where it picks up angular momentum and is quickly ejected fro...

  3. Hydrogen-loosing planets in transition discs around young protostars

    Nayakshin, Sergei

    2013-01-01

    We point out that protoplanets created in the framework of the Tidal Downsizing (TD) theory for planet formation play a very important role for the evolution of accretion discs hosting them. Since all TD protoplanets are initially as massive as $\\sim 10$ Jupiter masses, they are able to open very deep gaps in their discs, and even completely isolate the inner disc flows from the outer ones. Furthermore, in contrast to other planet formation theories, TD protoplanets are mass donors for their protostars. One potentially observable signature of planets being devoured by their protostars are FU Ori like outbursts, and episodic protostar accretion more generally, as discussed by a number of authors recently. Here we explore another observational implication of TD hypothesis: dust poor inner accretion flows, which we believe may be relevant to some of the observed mm-bright transitional discs around protostars. In our model, a massive protoplanet interrupts the flow of the outer dust-rich disc on its protostar, an...

  4. Accretion Disks

    Spruit, H.C.

    1995-01-01

    This is an introduction to accretion disk theory, with emphasis on aspects relevant for X-ray Binaries and Cataclysmic Variables. The text corrects some mistakes in an earlier version, which appeared in 'Lives of Neutron Stars', A. Alpar, \\"U. Kizilo\\u glu and J. van Paradijs (eds.), Kluwer, Dordrecht (NATO ASI series, 1994).

  5. Investigating the disc-jet coupling in accreting compact objects using the black hole candidate Swift J1753.5-0127

    Soleri, P.; Fender, R.; Tudose, V.; Maitra, D.; Bell, M.; Linares, M.; Altamirano, D.; Wijnands, R.; Belloni, T.; Casella, P.; Miller-Jones, J. C. A.; Muxlow, T.; Klein-Wolt, M.; Garrett, M.; van der Klis, M.

    2010-01-01

    In studies of accreting black holes in binary systems, empirical relations have been proposed to quantify the coupling between accretion processes and ejection mechanisms. These processes are probed, respectively, by means of X-ray and radio/optical-infrared observations. The relations predict, give

  6. Theory of quasi-spherical accretion in X-ray pulsars

    Shakura, N; Kochetkova, A; Hjalmarsdotter, L

    2011-01-01

    A theoretical model for quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars is constructed. In this model the accreting matter subsonically settles down onto the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere. The settling regime of accretion can be realized for moderate accretion rates $\\dot M< \\dot M_*\\simeq 4\\times 10^{16}$ g/s. At higher accretion rates a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of the spin-up/spin-down rates (the angular rotation frequency derivative $\\dot \\omega^*$, and $\\partial\\dot\\omega^*/\\partial\\dot M$ near the torque reversal) of X-ray pulsars with known orbital perio...

  7. Evolution of Protoplanetary Discs with Magnetically Driven Disc Winds

    Suzuki, Takeru K; Morbidelli, Alessandro; Crida, Aurélien; Guillot, Tristan

    2016-01-01

    Aims: We investigate the evolution of protoplanetary discs (PPDs hereafter) with magnetically driven disc winds and viscous heating. Methods: We consider an initially massive disc with ~0.1 Msun to track the evolution from the early stage of PPDs. We solve the time evolution of surface density and temperature by taking into account viscous heating and the loss of the mass and the angular momentum by the disc winds within the framework of a standard alpha model for accretion discs. Our model parameters, turbulent viscosity, disc wind mass loss, and disc wind torque, which are adopted from local magnetohydrodynamical simulations and constrained by the global energetics of the gravitational accretion, largely depends on the physical condition of PPDs, particularly on the evolution of the vertical magnetic flux in weakly ionized PPDs. Results: Although there are still uncertainties concerning the evolution of the vertical magnetic flux remaining, surface densities show a large variety, depending on the combinatio...

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

    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.

  9. Appearance of Keplerian discs orbiting Kerr superspinars

    Stuchlik, Zdenek; Schee, Jan, E-mail: zdenek.stuchlik@fpf.slu.c, E-mail: jan.schee@fpf.slu.c [Institute of Physics, Faculty of Philosophy and Science, Silesian University in Opava, Bezrucovo nam. 13, Opava (Czech Republic)

    2010-11-07

    We study optical phenomena related to the appearance of Keplerian accretion discs orbiting Kerr superspinars predicted by string theory. The superspinar exterior is described by standard Kerr naked singularity geometry breaking the black hole limit on the internal angular momentum (spin). We construct local photon escape cones for a variety of orbiting sources that enable us to determine the superspinars silhouette in the case of distant observers. We show that the superspinar silhouette depends strongly on the assumed edge where the external Kerr spacetime is joined to the internal spacetime governed by string theory and significantly differs from the black hole silhouette. The appearance of the accretion disc is strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile. Apparent extension of the disc grows significantly with the growing spin, while the frequency shift grows with the descending spin. This behaviour differs substantially from the appearance of discs orbiting black holes enabling thus, at least in principle, to distinguish clearly the Kerr superspinars and black holes. In vicinity of a Kerr superspinar the non-escaped photons have to be separated to those captured by the superspinar and those being trapped in its strong gravitational field leading to self-illumination of the disc that could even influence its structure and cause self-reflection effect of radiation of the disc. The amount of trapped photons grows with descending superspinar spin. We thus can expect significant self-illumination effects in the field of Kerr superspinars with near-extreme spin a {approx} 1.

  10. Appearance of Keplerian discs orbiting Kerr superspinars

    Stuchlík, Zdeněk; Schee, Jan

    2010-11-01

    We study optical phenomena related to the appearance of Keplerian accretion discs orbiting Kerr superspinars predicted by string theory. The superspinar exterior is described by standard Kerr naked singularity geometry breaking the black hole limit on the internal angular momentum (spin). We construct local photon escape cones for a variety of orbiting sources that enable us to determine the superspinars silhouette in the case of distant observers. We show that the superspinar silhouette depends strongly on the assumed edge where the external Kerr spacetime is joined to the internal spacetime governed by string theory and significantly differs from the black hole silhouette. The appearance of the accretion disc is strongly dependent on the value of the superspinar spin in both their shape and frequency shift profile. Apparent extension of the disc grows significantly with the growing spin, while the frequency shift grows with the descending spin. This behaviour differs substantially from the appearance of discs orbiting black holes enabling thus, at least in principle, to distinguish clearly the Kerr superspinars and black holes. In vicinity of a Kerr superspinar the non-escaped photons have to be separated to those captured by the superspinar and those being trapped in its strong gravitational field leading to self-illumination of the disc that could even influence its structure and cause self-reflection effect of radiation of the disc. The amount of trapped photons grows with descending superspinar spin. We thus can expect significant self-illumination effects in the field of Kerr superspinars with near-extreme spin a ~ 1.

  11. AGN flickering and chaotic accretion

    King, Andrew; Nixon, Chris

    2015-10-01

    Observational arguments suggest that the growth phases of the supermassive black holes in active galactic nuclei have a characteristic time-scale ˜105 yr. We show that this is the time-scale expected in the chaotic accretion picture of black hole feeding, because of the effect of self-gravity in limiting the mass of any accretion-disc feeding event.

  12. AGN Flickering and Chaotic Accretion

    King, Andrew

    2015-01-01

    Observational arguments suggest that the growth phases of the supermassive black holes in active galactic nuclei have a characteristic timescale $\\sim 10^5$ yr. We show that this is the timescale expected in the chaotic accretion picture of black hole feeding, because of the effect of self-gravity in limiting the mass of any accretion disc feeding event.

  13. A CFD analysis of the actuator disc flow compared with momentum theory results

    Aagaard Madsen, H. [Risoe National Laboratory, Roskilde (Denmark)

    1997-08-01

    The blade element momentum (BEM) model is still used in many aerodynamic and aeroelastic models for design and load calculations. This is due to its simplicity, robustness, computational speed and good accuracy for a wide range of applications. The question about accuracy is however closely connected to the airfoil section data and therefore correlation/lack of correlation with experimental results can both be due to the specific input data used and due to the induced velocity field predicted by the BEM method. It is also well-known that the BEM method for some applications is used under operational conditions that violates the assumptions made for the development of the model, e.g. operation in yaw and operation at high loading. The main objective with the present study is to investigate this part of the BEM method (the momentum strip theory MST) on which the determination of the induced velocities is based. This is done by comparing the results of the MST model with velocities predicted on basis of the Navier Stokes equations for the flow through an actuator disc. (au)

  14. Theory of quasi-spherical accretion in X-ray pulsars

    Shakura, N.; Postnov, K.; Kochetkova, A.; Hjalmarsdotter, L.

    2012-02-01

    A theoretical model for quasi-spherical subsonic accretion on to slowly rotating magnetized neutron stars is constructed. In this model, the accreting matter subsonically settles down on to the rotating magnetosphere forming an extended quasi-static shell. This shell mediates the angular momentum removal from the rotating neutron star magnetosphere during spin-down episodes by large-scale convective motions. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere. The settling regime of accretion can be realized for moderate accretion rates ? g s-1. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and accretion becomes highly non-stationary. From observations of the spin-up/spin-down rates (the angular rotation frequency derivative ?, and ? near the torque reversal) of X-ray pulsars with known orbital periods, it is possible to determine the main dimensionless parameters of the model, as well as to estimate the magnetic field of the neutron star. We illustrate the model by determining these parameters for three wind-fed X-ray pulsars GX 301-2, Vela X-1 and GX 1+4. The model explains both the spin-up/spin-down of the pulsar frequency on large time-scales and the irregular short-term frequency fluctuations, which can correlate or anticorrelate with the X-ray flux fluctuations in different systems. It is shown that in real pulsars an almost iso-angular-momentum rotation law with ω˜ 1/R2, due to strongly anisotropic radial turbulent motions sustained by large-scale convection, is preferred.

  15. On the formation of a quasi-stationary twisted disc after a tidal disruption event

    Xiang-Gruess, M.; Ivanov, P. B.; Papaloizou, J. C. B.

    2016-08-01

    We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the α model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from ˜1 to ˜10-3 which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter α > ˜0.01 - 0.1 the ratio of the disc inclination to that of the initial stellar orbit is estimated to be 0.1 - 0.2 in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of α, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.

  16. Quasi-spherical accretion in low-luminosity X-ray pulsars: Theory vs. observations

    Postnov, K; Kochetkova, A; Hjalmarsdotter, L

    2012-01-01

    Quasi-spherical subsonic accretion can be realized in slowly rotating wind-fed X-ray pulsars (XPSRs) at X-ray luminosities <4 10^{36} erg/s. In this regime the accreting matter settles down subsonically onto the rotating magnetosphere, forming an extended quasi-static shell. The shell mediates the angular momentum removal from the rotating NS magnetosphere by shear turbulent viscosity in the boundary layer or via large-scale convective motions. In the last case the differential rotation law in the shell is close to iso-angular-momentum rotation. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instabilities while taking cooling into account. Measurements of spin-up/spin-down rates of quasi-spherically wind accreting XPSRs in equilibrium with known orbital periods (like e.g. GX 301-2 and Vela X-1) enable determination of the main dimensionless parameters of the model and the NS magnetic field. For equilibrium pulsars with indep...

  17. Mass Accretion Rate of Rotating Viscous Accretion Flow

    Park, Myeong-Gu

    2009-01-01

    The mass accretion rate of transonic spherical accretion flow onto compact objects such as black holes is known as the Bondi accretion rate(Mdot_B), which is determined only by the density and the temperature of gas at the outer boundary. But most work on disc accretion has taken the mass flux to be a given with the relation between that parameter and external conditions left uncertain. Within the framework of a slim alpha disk, we have constructed global solutions of the rotating, viscous ho...

  18. Magnetic field structure in accretion columns on HMXB and effects on CRSF

    Mukherjee Dipanjan; Bhattacharya Dipankar; Mignone Andrea

    2013-01-01

    In accreting neutron star binaries, matter is channelled by the magnetic fields from the accretion disc to the poles of neutron stars forming an accretion mound. We model such mounds by numerically solving the Grad-Shafranov equation for axisymmetric static MHD equilibria. From our solutions we infer local distortion of field lines due to the weight of accreted matter. Variation in mass loading at the accretion disc will alter the shape of the accretion mound which will also affect the local ...

  19. Validation and modification of the Blade Element Momentum theory based on comparisons with actuator disc simulations

    Aagaard Madsen, Helge; Bak, Christian; Døssing, Mads;

    2010-01-01

    A comprehensive investigation of the Blade Element Momentum (BEM) model using detailed numerical simulations with an axis symmetric actuator disc (AD) model has been carried out. The present implementation of the BEM model is in a version where exactly the same input in the form of non-dimensiona......A comprehensive investigation of the Blade Element Momentum (BEM) model using detailed numerical simulations with an axis symmetric actuator disc (AD) model has been carried out. The present implementation of the BEM model is in a version where exactly the same input in the form of non...

  20. A Double Outburst from IGR J00291+5934: Implications for Accretion Disk Instability Theory

    Hartman, Jacob M; Chakrabarty, Deepto

    2010-01-01

    The accretion-powered millisecond pulsar IGR J00291+5934 underwent two ~10 d long outbursts during 2008, separated by 30 d in quiescence. Such a short quiescent period between outbursts has never been seen before from a neutron star X-ray transient. X-ray pulsations at the 599 Hz spin frequency are detected throughout both outbursts. For the first time, we derive a pulse phase model that connects two outbursts, providing a long baseline for spin frequency measurement. Comparison with the frequency measured during the 2004 outburst of this source gives a spin-down during quiescence of -4(1)x10^-15 Hz/s, approximately an order of magnitude larger than the long-term spin-down observed in the 401 Hz accretion-powered pulsar SAX J1808.4-3658. If this spin-down is due to magnetic dipole radiation, it requires a 2x10^8 G field strength, and its high spin-down luminosity may be detectable with the Fermi Large Area Telescope. Alternatively, this large spin-down could be produced by gravitational wave emission from a f...

  1. Planet formation in evolving protoplanetary discs

    Alexander, Richard

    2013-01-01

    I attempt to summarize our knowledge of planet formation in evolving protoplanetary discs. I first review the physics of disc evolution and dispersal. For most of the disc lifetime evolution is driven by accretion and photoevaporation, and I discuss how the interplay between these processes shapes protoplanetary discs. I also discuss the observations that we use to test these models, and the major uncertainties that remain. I will then move on to consider planet formation and migration in evo...

  2. Demographics of Transition Discs in Ophiuchus and Taurus

    Najita, Joan R; Muzerolle, James

    2015-01-01

    Transition disc systems are young stars that appear to be on the verge of dispersing their protoplanetary discs. We explore the nature of these systems by comparing the stellar accretion rates and disc masses of transition discs and normal T Tauri stars in Taurus and Ophiuchus. After controlling for the known dependencies of stellar accretion rate and disc mass and on age, stellar accretion rate on stellar mass, and disc mass on the presence of stellar or sub-stellar companions, we find that the normal T Tauri stars show a trend of stellar accretion rate increasing with disc mass. The transition discs tend to have higher average disc masses than normal T Tauri stars as well as lower accretion rates than normal T Tauri stars of the same disc mass. These results are most consistent with the interpretation that the transition discs have formed objects massive enough to alter the accretion flow, i.e., single or multiple giant planets. Several Ophiuchus T Tauri stars that are not known transition disc systems also...

  3. Spherical Accretion

    Sari, Re'em; Goldreich, Peter

    2006-01-01

    We compare different examples of spherical accretion onto a gravitating mass. Limiting cases include the accretion of a collisionally dominated fluid and the accretion of collisionless particles. We derive expressions for the accretion rate and density profile for semi-collisional accretion which bridges the gap between these limiting cases. Particle crossing of the Hill sphere during the formation of the outer planets is likely to have taken place in the semi-collisional regime.

  4. Accretion funnels onto weakly magnetized young stars

    Bessolaz, N.; Zanni, C.; Ferreira, J.; Keppens, R.; Bouvier, J.

    2007-01-01

    Aims : We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods : New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipo...

  5. Academician Zeldovich and the foundations of disk accretion

    The author draws on his memories to review the decisive contributions of Ya B Zeldovich to the formation and development of the theory of disc accretion onto black holes and neutron stars in binaries. A theory developed by N I Shakura and R A Sunyaev in the early 1970s under the guidance of Ya B Zeldovich predicted these objects to be the brightest X-ray sources in the sky and defined the prospects for research in X-ray astronomy and high-energy astrophysics for decades ahead. (from the history of physics)

  6. Approximated Vector Diffraction Theory for Readout Signal of Multi-Level Run-Length-Limited Discs

    Multi-level run-length-limited read-only optical storage is a kind of high density storage method. The width and height of recording marks are varied with different levels, which is the key technology for the optical storage. When the readout signal of these discs with complex recording marks is computed by vector analysis method, it is very hard and time-consuming. Approximated vector computation combines the convenience of scalar method and precision of vector method, which is effective for multi-level run-length-limited read-only optical storage. (fundamental areas of phenomenology (including applications))

  7. Design of a fibre-optic disc accelerometer: theory and experiment

    Wang, Yongjie; Xiao, Hao; Zhang, Songwei; Li, Fang; Liu, Yuliang

    2007-06-01

    Mechanical principles of fibre-optic disc accelerometers (FODA) different from those assumed in previous calculation methods are presented. An FODA with a high sensitivity of 82 rad/g and a resonance frequency of 360 Hz is designed and tested. In this system, the minimum measurable demodulation phase of the phase-generated carrier (PGC) is 10-5 rad, and the minimum acceleration reaches 120 ng theoretically. This kind of FODA, with its high responsivity, all-optic-fibre configuration, small size, light weight and stiff shell housing, ensures effective performance in practice.

  8. Apsidal precession, disc breaking and viscosity in warped discs

    Nealon, Rebecca; Price, Daniel J; King, Andrew

    2015-01-01

    We demonstrate the importance of general relativistic apsidal precession in warped black hole accretion discs by comparing three - dimensional smoothed particle hydrodynamic simulations in which this effect is first neglected, and then included. If apsidal precession is neglected, we confirm the results of an earlier magnetohydrodynamic simulation which made this assumption, showing that at least in this case the $\\alpha$ viscosity model produces very similar results to those of simulations where angular momentum transport is due to the magnetorotational instability. Including apsidal precession significantly changes the predicted disc evolution. For moderately inclined discs thick enough that tilt is transported by bending waves, we find a disc tilt which is nonzero at the inner disc edge and oscillates with radius, consistent with published analytic results. For larger inclinations we find disc breaking.

  9. The vertical effects of disc non-axisymmetries from perturbation theory: the case of the Galactic bar

    Monari, Giacomo; Siebert, Arnaud

    2015-01-01

    Evidence for non-zero mean stellar velocities in the direction perpendicular to the Galactic plane has been accumulating from various recent large spectroscopic surveys. Previous analytical and numerical work has shown that a "breathing mode" of the Galactic disc, similar to what is observed in the Solar vicinity, can be the natural consequence of a non-axisymmetric internal perturbation of the disc. Here we provide a general analytical framework, in the context of perturbation theory, allowing us to compute the vertical bulk motions generated by a single internal perturber (bar or spiral pattern). In the case of the Galactic bar, we show that these analytically predicted bulk motions are well in line with the outcome of a numerical simulation. The mean vertical motions induced by the Milky Way bar are small (mean velocity of less than 1 km/sec) and cannot be responsible alone for the observed breathing mode, but they are existing. Our analytical treatment is valid close to the plane for all the non-axisymmet...

  10. Chondrule transport in protoplanetary discs

    Goldberg, Aaron Z.; Owen, James E.; Jacquet, Emmanuel

    2015-10-01

    Chondrule formation remains one of the most elusive early Solar system events. Here, we take the novel approach of employing numerical simulations to investigate chondrule origin beyond purely cosmochemical methods. We model the transport of generically produced chondrules and dust in a 1D viscous protoplanetary disc model in order to constrain the chondrule formation events. For a single formation event we are able to match analytical predictions of the memory they retain of each other (complementarity), finding that a large mass accretion rate (≳10-7 M⊙ yr-1) allows for delays on the order of the disc's viscous time-scale between chondrule formation and chondrite accretion. Further, we find older discs to be severely diminished of chondrules, with accretion rates ≲10-9 M⊙ yr-1 for nominal parameters. We then characterize the distribution of chondrule origins in both space and time, as functions of disc parameters and chondrule formation rates, in runs with continuous chondrule formation and both static and evolving discs. Our data suggest that these can account for the observed diversity between distinct chondrite classes, if some diversity in accretion time is allowed for.

  11. Generalized Shock Solutions for Hydrodynamic Black Hole Accretion

    Das, Tapas Kumar

    2002-01-01

    For the first time, {\\it all} available pseudo-Schwarzschild potentials are exhaustively used to investigate the possibility of shock formation in hydrodynamic, invicid, black hole accretion discs. It is shown that a significant region of parameter space spanned by important accretion parameters allows shock formation for flow in {\\it all} potentials used in this work. This leads to the conclusion that the standing shocks are essential ingredients in accretion discs around non-rotating black ...

  12. The thickness of HI in galactic discs under MOND: theory and application to the Galaxy

    Sánchez-Salcedo, F J; Narayan, C A

    2007-01-01

    The outskirts of galaxies are a very good laboratory for testing the nature of the gravitational field at low accelerations. By assuming that the neutral hydrogen gas is in hydrostatic equilibrium in the gravitational potential of the host galaxy, the observed flaring of the gas layer can be used to test modified gravities. For the first time we construct a simple framework to derive the scaleheight of the neutral hydrogen gas disc in the MOND scenario and apply this to the Milky Way. It is shown that using a constant gas velocity dispersion of ~9 km/s, MOND is able to give a very good fit to the observed HI flaring beyond a galactocentric distance of 17 kpc up to the last measured point (~40 kpc). Between 10 and 16 kpc, however, the observed scaleheight is about 40% more than what MOND predicts for the standard interpolating function and 70% for the form suggested by Famaey & Binney. Given the uncertainties in the non-thermal pressure support by cosmic rays and magnetic fields, MOND seems to be a plausib...

  13. Understanding X-ray Reflection as a Probe of Accreting Black Holes

    Wilkins, Dan

    2014-01-01

    Active galactic nuclei (AGN) are some of the most luminous objects we see in the Universe, powered by the accretion of matter onto a supermassive black hole in the centre of a galaxy, yet many of the physical processes by which the energy is released and injected into the surroundings remain a mystery. X-rays are emitted from a ‘corona’ of energetic particles surrounding the black hole and as well as being observed directly, they are seen to be reflected from the accreting disc, producing a number of spectral features including emission lines that are broadened by relativistic effects in the proximity of the black hole. In my thesis, I develop methods through which detailed measurement of the reflected X-rays from the accretion disc can be used to probe the innermost regions of accretion flow and corona, right down to the innermost stable orbit and the event horizon. Novel spectral analysis techniques allow us to reconstruct, from the observed relativistic X-ray reflection spectrum the spatially resolved illumination pattern of the accretion disc and will discuss how comparing this to the results of systematic general relativistic ray tracing simulations I have developed, we are able to constrain the location and geometry of the X-ray emitting corona and understand the dramatic change of the narrow line Seyfert 1 galaxy 1H 0707-495 into an extremely low flux state in terms of a collapse in the corona. I will discuss how measurements of the X-ray variability, specifically the reverberation time lags that are observed between variability in the directly observed X-rays from the corona and those reflected from the accretion disc add a further dimension to the study of accreting black holes, letting us not only build up a three dimensional image of the immediate vicinity of the black hole but also to probe mechanisms by which the energy is released from the accretion flow; techniques that will let us exploit not just current instrumentation but future proposed X

  14. Thin, thick and dark discs in LCDM

    Read, J I; Agertz, O; Debattista, Victor P

    2008-01-01

    In a LCDM cosmology, the Milky Way accretes satellites into the stellar disc. We use cosmological simulations to assess the frequency of near disc plane and higher inclination accretion events, and collisionless simulations of satellite mergers to quantify the final state of the accreted material and the effect on the thin disc. On average, a Milky Way-sized galaxy has 1.5 subhalos with vmax>80km/s; 5 with vmax>60km/s; and 13 with vmax>40km/s merge at redshift z>1. A third of these merge at an impact angle 20 degrees) are twice as likely as low inclination ones. These lead to structures that closely resemble the recently discovered inner/outer stellar halos. They also do more damage to the Milky Way stellar disc creating a more pronounced flare, and warp; both long-lived and consistent with current observations. The most massive mergers (vmax > 80km/s) heat t he thin disc enough to produce a thick disc. These heated thin disc stars are essential for obtaining a thick disc as massive as that seen in the Milky ...

  15. Accretion bursts in young stars driven by cluster environment

    Pfalzner, S; Tackenberg, J.; Steinhausen, M.

    2008-01-01

    The standard picture of accretion is a steady flow of matter from the disc onto the young star - a concept which assumes the star-disc system to be completely isolated. However, in a dense cluster environment star-disc systems do interact gravitationally. The aim here is to estimate the encounter-induced accretion rate in an ONC-like environment. Combining simulations of the cluster dynamics with simulations of the effect of encounters on star-disc systems we determine the likelihood and degr...

  16. Black hole mergers: do gas discs lead to spin alignment?

    Lodato, Giuseppe; Gerosa, Davide

    2012-01-01

    In this Letter we revisit arguments suggesting that the Bardeen-Petterson effect can coalign the spins of a central supermassive black hole binary accreting from a circumbinary (or circumnuclear) gas disc. We improve on previous estimates by adding the dependence on system parameters, and noting that the nonlinear nature of warp propagation in a thin viscous disc affects alignment. This reduces the disc's ability to communicate the warp, and can severely reduce the effectiveness of disc-assis...

  17. Relativistic Disc lines

    Fabian, A C; Parker, M L

    2014-01-01

    Broad emission lines, particularly broad iron-K lines, are now commonly seen in the X-ray spectra of luminous AGN and Galactic black hole binaries. Sensitive NuSTAR spectra over the energy range of 3-78 keV and high frequency reverberation spectra now confirm that these are relativistic disc lines produced by coronal irradiation of the innermost accretion flow around rapidly spinning black holes. General relativistic effects are essential in explaining the observations. Recent results are briefly reviewed here.

  18. Accretion, radial flows and abundance gradients in spiral galaxies

    Pezzulli, Gabriele

    2015-01-01

    The metal-poor gas continuously accreting onto the discs of spiral galaxies is unlikely to arrive from the intergalactic medium (IGM) with exactly the same rotation velocity as the galaxy itself and even a small angular momentum mismatch inevitably drives radial gas flows within the disc, with significant consequences to galaxy evolution. Here we provide some general analytic tools to compute accretion profiles, radial gas flows and abundance gradients in spiral galaxies as a function of the angular momentum of accreting material. We generalize existing solutions for the decomposition of the gas flows, required to reproduce the structural properties of galaxy discs, into direct accretion from the IGM and a radial mass flux within the disc. We then solve the equation of metallicity evolution in the presence of radial gas flows with a novel method, based on characteristic lines, which greatly reduces the numerical demand on the computation and sheds light on the crucial role of boundary conditions on the abunda...

  19. SMBH accretion and mergers: removing the symmetries

    We review recent progress in studying accretion flows on to supermassive black holes (SMBH). Much of this removes earlier assumptions of symmetry and regularity, such as aligned and prograde disc rotation. This allows a much richer variety of effects, often because cancellation of angular momentum allows rapid infall. Potential applications include lower SMBH spins allowing faster mass growth and suppressing gravitational-wave reaction recoil in mergers, gas-assisted SMBH mergers, and near-dynamical accretion in galaxy centres. (paper)

  20. Stationary axisymmetric configuration of the resistive thick accretion tori around a Schwarzschild black hole

    Shaghaghian, M.

    2016-02-01

    We examine a thick accretion disc in the presence of external gravity and intrinsic dipolar magnetic field due to a non-rotating central object. In this paper, we generalize the Newtonian theory of stationary axisymmetric resistive tori of Tripathy, Prasanna & Das by including the fully general relativistic features. If we are to obtain the steady state configuration, we have to take into account the finite resistivity for the magnetofluid in order to avoid the piling up of the field lines anywhere in the accretion discs. The efficient value of conductivity must be much smaller than the classical conductivity to be astrophysically interesting. The accreting plasma in the presence of an external dipole magnetic field gives rise to a current in the azimuthal direction. The azimuthal current produced due to the motion of the magnetofluid modifies the magnetic field structure inside the disc and generates a poloidal magnetic field for the disc. The solutions we have found show that the radial inflow, pressure and density distributions are strongly modified by the electrical conductivity both in relativistic and Newtonian regimes. However, the range of conductivity coefficient is different for both regimes, as well as that of the angular momentum parameter and the radius of the innermost stable circular orbit. Furthermore, it is shown that the azimuthal velocity of the disc which is not dependent on conductivity is sub-Keplerian in all radial distances for both regimes. Owing to the presence of pressure gradient and magnetic forces. This work may also be important for the general relativistic computational magnetohydrodynamics that suffers from the lack of exact analytic solutions that are needed to test computer codes.

  1. Hoyle-Lyttleton Accretion onto Accretion Disks

    Fukue, Jun; Ioroi, Masayuki

    1999-01-01

    We investigate Hoyle-Lyttleton accretion for the case where the central source is a luminous accretion disk. %In classical Hoyle-Lyttleton accretion onto a ``spherical'' source, accretion takes place in an axially symmetric manner around a so-called accretion axis. The accretion rate of the classical Hoyle-Lyttleton accretion onto a non-luminous object and $\\Gamma$ the luminosity of the central object normalized by the Eddington luminosity. %If the central object is a compact star with a lumi...

  2. The properties of discs around planets and brown dwarfs as evidence for disc fragmentation

    Stamatellos, Dimitris

    2015-01-01

    Direct imaging searches have revealed many very low-mass objects, including a small number of planetary mass objects, as wide-orbit companions to young stars. The formation mechanism of these objects remains uncertain. In this paper we present the predictions of the disc fragmentation model regarding the properties of the discs around such low-mass objects. We find that the discs around objects that have formed by fragmentation in discs hosted by Sun-like stars (referred to as 'parent' discs and 'parent' stars) are more massive than expected from the ${M}_{\\rm disc}-M_*$ relation (which is derived for stars with masses $M_*>0.2 {\\rm M}_{\\odot}$). Accordingly, the accretion rates onto these objects are also higher than expected from the $\\dot{M}_*-M_*$ relation. Moreover there is no significant correlation between the mass of the brown dwarf or planet with the mass of its disc nor with the accretion rate from the disc onto it. The discs around objects that form by disc fragmentation have larger than expected m...

  3. Simulations of the Galactic Centre Stellar Discs In a Warped Disc Origin Scenario

    The Galactic Center (GC) hosts a population of young stars some of which seem to form a system of mutually inclined warped discs. While the presence of young stars in the close vicinity of the massive black hole is already problematic, their orbital configuration makes the situation even more puzzling. We present a possible warped disc origin scenario for these stars, which assumes an initially flat accretion disc which develops a warp through Pringle instability, or Bardeen-Petterson Effect. By working out the critical radii and the time scales involved, we argue that disc warping is plausible for GC parameters. We construct time evolution models for such discs considering the discs' self-gravity, and the torques exerted by the surrounding old star cluster. Our simulations suggest that the best agreement for a purely self-gravitating model is obtained for a disc-to-black hole mass ratio of Md/Mbh ∼ 0.001.

  4. Simulations of the Galactic Centre Stellar Discs In a Warped Disc Origin Scenario

    Ulubay-Siddiki, A.; Bartko, H.

    2012-07-01

    The Galactic Center (GC) hosts a population of young stars some of which seem to form a system of mutually inclined warped discs. While the presence of young stars in the close vicinity of the massive black hole is already problematic, their orbital configuration makes the situation even more puzzling. We present a possible warped disc origin scenario for these stars, which assumes an initially flat accretion disc which develops a warp through Pringle instability, or Bardeen-Petterson Effect. By working out the critical radii and the time scales involved, we argue that disc warping is plausible for GC parameters. We construct time evolution models for such discs considering the discs' self-gravity, and the torques exerted by the surrounding old star cluster. Our simulations suggest that the best agreement for a purely self-gravitating model is obtained for a disc-to-black hole mass ratio of Md/Mbh ~ 0.001.

  5. Radially truncated galactic discs

    De Grijs, R; Wesson, K H; Grijs, Richard de; Kregel, Michiel; Wesson, Karen H.

    2000-01-01

    We present the first results of a systematic analysis of radially truncatedexponential discs for four galaxies of a sample of disc-dominated edge-onspiral galaxies. Edge-on galaxies are very useful for the study of truncatedgalactic discs, since we can follow their light distributions out to largerradii than in less highly inclined galaxies. The origin of these truncationsand their asymmetry and sharpness are helpful to better constrain theories ofgalaxy formation. In general, the discs of our sample galaxies are truncated at similar radiion either side of their centres. With the exception of the disc of ESO 416-G25,it appears that our sample galaxies are closely symmetric, in terms of both thesharpness of their disc truncations and the truncation length. However, thetruncations occur over a larger region and not as abruptly as found by van derKruit & Searle (KS1-4). We show that the truncated luminosity distributions of our sample galaxies,if also present in the mass distributions, comfortably meet the r...

  6. Gravitating discs around black holes

    Karas, Vladimír; Huré, J.-M.; Semerák, O.

    2004-01-01

    Roč. 21, č. 7 (2004), R1-R5. ISSN 0264-9381 R&D Projects: GA ČR GA205/03/0902; GA AV ČR KSK1048102 Institutional research plan: CEZ:AV0Z1003909 Keywords : black holes * accretion discs * general relativity Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 2.941, year: 2004

  7. Episodic accretion, protostellar radiative feedback, and their role in low-mass star formation

    Stamatellos, Dimitris; Hubber, David A

    2012-01-01

    Protostars grow in mass by accreting material through their discs, and this accretion is initially their main source of luminosity. The resulting radiative feedback heats the environments of young protostars, and may thereby suppress further fragmentation and star formation. There is growing evidence that the accretion of material onto protostars is episodic rather than continuous; most of it happens in short bursts that last up to a few hundred years, whereas the intervals between these outbursts of accretion could be thousands of years. We have developed a model to include the effects of episodic accretion in simulations of star formation. Episodic accretion results in episodic radiative feedback, which heats and temporarily stabilises the disc, suppressing the growth of gravitational instabilities. However, once an outburst has been terminated, the luminosity of the protostar is low, and the disc cools rapidly. Provided that there is enough time between successive outbursts, the disc may become gravitation...

  8. Have proto-planetary discs formed planets?

    Greaves, J S

    2010-01-01

    It has recently been noted that many discs around T Tauri stars appear to comprise only a few Jupiter-masses of gas and dust. Using millimetre surveys of discs within six local star-formation regions, we confirm this result, and find that only a few percent of young stars have enough circumstellar material to build gas giant planets, in standard core accretion models. Since the frequency of observed exo-planets is greater than this, there is a `missing mass' problem. As alternatives to simply adjusting the conversion of dust-flux to disc mass, we investigate three other classes of solution. Migration of planets could hypothetically sweep up the disc mass reservoir more efficiently, but trends in multi-planet systems do not support such a model, and theoretical models suggest that the gas accretion timescale is too short for migration to sweep the disc. Enhanced inner-disc mass reservoirs are possible, agreeing with predictions of disc evolution through self-gravity, but not adding to millimetre dust-flux as t...

  9. The formation of planets by disc fragmentation

    Stamatellos Dimitris

    2013-04-01

    Full Text Available I discuss the role that disc fragmentation plays in the formation of gas giant and terrestrial planets, and how this relates to the formation of brown dwarfs and low-mass stars, and ultimately to the process of star formation. Protostellar discs may fragment, if they are massive enough and can cool fast enough, but most of the objects that form by fragmentation are brown dwarfs. It may be possible that planets also form, if the mass growth of a proto-fragment is stopped (e.g. if this fragment is ejected from the disc, or suppressed and even reversed (e.g by tidal stripping. I will discuss if it is possible to distinguish whether a planet has formed by disc fragmentation or core accretion, and mention of a few examples of observed exoplanets that are suggestive of formation by disc fragmentation.

  10. From birth to death of protoplanetary discs: modelling their formation, evolution and dispersal

    Kimura, Shigeo S.; Kunitomo, Masanobu; Takahashi, Sanemichi Z.

    2016-09-01

    The formation, evolution and dispersal processes of protoplanetary discs are investigated and the disc lifetime is estimated. The gravitational collapse of a pre-stellar core forms both a central star and a protoplanetary disc. The central star grows by accretion from the disc and irradiation by the central star heats up the disc and generates a thermal wind, which results in the disc's dispersal. Using the one-dimensional diffusion equation, we calculate the evolution of protoplanetary discs numerically. To calculate the disc evolution from formation to dispersal, we add source and sink terms that represent gas accretion from pre-stellar cores and photoevaporation, respectively. We find that the disc lifetimes of typical pre-stellar cores are around 2-4 million years (Myr). A pre-stellar core with high angular momentum forms a larger disc with a long lifetime, while a disc around an X-ray-luminous star has a short lifetime. Integrating disc lifetimes under various masses and angular velocities of pre-stellar cores and X-ray luminosities of young stellar objects, we obtain the disc fraction at a given stellar age and mean lifetime of the disc. Our model indicates that the mean lifetime of a protoplanetary disc is 3.7 Myr, which is consistent with the observational estimate from young stellar clusters. We also find that the dispersion of X-ray luminosity is needed to reproduce the observed disc fraction.

  11. Compressive vs Solenoidal Turbulence and Violent Disc Instability

    Mandelker, Nir; Dekel, Avishai; Inoue, Shigeki; Ceverino, Daniel; Primack, Joel

    2015-08-01

    High redshift star-forming galaxies (SFGs) exhibit star-formation rates (SFR) 20-100 times higher than local SFGs. This increased SFR is due to much higher gas fractions and surface densities prevalent in high redshift SFG. Such high gas fractions cause the disc to become violently unstable and undergo a phase of violent disc instability (VDI). During the VDI phase, there is rapid inflow of gas towards the galactic centre with inflow times comparable to the disc orbital time, which can lead to a compact "blue nugget". In addition, giant ~kpc scale star-forming clumps are formed, the most massive of which survive dtellar feedback and migrate towards the centre, where they coalesce in the growing bulge. While the existence of giant clumps is commonly associated with Toomre instability, high resolution cosmological simulations of galaxy formation show that active clump formation occurs even in regions where the Toomre Q parameter is well above the threshold for stability. The simulations suggest instead a non-linear instability, where frequent minor mergers and intense inflow by cold streams increase the compressive mode of turbulence relative to the solenoidal mode. We find this can induce clump formation even in regions that should be stable according to linear Toomre theory. Thus we show that mergers, smooth accretion from the cosmic web and VDI all go hand in hand in driving galaxy evolution at high redshift.

  12. Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances

    Snaith, O.; Haywood, M.; Di Matteo, P.; Lehnert, M. D.; Combes, F.; Katz, D.; Gómez, A.

    2015-06-01

    We develop a chemical evolution model to study the star formation history of the Milky Way. Our model assumes that the Milky Way has formed from a closed-box-like system in the inner regions, while the outer parts of the disc have experienced some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age to recover the star formation history of the Galaxy. Our method enables us to recover the star formation history of the Milky Way in the first Gyrs with unprecedented accuracy in the inner (R 9-10 kpc) discs, as sampled in the solar vicinity. We show that half the stellar mass formed during the thick-disc phase in the inner galaxy during the first 4-5 Gyr. This phase was followed by a significant dip in star formation activity (at 8-9 Gyr) and a period of roughly constant lower-level star formation for the remaining 8 Gyr. The thick-disc phase has produced as many metals in 4 Gyr as the thin-disc phase in the remaining 8 Gyr. Our results suggest that a closed-box model is able to fit all the available constraints in the inner disc. A closed-box system is qualitatively equivalent to a regime where the accretion rate maintains a high gas fraction in the inner disc at high redshift. In these conditions the SFR is mainly governed by the high turbulence of the interstellar medium. By z ~ 1 it is possible that most of the accretion takes place in the outer disc, while the star formation activity in the inner disc is mostly sustained by the gas that is not consumed during the thick-disc phase and the continuous ejecta from earlier generations of stars. The outer disc follows a star formation history very similar to that of the inner disc, although initiated at z ~ 2, about 2 Gyr before the onset of the thin-disc formation in the inner disc.

  13. Understanding X-ray reflection as a probe of accreting black holes

    Wilkins, Daniel Richard

    2013-01-01

    The reflection of the X-rays emitted from a corona of energetic particles surrounding an accreting black hole from the accretion disc is investigated in the context of probing the structure of the central regions as well as the physical processes that power some of the brightest objects seen in the Universe. A method is devised to measure the emissivity profile of the accretion disc, that is the reflected flux as a function of radius in the disc. This method exploits the variation in the D...

  14. Planet-disc interaction on a freely moving mesh

    Munoz, Diego J; Springel, Volker; Hernquist, Lars

    2014-01-01

    General-purpose, moving-mesh schemes for hydrodynamics have opened the possibility of combining the accuracy of grid-based numerical methods with the flexibility and automatic resolution adaptivity of particle-based methods. Due to their supersonic nature, Keplerian accretion discs are in principle a very attractive system for applying such freely moving mesh techniques. However, the high degree of symmetry of simple accretion disc models can be difficult to capture accurately by these methods, due to the generation of geometric grid noise and associated numerical diffusion, which is absent in polar grids. To explore these and other issues, in this work we study the idealized problem of two-dimensional planet-disc interaction with the moving-mesh code AREPO. We explore the hydrodynamic evolution of discs with planets through a series of numerical experiments that vary the planet mass, the disc viscosity and the mesh resolution, and compare the resulting surface density, vortensity field and tidal torque with ...

  15. Numerical simulations of disc-planet interactions

    Paardekooper, Richard P Nelson Sijme-Jan

    2009-01-01

    The gravitational interaction between a protoplanetary disc and planetary sized bodies that form within it leads to the exchange of angular momentum, resulting in migration of the planets and possible gap formation in the disc for more massive planets. In this article, we review the basic theory of disc-planet interactions, and discuss the results of recent numerical simulations of planets embedded in protoplanetary discs. We consider the migration of low mass planets and recent developments in our understanding of so-called type I migration when a fuller treatment of the disc thermodynamics is included. We discuss the runaway migration of intermediate mass planets (so-called type III migration), and the migration of giant planets (type II migration) and the associated gap formation in the disc. The availability of high performance computing facilities has enabled global simulations of magnetised, turbulent discs to be computed, and we discuss recent results for both low and high mass planets embedded in such...

  16. Accretion by the Galaxy

    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.”

  17. Intermediate mass black holes in AGN discs - I. Production and growth

    McKernan, B.; Ford, K. E. S.; Lyra, W.; Perets, H. B.

    2012-09-01

    Here we propose a mechanism for efficiently growing intermediate mass black holes (IMBH) in discs around supermassive black holes. Stellar mass objects can efficiently agglomerate when facilitated by the gas disc. Stars, compact objects and binaries can migrate, accrete and merge within discs around supermassive black holes. While dynamical heating by cusp stars excites the velocity dispersion of nuclear cluster objects (NCOs) in the disc, gas in the disc damps NCO orbits. If gas damping dominates, NCOs remain in the disc with circularized orbits and large collision cross-sections. IMBH seeds can grow extremely rapidly by collisions with disc NCOs at low relative velocities, allowing for super-Eddington growth rates. Once an IMBH seed has cleared out its feeding zone of disc NCOs, growth of IMBH seeds can become dominated by gas accretion from the active galactic nucleus (AGN) disc. However, the IMBH can migrate in the disc and expand its feeding zone, permitting a super-Eddington accretion rate to continue. Growth of IMBH seeds via NCO collisions is enhanced by a pile-up of migrators. We highlight the remarkable parallel between the growth of IMBH in AGN discs with models of giant planet growth in protoplanetary discs. If an IMBH becomes massive enough it can open a gap in the AGN disc. IMBH migration in AGN discs may stall, allowing them to survive the end of the AGN phase and remain in galactic nuclei. Our proposed mechanisms should be more efficient at growing IMBH in AGN discs than the standard model of IMBH growth in stellar clusters. Dynamical heating of disc NCOs by cusp stars is transferred to the gas in an AGN disc helping to maintain the outer disc against gravitational instability. Model predictions, observational constraints and implications are discussed in a companion paper (Paper II).

  18. Inner disc obscuration in GRS 1915+105 based on relativistic slim disc model

    Vierdayanti, K.; Sądowski, A.; Mineshige, L.S.; Bursa, Michal

    2013-01-01

    Roč. 436, č. 1 (2013), s. 71-81. ISSN 0035-8711 Institutional support: RVO:67985815 Keywords : accretion discs * black hole physics * GRS 1915+105 Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.226, year: 2013

  19. Misaligned accretion on to supermassive black hole binaries

    Dunhill, Alex; Nixon, Chris; King, Andrew

    2014-01-01

    We present the results of high-resolution numerical simulations of gas clouds falling onto binary supermassive black holes to form circumbinary accretion discs, with both prograde and retrograde cloud orbits. We explore a range of clouds masses and cooling rates. We find that for low mass discs that cool fast enough to fragment, prograde discs are significantly shorter-lived than similar discs orbiting retrograde with respect to the binary. For fragmenting discs of all masses, we also find that prograde discs fragment across a narrower radial region. If the cooling is slow enough that the disc enters a self-regulating gravitoturbulent regime, we find that alignment between the disc and binary planes occurs on a timescale primarily dictated by the disc thickness. We estimate realistic cooling times for such discs, and find that in the majority of cases we expect fragmentation to occur. The longer lifetime of low-mass fragmenting retrograde discs allows them to drive significant binary evolution, and may provid...

  20. Accreting Protoplanets in the LkCa 15 Transition Disk

    Sallum, S; Eisner, J A; Close, L M; Hinz, P; Kratter, K; Males, J; Skemer, A; Macintosh, B; Tuthill, P; Bailey, V; Defrère, D; Morzinski, K; Rodigas, T; Spalding, E; Vaz, A; Weinberger, A J

    2015-01-01

    Exoplanet detections have revolutionized astronomy, offering new insights into solar system architecture and planet demographics. While nearly 1900 exoplanets have now been discovered and confirmed, none are still in the process of formation. Transition discs, protoplanetary disks with inner clearings best explained by the influence of accreting planets, are natural laboratories for the study of planet formation. Some transition discs show evidence for the presence of young planets in the form of disc asymmetries or infrared sources detected within their clearings, as in the case of LkCa 15. Attempts to observe directly signatures of accretion onto protoplanets have hitherto proven unsuccessful. Here we report adaptive optics observations of LkCa 15 that probe within the disc clearing. With accurate source positions over multiple epochs spanning 2009 - 2015, we infer the presence of multiple companions on Keplerian orbits. We directly detect H{\\alpha} emission from the innermost companion, LkCa 15 b, evincing...

  1. Numerical models of rotating accretion flows around black holes

    Igumenshchev, I V

    1999-01-01

    Numerical, two-dimensional, time-dependent hydrodynamical models of geometrically thick accretion discs around black holes are presented. Accretion flows with non-effective radiation cooling (ADAFs) can be both convectively stable or unstable depending on the value of the viscosity parameter \\alpha. The high viscosity flows (\\alpha~1) are stable and have a strong equatorial inflow and bipolar outflows. The low viscosity flows (\\alpha<0.1) are convectively unstable and this induces quasi-periodic variability.

  2. Evolution of Disk Accretion

    Calvet, Nuria; Hartmann, Lee; Strom, Stephen E.

    1999-01-01

    We review the present knowledge of disk accretion in young low mass stars, and in particular, the mass accretion rate and its evolution with time. The methods used to obtain mass accretion rates from ultraviolet excesses and emission lines are described, and the current best estimates of mass accretion rate for Classical T Tauri stars and for objects still surrounded by infalling envelopes are given. We argue that the low mass accretion rates of the latter objects require episodes of high mas...

  3. Herniated lumbar disc

    Jordon, Jo; Konstantinou, Kika; O'Dowd, John

    2011-01-01

    Herniated lumbar disc is a displacement of disc material (nucleus pulposus or annulus fibrosis) beyond the intervertebral disc space. The highest prevalence is among people aged 30 to 50 years, with a male to female ratio of 2:1.

  4. Herniated Lumbar Disc

    Herniated Lumbar Disc What is a herniated disc? Nonsurgical treatment Medication and pain management Surgery What can I expect after ... at and just below the waist. A herniated lumbar disc can press on the nerves in the spine ...

  5. The accretion flow to the intermittent accreting ms pulsar, HETE J1900.1-2455, as observed by XMM-Newton and RXTE

    Papitto, A.; D'Aì, A.; T. Di Salvo; Egron, E.; Bozzo, E.; Burderi, L.; Iaria, R.; Riggio, A.; Menna, M. T.

    2012-01-01

    We present a study of the accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1-2455, based on observations performed simultaneously by XMM-Newton and RXTE. The 0.33-50 keV spectrum is described by the sum of a hard Comptonized component originated in an optically thin {\\tau}~1 corona, a soft kTin~0.2 keV component interpreted as accretion disc emission, and of disc reflection of the hard component. Two emission features are detected at energies of 0.98(1) and 6.58(7) ...

  6. The disc-jet-spin connection: 3C273

    Done, C.

    2014-07-01

    Black hole spin is difficult to measure as it leaves an imprint only close to the horizon, but it may be required to produce most dramatic relativistic jets. For stellar mass black holes there are two established methods to measure spin, from the disc continuum peak temperature in disc dominated states, and from the iron line profile in states with more hard X-ray flux. However, these two methods do not always agree! In AGN the higher black hole mass means a lower disc temperature, so the peak is in the unobservable EUV region, and only the iron line method can be used, but in very high mass AGN, the disc temperature is so low that the peak starts to be visible in the far UV. We use archival data from 3C273 where the observed far UV emission clearly requires a disc around a high spin black hole. The accretion flow dissipates all the available accretion energy, yet the jet in this system is known to be as powerful as the observed accretion flow. Since there is no accretion power left, the jet must be powered by another source of energy - and the only one remaining is black hole spin.

  7. Origin of the X-ray disc-reflection steep radial emissivity

    Svoboda, Jiří; Dovčiak, Michal; Goosmann, René W.; Jethwa, Prashin; Karas, Vladimír; Miniutti, Giovanni; Guainazzi, Matteo

    2012-01-01

    X-ray reflection off the accretion disc surrounding a black hole, together with the associated broad iron K$\\alpha$ line, has been widely used to constrain the innermost accretion-flow geometry and black hole spin. Some recent measurements have revealed steep reflection emissivity profiles in a number of active galactic nuclei and X-ray binaries. We explore the physically motivated conditions that give rise to the observed steep disc-reflection emissivity profiles. We perform a set of simulat...

  8. Dynamics of perturbation modes in protoplanetary discs : new effects of self-gravity and velocity shear

    Mamatsashvili, George

    2011-01-01

    Protoplanetary discs, composed of gas and dust, usually surround young stellar objects and serve two main purposes: they determine the accretion of matter onto the central object and also represent sites of planet formation. The accretion proceeds through the transport of angular momentum outwards allowing the disc matter to fall towards the centre. A mechanism responsible for the transport can be turbulence, waves or other coherent structures originating from various instabili...

  9. MHD Simulations of Magnetized Stars in the Propeller Regime of Accretion

    Lii Patrick

    2014-01-01

    Full Text Available Accreting magnetized stars may be in the propeller regime of disc accretion in which the angular velocity of the stellar magnetosphere exceeds that of the inner disc. In these systems, the stellar magnetosphere acts as a centrifugal barrier and inhibits matter accretion onto the rapidly rotating star. Instead, the matter accreting through the disc accumulates at the disc-magnetosphere interface where it picks up angular momentum and is ejected from the system as a wide-angled outflow which gradually collimates at larger distances from the star. If the ejection rate is lower than the accretion rate, the matter will accumulate at the boundary faster than it can be ejected; in this case, accretion onto the star proceeds through an episodic accretion instability in which the episodes of matter accumulation are followed by a brief episode of simultaneous ejection and accretion of matter onto the star. In addition to the matter dominated wind component, the propeller outflow also exhibits a well-collimated, magnetically-dominated Poynting jet which transports energy and angular momentum away from the star. The propeller mechanism may explain some of the weakly-collimated jets and winds observed around some T Tauri stars as well as the episodic variability present in their light curves. It may also explain some of the quasi-periodic variability observed in cataclysmic variables, millisecond pulsars and other magnetized stars.

  10. The dispersal of protoplanetary discs

    Ercolano Barbara

    2013-04-01

    Full Text Available Protoplanetary discs are a natural consequence of the star formation process and as such are ubiquitous around low-mass stars. They are fundamental to planet formation as they hold the reservoir of material from which planets form. Their evolution and final dispersal and the timescales that regulate these process are therefore of particular interest. In this contribution I will review the observational evidence for the dispersal of discs being dominated by two timescales and for the final dispersal to occur quickly and from the inside out. I will discuss the current theoretical models, including X-ray photoevaporation, showing that the latter provides a natural explanation to the observed behaviour and review supporting and contrasting evidence. I will finally introduce a new mechanism based on the interaction between planet formation and photoevaporation that may explain a particular class of transition discs with large inner holes and high accretion rates that are problematic for photoevaporation models and planet formation models alone.

  11. Magnetic white dwarfs with debris discs

    Külebi, Baybars; Lorén-Aguilar, Pablo; Isern, Jordi; García-Berro, Enrique

    2013-01-01

    It has long been accepted that a possible mechanism for explaining the existence of magnetic white dwarfs is the merger of a binary white dwarf system, as there are viable mechanisms for producing sustainable magnetic fields within the merger product. However, the lack of rapid rotators in the magnetic white dwarf population has been always considered a problematic issue of this scenario. Smoothed Particle Hydrodynamics simulations show that in mergers in which the two white dwarfs have different masses a disc around the central compact object is formed. If the central object is magnetized it can interact with the disc through its magnetosphere. The torque applied by the disc changes the spin of the star, whereas the transferred angular momentum from the star to the disc determines the properties of the disc. In this work we build a model for the disc evolution under the effect of magnetic accretion, and for the angular momentum evolution of the star, which can be compared with the observations. Our model pre...

  12. The origin and evolution of transition discs: successes, problems and open questions

    Owen, James E

    2015-01-01

    Transition discs are protoplanetary discs that show evidence for large holes or wide gaps (with widths comparable to their radii) in their dust component. These discs could be giving us clues about the disc destruction mechanism or hints about the location and time-scales for the formation of planets. However, at the moment there remain key gaps in our theoretical understanding. The vast majority of transition discs are accreting onto their central stars, indicating that - at least close to the star - dust has been depleted from the gas by a very large amount. In this review, we discuss evidence for two distinct populations of transition discs: mm-faint - those with low mm-fluxes, small holes ($\\lesssim$10AU) and low accretion rates (~1e-10-1e-9 Msun/yr) and mm-bright - discs with large mm-fluxes, large holes ($\\gtrsim$20 AU) and high accretion rates ~1e-8 Msun/yr. MM-faint transition discs are consistent with what would naively be expected from a disc undergoing dispersal; however, mm-bright discs are not, a...

  13. Infall and accretion

    Combes, F.

    2007-01-01

    Gas infall and accretion play a fundamental role in galaxy formation, and several processes of accretion are reviewed. In particular the cold accretion may solve to some extent the angular momentum problem in disk formation, while it is aggravated by mergers. Gas accretion is one of the main actor in secular evolution: it is required to account for recurrent bar formation, and to explain the feedback cycles of formation of bulges and black holes, with correlated masses. Infall is also require...

  14. Neutrino transport in accretion disks

    Sawyer, R F

    2003-01-01

    We test approximate approaches to solving a neutrino transport problem that presents itself in the analysis of some accretion-disk models. Approximation #1 consists of replacing the full, angular- dependent, distribution function by a two-stream simulation, where the streams are respectively outwardly and inwardly directed, with angles $\\cos \\theta=\\pm 1/\\sqrt{3}$ to the vertical. In this approximation the full energy dependence of the distribution function is retained, as are the energy and temperature dependences of the scattering rates. Approximation #2, used in recent works on the subject, replaces the distribution function by an intensity function and the scattering rates by temperature-energy-averaged quantities. We compare the approximations to the results of solving the full Boltzmann equation. Under some interesting conditions, approximation #1 passes the test; approximation #2 does not. We utilize the results of our analysis to construct a toy model of a disc at a temperature and density such that r...

  15. Spectra of thermally unstable slim discs

    Szuszkiewicz, E; Zampieri, L; Szuszkiewicz, Ewa; Turolla, Roberto; Zampieri, Luca

    2000-01-01

    Thermal instability driven by radiation pressure might be relevant for intrinsically bright accreting sources. The most promising candidate where this instability seems to be at work is one of the two known galactic superluminal sources, GRS 1915+105 (Belloni et al. 1997). In spite of being of relevance, this scenario has not yet been confirmed by proper time-dependent modelling. Non-linear time-dependent calculations performed by Szuszkiewicz and Miller (1998) show that thermally unstable discs undergo limit-cycle behaviour with successive evacuation and refilling of the central parts of the disc. This evolution is very similar to the one proposed by Belloni et al. (1997) in their phenomenological model. Further investigations are needed to confirm the thermal instability being operational in this source. First of all the spectra emitted from the disc during its evolution should be calculated and compared with observations. Here such spectra are computed assuming local blackbody emission from the best studie...

  16. Multiple accretion events as a trigger for Sagittarius A* activity

    Kunneriath, Devaky; Czerny, B.; Karas, Vladimír; Das, T. K.

    Cambridge: Cambridge University Press, 2014 - (Sjouwerman, L.), s. 320-321. (IAU Symposium Proceedings Series. 303). ISBN 9781107044616. ISSN 1743-9213. [Symposium of the International Astronomical Union /303./. Santa Fe (US), 30.09.2013-04.10.2013] Institutional support: RVO:67985815 Keywords : accretion discs * galaxy center Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  17. The Role of Discs in the Collapse and Fragmentation of Prestellar Cores

    Lomax, O; Hubber, D A

    2015-01-01

    Disc fragmentation provides an important mechanism for producing low mass stars in prestellar cores. Here, we describe Smoothed Particle Hydrodynamics simulations which show how populations of prestellar cores evolve into stars. We find the observed masses and multiplicities of stars can be recovered under certain conditions. First, protostellar feedback from a star must be episodic. The continuous accretion of disc material on to a central protostar results in local temperatures which are too high for disc fragmentation. If, however, the accretion occurs in intense outbursts, separated by a downtime of $\\sim10^4\\,\\mathrm{years}$, gravitational instabilities can develop and the disc can fragment. Second, a significant amount of the cores' internal kinetic energy should be in solenoidal turbulent modes. Cores with less than a third of their kinetic energy in solenoidal modes have insufficient angular momentum to form fragmenting discs. In the absence of discs, cores can fragment but results in a top heavy dist...

  18. Two-dimensional models of layered protoplanetary discs. I. - The ring instability

    Wünsch, Richard; Klahr, Hubert; Rózycka, M.

    2005-01-01

    Roč. 362, č. 1 (2005), s. 361-368. ISSN 0035-8711 R&D Projects: GA AV ČR IAB3003106 Institutional research plan: CEZ:AV0Z10030501 Keywords : accretion, accretion discs * hydrodynamics * instabilities * Solar system: formation Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.352, year: 2005

  19. Chemodynamical analysis of bulge stars for simulated disc galaxies

    A. Rahimi; Kawata, D.; Brook, Chris B.; Gibson, Brad K.

    2009-01-01

    We analyse the kinematics and chemistry of the bulge stars of two simulated disc galaxies using our chemodynamical galaxy evolution code GCD+. First we compare stars that are born inside the galaxy with those that are born outside the galaxy and are accreted into the centre of the galaxy. Stars that originate outside of the bulge are accreted into it early in its formation within 3 Gyrs so that these stars have high [alpha/Fe] as well as having a high total energy reflecting their accretion t...

  20. Accretion on to Magnetic White Dwarfs

    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. Accretion of the Moon from non-canonical disks

    Salmon, Julien

    2014-01-01

    Impacts that leave the Earth-Moon system with a large excess in angular momentum have recently been advocated as a means of generating a protolunar disc with a composition that is nearly identical to that of the Earth's mantle. We here investigate the accretion of the Moon from discs generated by such "non-canonical" impacts, which are typically more compact than discs produced by canonical impacts and have a higher fraction of their mass initially located inside the Roche limit. Our model predicts a similar overall accretional history for both canonical and non-canonical discs, with the Moon forming in three consecutive steps over hundreds of years. However, we find that, to yield a lunar-mass Moon, the more compact non-canonical discs must initially be more massive than implied by prior estimates, and only a few of the discs produced by impact simulations to date appear to meet this condition. Non-canonical impacts require that capture of the Moon into the evection resonance with the Sun reduced the Earth-M...

  2. Binary accretion rates: dependence on temperature and mass-ratio

    Young, Matthew D

    2015-01-01

    We perform a series of 2D smoothed particle hydrodynamics (SPH) simulations of gas accretion onto binaries via a circumbinary disc, for a range of gas temperatures and binary mass ratios ($q$). We show that increasing the gas temperature increases the accretion rate onto the primary for all values of the binary mass ratio: for example, for $q=0.1$ and a fixed binary separation, an increase of normalised sound speed by a factor of $5$ (from our "cold" to "hot" simulations) changes the fraction of the accreted gas that flows on to the primary from $ 10\\%$ to $\\sim40\\%$. We present a simple parametrisation for the average accretion rate of each binary component accurate to within a few percent and argue that this parametrisation (rather than those in the literature based on warmer simulations) is relevant to supermassive black hole accretion and all but the widest stellar binaries. We present trajectories for the growth of $q$ during circumbinary disc accretion and argue that the period distribution of stellar "...

  3. Core-assisted gas capture instability: a new mode of giant planet formation by gravitationally unstable discs

    Nayakshin, Sergei; Boley, Aaron C

    2014-01-01

    Giant planet formation in the core accretion (CA) paradigm is predicated by the formation of a core, assembled by the coagulation of grains and later by planetesimals within a protoplanetary disc. In contrast, in the disc instability paradigm, giant planet formation is believed to be independent of core formation: massive self-gravitating gas fragments cool radiatively and collapse as a whole. We show that giant planet formation in the disc instability model may be also enhanced by core formation for reasons physically very similar to the CA paradigm. In the model explored here, efficient grain sedimentation within an initial fragment (rather than the disc) leads to the formation of a core composed of heavy elements. We find that massive atmospheres form around cores and undergo collapse as a critical core mass is exceeded, analogous to CA theory. The critical mass of the core to initiate such a collapse depends on the fragment mass and metallicity, as well as core luminosity, but ranges from less than 1 to a...

  4. A Review of a Study on Disc Brake Noise

    2006-01-01

    With the development of the automotive industry, disc brake noise has become an issue of growing concern to the automotive industry and customers. In this paper, the types of disc brake noise have been discussed. Ajter that, the theories and models that have been proposed as an explanation of brake squeal are reviewed. On the basis of these theories and models, some example simulations of disc brake squeal which use the Finite Element method and mathematical model have been introduced.

  5. Asymmetric MHD Outflows/Jets from Accreting T Tauri Stars

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

    2015-01-01

    A large set of 2.5D MHD simulations has been carried out for axisymmetric viscous/diffusive disc accretion to rotating magnetized stars for the purpose of assessing the conditions where the outflows or jets are asymmetric relative to the equatorial plane. Observations of jets from young stellar objects reveal the asymmetric outflows from some sources. The considered initial magnetic fields are symmetric about the equatorial plane and consist of a radially distributed field threading the disc (disc-field) and a stellar dipole field. (1). For pure disc-fields the symmetry or asymmetry of the outflows is affected by the midplane plasma $\\beta$ of the disc. For the low density discs with small plasma $\\beta$ values, outflows are observed to be symmetric about the equatorial plane to within 10% over timescales of hundreds of inner disc orbits. For the denser higher $\\beta$ discs, the coupling of the upper and lower coronal plasmas is broken, and quasi-periodic field motion in the two hemispheres becomes different....

  6. The inner cavity of the circumnuclear disc

    Blank, Marvin; Frank, Adam; Carroll-Nellenback, Jonathan J; Duschl, Wolfgang J

    2016-01-01

    The circumnuclear disc (CND) orbiting the Galaxy's central black hole is a reservoir of material that can ultimately provide energy through accretion, or form stars in the presence of the black hole, as evidenced by the stellar cluster that is presently located at the CND's centre. In this paper, we report the results of a computational study of the dynamics of the CND. The results lead us to question two paradigms that are prevalent in previous research on the Galactic Centre. The first is that the disc's inner cavity is maintained by the interaction of the central stellar cluster's strong winds with the disc's inner rim, and second, that the presence of unstable clumps in the disc implies that the CND is a transient feature. Our simulations show that, in the absence of a magnetic field, the interaction of the wind with the inner disc rim actually leads to a filling of the inner cavity within a few orbital time-scales, contrary to previous expectations. However, including the effects of magnetic fields stabi...

  7. The angular momentum of hot coronae around spiral galaxies and its impact on the evolution of star forming discs

    Pezzulli, G.; Fraternali, F.; Binney, J.

    2016-06-01

    Galaxy formation theory and recent observations indicate that spiral galaxies are surrounded by massive and hot coronae, which potentially constitute a huge source of mass and angular momentum for the star forming discs embedded within them. Accretion from these reservoirs is likely a key ingredient for the evolution of spiral galaxies, but our understanding of the involved processes requires more observational and theoretical investigation, both at global and local scales. In this talk, I focus on some theoretical aspects of the angular momentum distribution of hot coronae. I address, in particular, whether these structures can sustain the inside-out growth of spiral galaxies and what are the dynamical consequences of the accretion of hot coronal gas onto the disc. These processes can have a big impact on observable quantities, most notably gas-phase abundance gradients, which can be used to put constraints on theory. I finally mention ongoing work to understand whether a cosmologically motivated angular momentum distribution for the hot gas is compatible with the constraints from galaxy evolution.

  8. Viscous and resistive accretion flows with radially self-similar and outflows

    Faghei, Kazem

    2012-01-01

    The existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamics (MHD) simulations. In this paper, we study outflows of accretion flows in the presence of resistivity and toroidal magnetic field. The mechanism of energy dissipation in the flow is assumed to be the viscosity and the magnetic diffusivity due to turbulence in the accretion flow. It is also assumed that the magnetic diffusivity and the kinematic viscosity are not constant and vary by position and $\\alpha$-prescription is used for them. The influence of outflow emanating from accretion disc is considered as a sink for mass, angular momentum and energy. The self-similar method is used to solve the integrated equations that govern the behavior of the accretion flow in the presence of outflow. The solutions represent the disc which rotates faster and becomes cooler for stronger outflows. Moreover, by adding the magnetic diffusivity, the surface density and rotational velocity decrease, while the radial velocity and te...

  9. Magnetic field structure in accretion columns on HMXB and effects on CRSF

    Mukherjee Dipanjan

    2014-01-01

    Full Text Available In accreting neutron star binaries, matter is channelled by the magnetic fields from the accretion disc to the poles of neutron stars forming an accretion mound. We model such mounds by numerically solving the Grad-Shafranov equation for axisymmetric static MHD equilibria. From our solutions we infer local distortion of field lines due to the weight of accreted matter. Variation in mass loading at the accretion disc will alter the shape of the accretion mound which will also affect the local field distortion. From simulations of cyclotron resonance scattering features from HMXBs, we conclude that local field distortion will greatly affect the shape and nature of the CRSF. From phase resolved spectral analysis one can infer the local field structure and hence the nature of mass loading of field lines at the accretion disc. We also study the stability of such mounds by performing MHD simulations using the PLUTO MHD code. We find that pressure and gravity driven instabilities depend on the total mass accreted and the nature of mass loading of the field lines.

  10. Magnetic field structure in accretion columns on HMXB and effects on CRSF

    Mukherjee, Dipanjan; Mignone, Andrea

    2013-01-01

    In accreting neutron star binaries, matter is channelled by the magnetic fields from the accretion disc to the poles of neutron stars forming an accretion mound. We model such mounds by numerically solving the Grad-Shafranov equation for axisymmetric static MHD equilibria. From our solutions we infer local distortion of field lines due to the weight of accreted matter. Variation in mass loading at the accretion disc will alter the shape of the accretion mound which will also affect the local field distortion. From simulations of cyclotron resonance scattering features from HMXBs, we conclude that local field distortion will greatly affect the shape and nature of the CRSF. From phase resolved spectral analysis one can infer the local field structure and hence the nature of mass loading of field lines at the accretion disc. We also study the stability of such mounds by performing MHD simulations using the PLUTO MHD code. We find that pressure and gravity driven instabilities depend on the total mass accreted an...

  11. Silicon and Nickel Enrichment in Planet-Host Stars: Observations and Implications for the Core-Accretion Theory of Planet Formation

    Robinson, Sarah E.; Laughlin, Gregory; Bodenheimer, Peter; Fischer, Debra

    2006-01-01

    We present evidence that stars with planets exhibit statistically significant silicon and nickel enrichment over the general metal-rich population. We also present simulations which predict silicon enhancement of planet hosts within the context of the core-accretion hypothesis for giant planet formation. Because silicon and oxygen are both alpha elements, [Si/Fe] traces [O/Fe], so the silicon enhancement in planet hosts predicts that these stars are oxygen-rich as well. We present new numeric...

  12. Inner Disc Obscuration in GRS 1915+105 Based on Relativistic Slim Disc Model

    Vierdayanti, K; Mineshige, S; Bursa, M

    2013-01-01

    We study the observational signatures of the relativistic slim disc of 10 M_sun black hole, in a wide range of mass accretion rate, mdot, dimensionless spin parameter, a_ast, and viewing angle, i. In general, the innermost temperature, T_in increases with the increase of i for a fixed value of mdot and a_ast, due to the Doppler effect. However, for i > 50 and mdot > mdot_turn, T_in starts to decrease with the increase of mdot. This is a result of self-obscuration -- the radiation from the innermost hot part of the disc is blocked by the surrounding cooler part. The value of mdot_turn and the corresponding luminosities depend on a_ast and i. Such obscuration effects cause an interesting behavior on the disc luminosity (L_disc) -- T_in plane for high inclinations. In addition to the standard-disc branch which appears below mdot_turn and which obeys L_disc propto T_in^4 -relation, another branch above mdot_turn, which is nearly horizontal, may be observed at luminosities close to the Eddington luminosity. We sho...

  13. Mineral accretion in seawater

    Bozak, Ronald Richard

    2000-10-01

    By performing electrolysis in seawater a concrete-like accretion of precipitating aragonite (one crystalline form of CaCO3) and brucite (Mg(OH) 2) slowly develops onto the cathode. The accretion forms by high pH conditions caused by the reduction reactions occurring at the cathode. A solid casing of accretions over a preformed cathodic mesh has the potential for many engineering applications such as artificial reefs, sub-surface breakwaters and pipe construction. To investigate using mineral accretion as an alternative means of construction, experiments in the open coast, laboratory and ocean harbor have resulted in tables that can projected into a feasibility study. Inevitable current density variations over the cathodic framework and sensitivity to seawater hydrodynamics make accretion thickness difficult to predict and control in practice. Ideal conditions for growing a large-scale mineral accretion structure are still, clean ocean waters where low DCV power can be delivered on the order of years.

  14. Herschel-PACS observations of discs in the η Chamaeleontis association

    Riviere-Marichalar, P.; Elliott, P.; Rebollido, I.; Bayo, A.; Ribas, A.; Merín, B.; Kamp, I.; Dent, W. R. F.; Montesinos, B.

    2015-12-01

    Context. Protoplanetary discs are the birthplace of planets. Studying protoplanetary discs is the key to constraining theories of planet formation. By observing dust and gas in associations at different ages we can study the evolution of these discs, their clearing timescales, and their physical and geometrical properties.The stellar association η Cha is peculiar; some members still retain detectable amounts of gas in their discs at the late age of ~7 Myr, making it one of the most interesting young stellar associations in the solar neighbourhood. Aims: We characterise the properties of dust and gas in protoplanetary and transitional discs in the η Cha young cluster, with special emphasis on explaining the peculiarities that lead to the observed high disc detection fraction and prominent IR excesses at an age of ~7 Myr. Methods: We observed 17 members of the η Cha association with Herschel-PACS in photometric mode and line spectroscopic mode. A subset of members were also observed in range spectroscopic mode. The observations trace [OI] and H2O emissions at 63.18 and 63.32 μm, respectively, as well as CO, OH, CH+, and [CII] at different wavelengths for those systems observed in range mode. The photometric observations were used to build complete spectral energy distributions (SEDs) from the optical to the far-IR. High-resolution multi-epoch optical spectra with high signal-to-noise ratios were also analysed to study the multiplicity of the sources and look for further gas (accreting) and outflow indicators. Results: We detect four out of fifteen sources observed at 70 μm, four out of six at 100 μm, and six out of sixteen at 160 μm. Only one system shows [OI] emission at 63 μm, namely RECX 15 or J0843.3-7905. None of them shows far-IR line emission at any other wavelength. The [OI] emission toward RECX 15 points to the presence of an outflow; however, the emission is not extended. We study Hα emission among η Cha members and conclude that RECX 4, 5, 9, 11

  15. Accretion radiation from nearby isolated black holes

    Recent work attempting to establish the presence of dark matter in the solar neighbourhood has led to renewed interest in the search for the nature of this matter. Previous authors attempt to exclude large (>=2 solar mass) objects by considering their tidal effect on wide binaries. Here independent constraints on such dark massive objects, if they are black holes, are provided by the requirement that their radiation due to accretion from the ISM should not make the nearest ones directly observable as optical objects. The expected infrared brightness is also predicted. It is shown that halo holes must be less massive than about 103 solar masses, and that the dark matter in the galactic disc cannot be made up of black holes of mass more than 10solar masses. Even if black holes do not make up the dark matter, they are expected to be present in the disc as remnants of massive stars. (author)

  16. Bulk Comptonization by Turbulence in Accretion Disks

    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...

  17. Only marginal alignment of disc galaxies

    Andrae, Rene

    2011-01-01

    Testing theories of angular-momentum acquisition of rotationally supported disc galaxies is the key to understand the formation of this type of galaxies. The tidal-torque theory tries to explain this acquisition process in a cosmological framework and predicts positive autocorrelations of angular-momentum orientation and spiral-arm handedness on distances of 1Mpc/h. This disc alignment can also cause systematic effects in weak-lensing measurements. Previous observations claimed discovering such correlations but did not account for errors in redshift, ellipticity and morphological classifications. We explain how to rigorously propagate all important errors. Analysing disc galaxies in the SDSS database, we find that positive autocorrelations of spiral-arm handedness and angular-momentum orientations on distances of 1Mpc/h are plausible but not statistically significant. This result agrees with a simple hypothesis test in the Local Group, where we find no evidence for disc alignment. Moreover, we demonstrate tha...

  18. Hard Discs on the Hyperbolic Plane

    Modes, Carl D.; Kamien, Randall D.

    2007-01-01

    We examine a simple hard disc fluid with no long range interactions on the two dimensional space of constant negative Gaussian curvature, the hyperbolic plane. This geometry provides a natural mechanism by which global crystalline order is frustrated, allowing us to construct a tractable model of disordered monodisperse hard discs. We extend free area theory and the virial expansion to this regime, deriving the equation of state for the system, and compare its predictions with simulation near...

  19. The migration of gas giant planets in gravitationally unstable discs

    Stamatellos, Dimitris

    2015-01-01

    Planets form in the discs of gas and dust that surround young stars. It is not known whether gas giant planets on wide orbits form the same way as Jupiter or by fragmentation of gravitationally unstable discs. Here we show that a giant planet, which has formed in the outer regions of a protostellar disc, initially migrates fast towards the central star (migration timescale ~10,000 yr) while accreting gas from the disc. However, in contrast with previous studies, we find that the planet eventually opens up a gap in the disc and the migration is essentially halted. At the same time, accretion-powered radiative feedback from the planet, significantly limits its mass growth, keeping it within the planetary mass regime (i.e. below the deuterium burning limit) at least for the initial stages of disc evolution. Giant planets may therefore be able to survive on wide orbits despite their initial fast inward migration, shaping the environment in which terrestrial planets that may harbour life form.

  20. Daughter Fragmentation is Unlikely To Occur in Self-Gravitating Circumstellar Discs

    Forgan, Duncan

    2016-01-01

    Circumstellar discs are thought to be self-gravitating at very early times. If the disc is relatively cool, extended and accreting sufficiently rapidly, it can fragment into bound objects of order a few Jupiter masses and upwards. Given that the fragment's initial angular momentum is non-zero, and it will continue to accrete angular momentum from the surrounding circumstellar disc, we should expect that the fragment will also possess a relatively massive disc at early times. Therefore, we can ask: is disc fragmentation a hierarchical process? Or, can a disc fragment go on to produce its own self-gravitating circumfragmentary disc that produces daughter fragments? We investigate this using a set of nested 1D self-gravitating disc models. We calculate the radial structure of a marginally stable, self-gravitating circumstellar disc, and compute its propensity to fragmentation. We use this data to construct the local fragment properties at this radius. For each circumstellar disc model that results in fragmentati...

  1. The gravo-magneto disc instability with a viscous dead zone

    Martin, Rebecca G

    2013-01-01

    We consider the evolution of accretion discs that contain some turbulence within a disc dead zone, a region about the disc midplane of a disc that is not sufficiently ionised for the magneto-rotational instability (MRI) to drive turbulence. In particular, we determine whether additional sources of turbulence within a dead zone are capable of suppressing gravo-magneto (GM) disc outbursts that arise from a rapid transition from gravitationally produced to MRI produced turbulence. With viscous $\\alpha$ disc models we consider two mechanisms that may drive turbulence within the dead zone. First, we examine a constant $\\alpha$ parameter within the dead zone. This may be applicable to hydrodynamical instability, such as baroclinic instability, where the turbulence level is independent of the MRI active surface layer properties. In this case, we find that the disc will not become stable to the outbursts unless the dead zone turbulent viscosity is comparable to that in the MRI active surface layers. Under such condit...

  2. Search for substellar-mass companions and asymmetries in their parent discs

    Willson, M; Kluska, J; Monnier, J D; Ireland, M; Aarnio, A; Sitko, M L; Calvet, N; Espaillat, C; Wilner, D J

    2016-01-01

    Transitional discs are a class of circumstellar discs around young stars with extensive clearing of dusty material within their inner regions on 10s of au scales. One of the primary candidates for this kind of clearing is the formation of planet(s) within the disc that then accrete or clear their immediate area as they migrate through the disc. Our sample included eight transitional discs. Using the Keck/NIRC2 instrument we utilised the Sparse Aperture Masking (SAM) interferometry technique to search for asymmetries indicative of ongoing planet formation. We searched for close-in companions using both model fitting and interferometric image reconstruction techniques. Using simulated data, we derived diagnostics that helped us to distinguish between point sources and extended asymmetric disc emission. In addition, we investigated the degeneracy between the contrast and separation that appear for marginally resolved companions. We found FP Tau to contain a previously unseen disc wall, and DM Tau, LkHa 330, and ...

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

    Muñoz-Darias, T.; Casares, J.; 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 i...

  4. The subcritical baroclinic instability in local accretion disc models

    Lesur, G

    2009-01-01

    (abridged) Aims: We present new results exhibiting a subcritical baroclinic instability (SBI) in local shearing box models. We describe the 2D and 3D behaviour of this instability using numerical simulations and we present a simple analytical model describing the underlying physical process. Results: A subcritical baroclinic instability is observed in flows stable for the Solberg-Hoiland criterion using local simulations. This instability is found to be a nonlinear (or subcritical) instability, which cannot be described by ordinary linear approaches. It requires a radial entropy gradient weakly unstable for the Schwartzchild criterion and a strong thermal diffusivity (or equivalently a short cooling time). In compressible simulations, the instability produces density waves which transport angular momentum outward with typically alpha<3e-3, the exact value depending on the background temperature profile. Finally, the instability survives in 3D, vortex cores becoming turbulent due to parametric instabilities...

  5. Origin and Ubiquity of Short-Period Earth-like Planets: Evidence for the Sequential-Accretion Theory of Planet Formation

    Zhou, J L; Lin, D N C; Nagasawa, M

    2005-01-01

    The formation of gas giant planets is assumed to be preceded by the emergence of solid cores in the conventional sequential-accretion paradigm. This hypothesis implies that the presence of earth-like planets can be inferred from the detection of gas giants. A similar prediction cannot be made with the gravitational instability (hereafter GI) model which assumes that gas giants (hereafter giants) formed from the collapse of gas fragments analogous to their host stars. We propose an observational test for the determination of the dominant planet-formation channel. Based on the sequential-accretion (hereafter SA) model, we identify several potential avenues which may lead to the prolific formation of a population of close-in earth-mass ($M_\\oplus$) planets (hereafter close-in earths) around stars with 1) short-period or 2) solitary eccentric giants and 3) systems which contain intermediate-period resonant giants. In contrast, these close-in earths are not expected to form in systems where giants originated rapid...

  6. The structure of protoplanetary discs around evolving young stars

    Bitsch, Bertram; Lambrechts, Michiel; Morbidelli, Alessandro

    2014-01-01

    The formation of planets with gaseous envelopes takes place in protoplanetary accretion discs on time-scales of several millions of years. Small dust particles stick to each other to form pebbles, pebbles concentrate in the turbulent flow to form planetesimals and planetary embryos and grow to planets, which undergo substantial radial migration. All these processes are influenced by the underlying structure of the protoplanetary disc, specifically the profiles of temperature, gas scale height and density. The commonly used disc structure of the Minimum Mass Solar Nebular (MMSN) is a simple power law in all these quantities. However, protoplanetary disc models with both viscous and stellar heating show several bumps and dips in temperature, scale height and density caused by transitions in opacity, which are missing in the MMSN model. These play an important role in the formation of planets, as they can act as sweet spots for the formation of planetesimals via the streaming instability and affect the direction...

  7. Self-gravitational collapse of polytropic mass-losing gaseous discs: effects of outer boundary condition dictated by parent cloud

    Nourbakhsh, Erfan; Abbassi, Shahram

    2012-01-01

    Given the fact that accretion discs are associated with their parent molecular cloud, we studied its effects as a constraint on the outer boundary of the viscous-resistive polytropic self-gravitating accretion flows subject to the mass and angular momentum loss.

  8. Angles in Fuzzy Disc and Angular Noncommutative Solitons

    Kobayashi, Shinpei

    2014-01-01

    The fuzzy disc, introduced by the authors of Ref.[1], is a disc-shaped region in a noncommutative plane, and is a fuzzy approximation of a commutative disc. In this paper we show that one can introduce a concept of angles to the fuzzy disc, by using the phase operator and phase states known in quantum optics. We gave a description of a fuzzy disc in terms of operators and their commutation relations, and studied properties of angular projection operators. A similar construction for a fuzzy annulus is also given. As an application, we constructed fan-shaped soliton solutions of a scalar field theory on a fuzzy disc, which corresponds to a fan-shaped D-brane. We also applied this concept to the theory of noncommutative gravity that we proposed in Ref.[2]. In addition, possible connections to black hole microstates, holography and an experimental test of noncommutativity by laser physics are suggested.

  9. On the possibility of a warped disc origin of the inclined stellar discs at the Galactic Centre

    Ulubay-Siddiki, A.; Bartko, H.; Gerhard, O.

    2013-01-01

    The central parsec of our Galaxy hosts a population of young stars. At distances of r ˜ 0.03-0.5 pc, most of these stars seem to form a system of mutually inclined discs of clockwise and counterclockwise rotating stars. We present a possible warped disc origin scenario for these stars assuming that an initially flat accretion disc becomes warped due to a central radiation source via the Pringle instability or due to a spinning black hole via the Bardeen-Petterson effect before it cools, fragments and forms stars. From simple arguments, we show that this is plausible if the star formation efficiency is high, ɛSF ≲ 1, and the viscosity parameter α ˜ 0.1. After fragmentation, we model the disc as a collection of concentric, circular rings tilted with respect to each other, and construct time evolution models of warped discs for mass ratios and other parameters relevant to the Galactic Centre environment, but also for more massive discs. We take into account the disc's self-gravity in the non-linear regime and the torques exerted by a slightly flattened surrounding star cluster. Our simulations show that a self-gravitating low-mass disc (Md/Mbh ˜ 0.001) precesses with its integrity maintained in the lifetime of the stars, but precesses essentially freely when the torques from a non-spherical cluster are included. An intermediate-mass disc (Md/Mbh ˜ 0.01) breaks into pieces, which precess as independent discs in the self-gravity-only case, and become disrupted in the presence of the star cluster torques. Finally, for a high-mass disc (Md/Mbh ˜ 0.1), the evolution is dominated by self-gravity and the disc is broken but not dissolved. The time-scale after which the disc breaks into pieces scales almost linearly with Md/Mbh for self-gravitating models. Typical values are longer than the age of the stars for Md/Mbh ˜ 0.001, and are in the range ˜8 × 104-105 yr for Md/Mbh ˜ 0.1-0.01, respectively. None of these discs explains the two Galactic Centre discs with

  10. The ins and outs of emission from accreting black holes

    S. Drappeau

    2013-01-01

    The most extreme physical conditions of space-time in the Universe happen in the vicinity of accreting black holes, which make them the perfect laboratory for testing extreme physics theories. The present thesis investigates accretion processes using radiation as a tracer of the physics occurring ve

  11. Bondi accretion in early-type galaxies

    Korol, Valeriya; Ciotti, Luca; Pellegrini, Silvia

    2016-05-01

    Accretion on to 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 systems with a low Eddington ratio), as well as for numerical simulations, where accretion rates are usually defined in terms of the gas properties near the black hole.

  12. Bondi accretion in early-type galaxies

    Korol, Valeriya; Ciotti, Luca; Pellegrini, Silvia

    2016-08-01

    Accretion on to 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 a 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 systems with a low Eddington ratio), as well as for numerical simulations, where accretion rates are usually defined in terms of the gas properties near the black hole.

  13. Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances

    Snaith, O; Di Matteo, P; Lehnert, M D; Combes, F; Katz, D; Gómez, A

    2014-01-01

    We develop a chemical evolution model in order to study the star formation history of the Milky Way. Our model assumes that the Milky Way is formed from a closed box-like system in the inner regions, while the outer parts of the disc experience some accretion. Unlike the usual procedure, we do not fix the star formation prescription (e.g. Kennicutt law) in order to reproduce the chemical abundance trends. Instead, we fit the abundance trends with age in order to recover the star formation history of the Galaxy. Our method enables one to recover with unprecedented accuracy the star formation history of the Milky Way in the first Gyrs, in both the inner (R9-10kpc) discs as sampled in the solar vicinity. We show that, in the inner disc, half of the stellar mass formed during the thick disc phase, in the first 4-5 Gyr. This phase was followed by a significant dip in the star formation activity (at 8-9 Gyr) and a period of roughly constant lower level star formation for the remaining 8 Gyr. The thick disc phase ha...

  14. Simulations of the magnetospheres of accreting millisecond pulsars

    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...

  15. Black hole mergers: do gas discs lead to spin alignment?

    Lodato, Giuseppe

    2012-01-01

    In this Letter we revisit arguments suggesting that the Bardeen-Petterson effect can coalign the spins of a central supermassive black hole binary accreting from a circumbinary (or circumnuclear) gas disc. We improve on previous estimates by adding the dependence on system parameters, and noting that the nonlinear nature of warp propagation in a thin viscous disc affects alignment. This reduces the disc's ability to communicate the warp, and can severely reduce the effectiveness of disc-assisted spin alignment. We test our predictions with a Monte Carlo realization of random misalignments and accretion rates and we find that the outcome depends strongly on the spin magnitude. We estimate a generous upper limit to the probability of alignment by making assumptions which favour it throughout. Even with these assumptions, about 40% of black holes with $a \\gtrsim 0.5$ do not have time to align with the disc. If the residual misalignment is not small and it is maintained down to the final coalescence phase this ca...

  16. Black hole mergers: do gas discs lead to spin alignment?

    Lodato, Giuseppe; Gerosa, Davide

    2013-02-01

    In this Letter, we revisit arguments suggesting that the Bardeen-Petterson effect can coalign the spins of a central supermassive black hole binary accreting from a circumbinary (or circumnuclear) gas disc. We improve on previous estimates by adding the dependence on system parameters and noting that the non-linear nature of warp propagation in a thin viscous disc affects alignment. This reduces the disc's ability to communicate the warp, and can severely reduce the effectiveness of disc-assisted spin alignment. We test our predictions with a Monte Carlo realization of random misalignments and accretion rates, and we find that the outcome depends strongly on the spin magnitude. We estimate a generous upper limit to the probability of alignment by making assumptions which favour it throughout. Even with these assumptions, about 40 per cent of black holes with a ≳ 0.5 do not have time to align with the disc. If the residual misalignment is not small and it is maintained down to the final coalescence phase, this can give a powerful recoil velocity to the merged hole. Highly spinning black holes are thus more likely being subject to strong recoils, the occurrence of which is currently debated.

  17. Numerical test of the method for revealing traces of deterministic chaos in the accreting black holes

    Suková, Petra

    2016-01-01

    The high energy radiation emitted by black hole X-ray binaries originates in an accretion disk, hence the variability of the lightcurves mirrors the dynamics of the disc. We study the time evolution of the emitted flux in order to find evidences, that low dimensional non-linear equations govern the accretion flow. Here we test the capabilities of our novel method to find chaotic behaviour on the two numerical time series describing the motion of a test particle around a black hole surrounded by a thin massive disc, one being regular and the other one chaotic.

  18. Double rupture disc experience

    Result of these observations, comparisons and evaluations can be summarized in the following list of concerns regarding the use of double rupture discs coupled to the liquid space of a steam generator that is subjected to a large leak sodium water reaction event. Single rupture disc show delayed collapse characteristics in LLTR Series I and double disc assemblies are presumed to be more complex with additional delay before opening to give pressure relief. Delayed failure increases pressures in the IHTS and must be adequately covered by design requirements. With CRBR design, the first disc may fail only partially reducing the loading on the second disc with the result that relief performance may not meet requirements

  19. The thickness of a weakly-magnetized accretion flow inside the last stable orbit of a Kerr black hole

    Abolmasov, P

    2014-01-01

    If accretion disc contains weak frozen-in entangled magnetic fields, their dynamical effect may be important inside the last stable orbit because of the decompression near the sonic point. Here, I consider the radial and vertical structure of a nearly free-falling flow inside the last stable orbit of a thin disc around a Kerr black hole. The thickness of such a flow is determined primarily by the vertical stress created by radial and azimuthal magnetic fields. The thickness is predicted to oscillate vertically around its equilibrium value determined by the magnetic field balance with gravity. For thin discs, this thickness is much larger than that of the accretion disc itself. Numerical simulations with HARM2d show the vertical structure is more complicated. In particular, magnetically supported disc seems to be unstable to segregation of matter into thinner streams with the vertical scale determined by thermal pressure or other processes.

  20. Magnetohydrodynamics of accretion disks

    The thesis consists of an introduction and summary, and five research papers. The introduction and summary provides the background in accretion disk physics and magnetohydrodynamics. The research papers describe numerical studies of magnetohydrodynamical processes in accretion disks. Paper 1 is a one-dimensional study of the effect of magnetic buoyancy on a flux tube in an accretion disk. The stabilizing influence of an accretion disk corona on the flux tube is demonstrated. Paper 2-4 present numerical simulations of mean-field dynamos in accretion disks. Paper 11 verifies the correctness of the numerical code by comparing linear models to previous work by other groups. The results are also extended to somewhat modified disk models. A transition from an oscillatory mode of negative parity for thick disks to a steady mode of even parity for thin disks is found. Preliminary results for nonlinear dynamos at very high dynamo numbers are also presented. Paper 3 describes the bifurcation behaviour of the nonlinear dynamos. For positive dynamo numbers it is found that the initial steady solution is replaced by an oscillatory solution of odd parity. For negative dynamo numbers the solution becomes chaotic at sufficiently high dynamo numbers. Paper 4 continues the studies of nonlinear dynamos, and it is demonstrated that a chaotic solution appears even for positive dynamo numbers, but that it returns to a steady solution of mixed parity at very high dynamo numbers. Paper 5 describes a first attempt at simulating the small-scale turbulence of an accretion disk in three dimensions. There is only find cases of decaying turbulence, but this is rather due to limitations of the simulations than that turbulence is really absent in accretion disks

  1. Star Formation and Gas Accretion in Nearby Galaxies

    Yim, Kijeong

    2016-01-01

    In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 {\\mu}m (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and HI in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.

  2. Rapid growth of gas-giant cores by pebble accretion

    Lambrechts, Michiel

    2012-01-01

    The observed lifetimes of gaseous protoplanetary discs place strong constraints on gas and ice giant formation in the core accretion scenario. The approximately 10-Earth-mass solid core responsible for the attraction of the gaseous envelope has to form before gas dissipation in the protoplanetary disc is completed within 1-10 million years. Building up the core by collisions between km-sized planetesimals fails to meet this time-scale constraint, especially at wide stellar separations. Nonetheless, gas-giant planets are detected by direct imaging at wide orbital distances. In this paper, we numerically study the growth of cores by the accretion of cm-sized pebbles loosely coupled to the gas. We measure the accretion rate onto seed masses ranging from a large planetesimal to a fully grown 10-Earth-mass core and test different particle sizes. The numerical results are in good agreement with our analytic expressions, indicating the existence of two accretion regimes, one set by the azimuthal and radial particle ...

  3. Particle dynamics in discs with turbulence generated by the vertical shear instability

    Stoll, Moritz H R

    2016-01-01

    Among the candidates for generating turbulence in accretion discs in situations with low intrinsic ionization the vertical shear instability (VSI) has become an interesting candidate, as it relies purely on a vertical gradient in the angular velocity. Existing simulations have shown that $\\alpha$-values a few times $10^{-4}$ can be generated. The particle growth in the early planet formation phase is determined by the dynamics of dust particles. Here, we address in particular the efficiency of VSI-turbulence in concentrating particles in order to generate overdensities and low collision velocities. We perform 3D numerical hydrodynamical simulations of accretion discs around young stars that include radiative transport and irradiation from the central star. The motion of particles within a size range of a fraction of mm up to several m is followed using standard drag formula. We confirm that under realistic conditions the VSI is able to generate turbulence in full 3D protoplanetary discs. The irradiated disc s...

  4. Optical effects related to Keplerian discs orbiting Kehagias-Sfetsos naked singularities

    Stuchlik, Zdenek

    2014-01-01

    We demonstrate possible optical signatures of the Kehagias-Sfetsos naked singularity spacetimes representing spherically symmetric vacuum solution of the modified Ho\\v{r}ava gravity. In such spacetimes, accretion structures significantly different from those present in the standard black hole spacetimes occur due to the "antigravity" effect causing existence of an internal static sphere surrounded by Keplerian discs. We focus our attention on the optical effects related to the Keplerian accretion discs, constructing the optical appearance of the Keplerian discs, the spectral continuum due to their thermal radiation, and spectral profiled lines generated in the innermost parts of such discs. The KS naked singularity signature is strongly encoded in the characteristics of predicted optical effects, especially in the case of the spectral continuum and spectral lines profiled by the strong gravity of the spacetimes, due to the region of the vanishing of the angular velocity gradient influencing the effectivity of...

  5. Constraining black-hole spin using disc tomography

    Middleton, Matthew

    2013-10-01

    The emission from the inner accretion disc of low mass, high mass accretion rate AGN extends into the soft X-ray bandpass. Where orbiting material covers or reveals each side of the disc in turn, we can study the region of strong gravity and determine the spin and inclination directly. RX J1301.9+2747, shows flares as a long-lived feature of its lightcurve which are most likely due to gaps in an obscuring shroud. By comparing to the predictions from ray-tracing codes, the data imply the spin and inclination to be low. We propose a 210 ks observation (including overheads and background flaring) to more robustly test our new technique and tightly constrain the spin and inclination, or otherwise provide a unique view of the mechanism responsible for the soft excess in this source.

  6. A Global Study of the Behaviour of Black Hole X-ray Binary Discs

    Dunn, Robert; Koerding, Elmar; Belloni, Tomaso; Merloni, Andrea

    2010-01-01

    We investigate the behaviour of the accretion discs in the outbursts of the low-mass black-hole X-ray binaries (BHXRB), an overview of which we have presented previously. Almost all of the systems in which there are sufficient observations in the most disc dominated states show a variation of the disc luminosity with temperature close to L ~\\propto T^4. This in turn implies that in these states, the disc radius, R_in, and the colour correction factor, f_col, are almost constant. Deviations away from the T^4 law are observed at the beginning and end of the most disc dominated states, during the intermediate states. Although these could be explained by an inward motion of the accretion disc, they are more likely to be the result of an increase in the value of f_col as the disc fraction decreases. By comparing the expected and observed disc luminosities, we place approximate limits on the allowed distances and masses of the BHXRB system. In a number of cases, the measured distances and masses of the BHXRB system...

  7. The long-term evolution of photoevaporating transition discs with giant planets

    Rosotti, Giovanni P; Owen, James E

    2015-01-01

    Photo-evaporation and planet formation have both been proposed as mechanisms responsible for the creation of a transition disc. We have studied their combined effect through a suite of 2d simulations of protoplanetary discs undergoing X-ray photoevaporation with an embedded giant planet. In a previous work we explored how the formation of a giant planet triggers the dispersal of the inner disc by photo-evaporation at earlier times than what would have happened otherwise. This is particularly relevant for the observed transition discs with large holes and high mass accretion rates that cannot be explained by photo-evaporation alone. In this work we significantly expand the parameter space investigated by previous simulations. In addition, the updated model includes thermal sweeping, needed for studying the complete dispersal of the disc. After the removal of the inner disc the disc is a non accreting transition disc, an object that is rarely seen in observations. We assess the relative length of this phase, to...

  8. Optic disc oedema

    Nielsen, Marianne Kromann; Hamann, Steffen

    2014-01-01

    Optic disc oedema describes the nonspecific, localized swelling of the optic nerve head regardless of aetiology. Therefore, differentiating among the various aetiologies depends on a thorough history and knowledge of the clinical characteristics of the underlying conditions. Papilloedema strictly...... refers to optic disc oedema as a consequence of elevated intracranial pressure. It is usually a bilateral condition and visual function is preserved until late. Optic disc oedema caused by an anterior optic neuropathy is usually unilateral and accompanied by the loss of visual function....

  9. Giant planet formation in radially structured protoplanetary discs

    Coleman, Gavin A. L.; Nelson, Richard P.

    2016-08-01

    Our recent N-body simulations of planetary system formation, incorporating models for the main physical processes thought to be important during the building of planets (i.e. gas disc evolution, migration, planetesimal/boulder accretion, gas accretion onto cores, etc.), have been successful in reproducing some of the broad features of the observed exoplanet population (e.g. compact systems of low mass planets, hot Jupiters), but fail completely to form any surviving cold Jupiters. The primary reason for this failure is rapid inward migration of growing protoplanets during the gas accretion phase, resulting in the delivery of these bodies onto orbits close to the star. Here, we present the results of simulations that examine the formation of gas giant planets in protoplanetary discs that are radially structured due to spatial and temporal variations in the effective viscous stresses, and show that such a model results in the formation of a population of cold gas giants. Furthermore, when combined with models for disc photoevaporation and a central magnetospheric cavity, the simulations reproduce the well-known hot-Jupiter/cold-Jupiter dichotomy in the observed period distribution of giant exoplanets, with a period valley between 10-100 days.

  10. Ringed accretion disks: instabilities

    Pugliese, D

    2016-01-01

    We analyze the possibility that several instability points may be formed, due to the Paczy\\'nski mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider recently proposed model of ringed accretion disk, made up by several tori (rings) which can be corotating or counterrotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  11. Ringed Accretion Disks: Instabilities

    Pugliese, D.; Stuchlík, Z.

    2016-04-01

    We analyze the possibility that several instability points may be formed, due to the Paczyński mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider a recently proposed model of a ringed accretion disk, made up by several tori (rings) that can be corotating or counter-rotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  12. Disc instability models for X-ray transients evidence for evaporation and low $\\alpha$-viscosity?

    Menou, K; Lasota, J P; Narayan, R; Menou, Kristen; Hameury, Jean-Marie; Lasota, Jean-Pierre; Narayan, Ramesh

    2000-01-01

    We construct time-dependent models of accretion discs around black holes and neutron stars. We investigate the effect of evaporating the disc inner regions during quiescence on the predictions of the Disc Instability Model (DIM) for these systems. We do not include irradiation of the disc in the models. Removing the inner, most unstable parts of the accretion disc increases the predicted recurrence times. However, DIMs with values of the viscosity parameter alpha_hot ~ 0.1 and alpha_cold ~ 0.02 (typical of applications of the DIM to standard dwarf nova outbursts) fail to reproduce the long recurrence times of SXTs (unless we resort to fine-tuning of the parameters) independent of the evaporation strength. We show that models with evaporation and a smaller value of alpha_cold (~ 0.005) do reproduce the long recurrence times and the accretion rates at the level of the Eddington rate observed in outburst. The large difference between the values of alpha_hot and alpha_cold, if confirmed when disc irradiation is i...

  13. Circumplanetary disc or circumplanetary envelope?

    Szulágyi, J.; Masset, F.; Lega, E.; Crida, A.; Morbidelli, A.; Guillot, T.

    2016-08-01

    We present three-dimensional simulations with nested meshes of the dynamics of the gas around a Jupiter mass planet with the JUPITER and FARGOCA codes. We implemented a radiative transfer module into the JUPITER code to account for realistic heating and cooling of the gas. We focus on the circumplanetary gas flow, determining its characteristics at very high resolution (80 per cent of Jupiter's diameter). In our nominal simulation where the temperature evolves freely by the radiative module and reaches 13000 K at the planet, a circumplanetary envelope was formed filling the entire Roche lobe. Because of our equation of state is simplified and probably overestimates the temperature, we also performed simulations with limited maximal temperatures in the planet region (1000, 1500, and 2000 K). In these fixed temperature cases circumplanetary discs (CPDs) were formed. This suggests that the capability to form a CPD is not simply linked to the mass of the planet and its ability to open a gap. Instead, the gas temperature at the planet's location, which depends on its accretion history, plays also fundamental role. The CPDs in the simulations are hot and cooling very slowly, they have very steep temperature and density profiles, and are strongly sub-Keplerian. Moreover, the CPDs are fed by a strong vertical influx, which shocks on the CPD surfaces creating a hot and luminous shock-front. In contrast, the pressure supported circumplanetary envelope is characterized by internal convection and almost stalled rotation.

  14. Physical Parameter Eclipse Mapping of the quiescent disc in V2051 Ophiuchi

    Vrielmann, S; Offutt, W; Stiening, Rae F.; Offutt, Warren

    2002-01-01

    We analyse simultaneous UBVR quiescent light curves of the cataclysmic variable V2051 Oph using the Physical Parameter Eclipse Mapping method in order to map the gas temperature and surface density of the disc for the first time. The disc appears optically thick in the central regions and gradually becomes optically thin towards the disc edge or shows a more and more dominating temperature inversion in the disc chromosphere. The gas temperatures in the disc range from about 13500 K near the white dwarf to about 6000 K at the disc edge. The intermediate part of the disc has temperatures of 9000 K to 6500 K. The quiescent disc (chromosphere) shows a prominent bright region with temperatures of 10500 K around the impact region of the stream from the secondary with an extension towards smaller azimuths. The disc has a size of 0.53 +- 0.03 R_L1 and a mass accretion rate of between dM/dt = 10^15 g/s to 10^17g/s. The light curves must include an uneclipsed component, a hot chromosphere and/or a disc wind. The PPEM m...

  15. Effect of Be disc evolution on global one-armed oscillations

    Oktariani, F.; Okazaki, A. T.; Kunjaya, C.; Aprilia

    2016-07-01

    We study the effect of density distribution evolution on the global one-armed oscillation modes in low-viscosity discs around isolated and binary Be stars. Observations show that some Be stars exhibit evidence of formation and dissipation of the equatorial disc. In this paper, we first calculate the density evolution in discs around isolated Be stars. To model the formation stage of the disc, we inject mass at a radius just outside the star at a constant rate for 30-50 yr. As the disc develops, the density distribution approaches the form of the steady disc solution. Then, we turn-off the mass injection to model the disc dissipation stage. The innermost part of the disc starts accretion, and a gap forms between the star and the disc. Next, we calculate the one-armed modes at several epochs. We neglect the effect of viscosity because the time-scale of oscillations is much shorter than the disc evolution time-scale for low viscosity. In the disc formation stage, the eigenfrequency increases with time towards the value for the steady state disc. On the other hand, one-armed eigenmodes in dissipating Be discs have significantly higher eigenfrequencies and narrower propagation regions. Observationally, such a change of mode characteristics can be taken as an evidence for gap opening around the star. In binary Be stars, the characteristics of the disc evolution and the eigenmodes are qualitatively the same as in isolated Be stars, but quantitatively, they have shorter evolution time-scales and higher eigenfrequencies, which is in agreement with the observed trend.

  16. Accretion of Chaplygin gas upon black holes: formation of faster outflowing winds

    We study the accretion of modified Chaplygin gas upon different types of black holes. Modified Chaplygin gas is one of the best candidates for a combined model of dark matter and dark energy. In addition, from a field theoretical point of view the modified Chaplygin gas model is equivalent to that of a scalar field having a self-interacting potential. We formulate the equations related to both spherical accretion and disc accretion, and respective winds. The corresponding numerical solutions of the flow, particularly of velocity, are presented and analysed. We show that the accretion-wind system of modified Chaplygin gas dramatically alters the wind solutions, producing faster winds, upon changes in physical parameters, while accretion solutions qualitatively remain unaffected. This implies that modified Chaplygin gas is more prone to produce outflow which is the natural consequence of the dark energy into the system.

  17. Line-driven ablation of circumstellar discs – I. Optically thin decretion discs of classical Oe/Be stars

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.

    2016-01-01

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars.

  18. Line-driven ablation of circumstellar discs - I. Optically thin decretion discs of classical Oe/Be stars

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.

    2016-05-01

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars.

  19. LUMBAR DISC HERNIATION

    Vialle, Luis Roberto; Vialle, Emiliano Neves; Suárez Henao, Juan Esteban; Giraldo, Gustavo

    2015-01-01

    Lumbar disc herniation is the most common diagnosis among the degenerative abnormalities of the lumbar spine (affecting 2 to 3% of the population), and is the principal cause of spinal surgery among the adult population. The typical clinical picture includes initial lumbalgia, followed by progressive sciatica. The natural history of disc herniation is one of rapid resolution of the symptoms (four to six weeks). The initial treatment should be conservative, managed through medication and physi...

  20. Quantification of stochastic fragmentation of self-gravitating discs

    Young, Matthew D

    2015-01-01

    Using 2D smoothed particle hydrodynamics, we investigate the distribution of wait times between strong shocks in a turbulent, self-gravitating accretion disc. We show the resulting distributions do not depend strongly on the cooling time or resolution of the disc and that they are consistent with the predictions of earlier work (Young & Clarke 2015; Cossins et al. 2009, 2010). We use the distribution of wait times between shocks to estimate the likelihood of stochastic fragmentation by gradual contraction of shear-resistant clumps on the cooling time scale. We conclude that the stochastic fragmentation mechanism (Paardekooper 2012) cannot change the radius at which fragmentation is possible by more than ~20%, restricting direct gravitational collapse as a mechanism for giant planet formation to the outer regions of protoplanetary discs.

  1. Radicular interdural lumbar disc herniation

    Akhaddar, Ali; Boulahroud, Omar; Elasri, Abad; Elmostarchid, Brahim; Boucetta, Mohammed

    2009-01-01

    Intraradicular lumbar disc herniation is a rare complication of disc disease that is generally diagnosed only during surgery. The mechanism for herniated disc penetration into the intradural space is not known with certainty, but adhesion between the radicular dura and the posterior longitudinal ligament was suggested as the most important condition. The authors report the first case of an intraradicular lumbar disc herniation without subdural penetration; the disc hernia was lodged between t...

  2. Black hole accretion rings revealed by future X-ray spectroscopy

    Sochora, Vjačeslav; Karas, Vladimír; Svoboda, Jiří; Dovčiak, Michal

    2011-01-01

    Roč. 418, č. 1 (2011), s. 276-283. ISSN 0035-8711 R&D Projects: GA ČR GA205/07/0052; GA MŠk ME09036 Institutional research plan: CEZ:AV0Z10030501 Keywords : black hole s * accretion discs Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 4.900, year: 2011

  3. Black hole spin dependence of general relativistic multi-transonic accretion close to the horizon

    Das, T. K.; Nag, S.; Hedge, S.; Bhattacharya, S.; Maity, I.; Czerny, B.; Barai, P.; Wiita, P. J.; Karas, Vladimír; Naskar, T.

    2015-01-01

    Roč. 37, May (2015), s. 81-104. ISSN 1384-1076 R&D Projects: GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : black holes * accretion discs Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 1.146, year: 2014

  4. Gas accretion by planetary cores

    Ayliffe, Ben A.; Bate, Matthew R.

    2009-01-01

    We present accretion rates obtained from three-dimensional self-gravitating radiation hydrodynamical models of giant planet growth. We investigate the dependence of accretion rates upon grain opacity and core/protoplanet mass. The accretion rates found for low mass cores are inline with the results of previous one-dimensional models that include radiative transfer.

  5. Composition of early planetary atmospheres - I. Connecting disc astrochemistry to the formation of planetary atmospheres

    Cridland, A. J.; Pudritz, R. E.; Alessi, M.

    2016-09-01

    We present a model of the early chemical composition and elemental abundances of planetary atmospheres based on the cumulative gaseous chemical species that are accreted on to planets forming by core accretion from evolving protoplanetary discs. The astrochemistry of the host disc is computed using an ionization-driven, non-equilibrium chemistry network within viscously evolving disc models. We accrete gas giant planets whose orbital evolution is controlled by planet traps using the standard core accretion model and track the chemical composition of the material that is accreted on to the protoplanet. We choose a fiducial disc model and evolve planets in three traps - water ice line, dead zone and heat transition. For a disc with a lifetime of 4.1 Myr, we produce two hot Jupiters (M = 1.43, 2.67 MJupiter, r = 0.15, 0.11 au) in the heat transition and ice line trap and one failed core (M = 0.003 MJupiter, r = 3.7 au) in the dead zone. These planets are found with mixing ratios for CO and H2O of 1.99 × 10-4 and 5.0 × 10-4, respectively, for both hot Jupiters. Additionally, for these planets we find CO2 and CH4, with mixing ratios of 1.8 × 10-6 → 9.8 × 10-10 and 1.1 × 10-8 → 2.3 × 10-10, respectively. These ranges correspond well with the mixing ratio ranges that have been inferred through the detection of emission spectra from hot Jupiters by multiple authors. We compute a carbon-to-oxygen ratio of 0.227 for the ice line planet and 0.279 for the heat transition planet. These planets accreted their gas inside the ice line, hence the sub-solar C/O.

  6. Spectral hardening as a viable alternative to disc truncation in black hole state transitions

    Salvesen, Greg; Reis, Rubens C; Begelman, Mitchell C

    2013-01-01

    Constraining the accretion flow geometry of black hole binaries in outburst is complicated by the inability of simplified multi-colour disc models to distinguish between changes in the inner disc radius and alterations to the emergent spectrum, parameterised by the phenomenological colour correction factor, f_col. We analyse Rossi X-ray Timing Explorer observations of the low mass Galactic black hole X-ray binary, GX 339-4, taken over seven epochs when the source was experiencing a state transition. The accretion disc component is isolated using a pipeline resulting in robust detections for disc luminosities, 0.001 < L_disc / L_Edd < 0.5. Assuming that the inner disc remains situated at the innermost stable circular orbit over the course of a state transition, we measure the relative degree of change in f_col required to explain the spectral evolution of the disc component. A variable f_col that increases by a factor of ~ 2.0 - 3.5 as the source transitions from the high/soft state to the low/hard state...

  7. Discovery of disc precession in the M31 dipping X-ray binary Bo 158

    Barnard, R; Haswell, C A; Kolb, U; Osborne, J P; Murray, J R

    2006-01-01

    We present results from three XMM-Newton observations of the M31 low mass X-ray binary XMMU J004314.4+410726.3 (Bo 158), spaced over 3 days in 2004, July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic intensity dips were previously seen to occur on a 2.78-hr period, due to absorption in material that is raised out of the plane of the accretion disc. The report of these observations stated that the dip depth was anti-correlated with source intensity. However, our new observations do not favour a strict intensity dependance, but rather suggest that the dip variation is due to precession of the accretion disc. This is to be expected in LMXBs with a mass ratio <~ 0.3 (period <~ 4 hr), as the disc reaches the 3:1 resonance with the binary companion, causing elongation and precession of the disc. A 3D smoothed particle hydrodynamics simulation of the disc in this system shows retrograde rotation of a disc warp on a period of ~11 P_orb, and prograde disc precession on a period of ~29 P_or...

  8. On the dynamics of planetesimals embedded in turbulent protoplanetary discs with dead zones

    Gressel, Oliver; Turner, Neal J

    2011-01-01

    (abridged) Accretion in protoplanetary discs is thought to be driven by [...] turbulence via the magnetorotational instability (MRI). Recent work has shown that a planetesimal swarm embedded in a fully turbulent disc is subject to strong excitation of the velocity dispersion, leading to collisional destruction of bodies with radii R_p < 100 km. Significant diffusion of planetesimal semimajor axes also arises, leading to large-scale spreading of the planetesimal population throughout the inner regions of the protoplanetary disc, in apparent contradiction of constraints provided by the distribution of asteroids within the asteroid belt. In this paper, we examine the dynamics of planetesimals embedded in vertically stratified turbulent discs, with and without dead zones. Our main aims are to examine the turbulent excitation of the velocity dispersion, and the radial diffusion, of planetesimals in these discs. We employ three dimensional MHD simulations [...], along with an equilibrium chemistry model [...] We...

  9. The accretion flow to the intermittent accreting ms pulsar, HETE J1900.1-2455, as observed by XMM-Newton and RXTE

    Papitto, A; Di Salvo, T; Egron, E; Bozzo, E; Burderi, L; Iaria, R; Riggio, A; Menna, M T

    2012-01-01

    We present a study of the accretion flow to the intermittent accreting millisecond pulsar, HETE J1900.1-2455, based on observations performed simultaneously by XMM-Newton and RXTE. The 0.33-50 keV spectrum is described by the sum of a hard Comptonized component originated in an optically thin {\\tau}~1 corona, a soft kTin~0.2 keV component interpreted as accretion disc emission, and of disc reflection of the hard component. Two emission features are detected at energies of 0.98(1) and 6.58(7) keV, respectively. The latter is identified as K{\\alpha} transition of Fe XXIII-XXV. A simultaneous detection in EPIC-pn, EPIC-MOS2, and RGS spectra favours an astrophysical origin also for the former, which has an energy compatible with Fe-L{\\alpha} and helium-like Ne-K{\\alpha} transitions. Broadness of the two features suggests a common origin, resulting from reflection in an accretion disc with inclination of (30+4{\\deg}), and extending down to Rin=25(+16,-11) gravitational radii from the compact object. However, the s...

  10. Magnetospheric accretion in EX Lupi

    Abraham, Peter; Kospal, Agnes; Bouvier, Jerome

    2016-08-01

    We propose to observe EX Lup, the prototype of the EXor class of young eruptive stars, in order to understand how the accretion process works in the quiescent system. Here, we request 2.6 hours of telescope time on Spitzer, to carry out a mid-infrared photometric monitoring, which we will supplement with simultaneous ground-based optical and near-infrared data. The multi-wavelength light curves will allow us to reliably separate the effects of fluctuating accretion rate from the rotation of the star. By analyzing the variations of the accretion rate we will determine whether EX Lup accretes through a few stable accretion columns or several short-lived random accretion streams. With this campaign, EX Lup will become one of the T Tauri systems where the accretion process is best understood.

  11. The vibration of rolling discs

    Ferguson, Neil Stuart

    1988-01-01

    The study presented is a fundamental investigation into the vibration of a rolling disc and an analysis of the sound which is then produced by the disc response. In order to characterise the disc's behaviour the free and forced vibration of a thin disc is analysed with the additional conditions which occur in rolling. These conditions include the geometric boundary conditions, the effect of the in-plane load, the change due to rotation of the disc and the effect of contact on the disc which i...

  12. Pseudoenhancement of intervertebral disc herniation

    Araki, Y.; Ootani, M.; Furukawa, T.; Tsukaguchi, I. (Dept. of Radiology, Osaka Rosai Hospital (Japan)); Mitomo, M. (Dept. of Radiology, Osaka Univ. Medical School (Japan))

    1992-08-01

    Two patients with intervertebral disc herniation appeared to demonstrate abnormally diffuse and intense enhancement of the disc after intravenous administration of gadolinium-DTPA for MRI. Surgery disclosed a dilated epidural venous plexus in one and vascular granulation tissue in the other, associated with the herniated disc material. The mechanism of this 'pseudoenhancement' of the disc appears to be a partial volume effect of disc material and the adjacent veins or granulation tissue. Pseudoenhancement of a herniated disc should be included in the differential diagnosis of a diffusely enhancing epidural mass. (orig.).

  13. Accretion of southern Alaska

    Hillhouse, J.W.

    1987-01-01

    Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 ?? as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation. ?? 1987.

  14. Accretion disk electrodynamics

    Coroniti, F. V.

    1985-01-01

    Accretion disk electrodynamic phenomena are separable into two classes: (1) disks and coronas with turbulent magnetic fields; (2) disks and black holes which are connected to a large-scale external magnetic field. Turbulent fields may originate in an alpha-omega dynamo, provide anomalous viscous transport, and sustain an active corona by magnetic buoyancy. The large-scale field can extract energy and angular momentum from the disk and black hole, and be dynamically configured into a collimated relativistic jet.

  15. Constraining the MOdified Newtonian Dynamics from spherically symmetrical hydrodynamic accretion

    Roy, Nirupam

    2011-01-01

    The MOdified Newtonian Dynamics (MOND) is an alternative to the dark matter assumption that can explain the observed flat rotation curve of galaxies. Here hydrodynamic accretion is considered to critically check the consistency and to constrain the physical interpretation of this theory. It is found that, in case of spherically symmetrical hydrodynamic accretion, the modified Euler's equation has real solution if the interpretation is assumed to be a modification of the law of dynamics. There...

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

    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.

    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 molecul...

  17. Unstable mass-outflows in geometrically thick accretion flows around black holes

    Okuda, Toru

    2015-01-01

    Accretion flows around black holes generally result in mass-outflows that exhibit irregular behavior quite often. Using 2D time-dependent hydrodynamical calculations, we show that the mass-outflow is unstable in the cases of thick accretion flows such as the low angular momentum accretion flow and the advection-dominated accretion flow. For the low angular momentum flow, the inward accreting matter on the equatorial plane interacts with the outflowing gas along the rotational axis and the centrifugally supported oblique shock is formed at the interface of both the flows, when the viscosity parameter $\\alpha$ is as small as $\\alpha \\le 10^{-3}$. The hot and rarefied blobs, which result in the eruptive mass-outflow, are generated in the inner shocked region and grow up toward the outer boundary. The advection-dominated accretion flow attains finally in the form of a torus disc with the inner edge of the disc at $3R_{\\rm g} \\le r \\le 6R_{\\rm g}$ and the center at $6R_ {\\rm g} \\le r \\le 10R_{\\rm g}$, and a series...

  18. Discovery and modelling of disc precession in the M31 X-ray binary Bo 158?

    Barnard, R; Haswell, C A; Kolb, U; Murray, J R

    2005-01-01

    The low mass X-ray binary (LMXB) associated with the M31 globular cluster Bo 158 is known to exhibit intensity dips on a ~2.78 hr period. This is due to obscuration of the X-ray source on the orbital period by material on the outer edge of the accretion disc. However, the depth of dipping varied from <10% to \\~83% in three archival XMM-Newton observations of Bo 158. Previous work suggested that the dip depth was anticorrelated with the X-ray luminosity. However, we present results from three new XMM-Newton observations that suggest that the evolution of dipping is instead due to precession of the accretion disc. Such precession is expected in neutron star LMXBs with mass ratios <0.3 (i.e. with orbital periods <4 hr), such as the Galactic dipping LMXB 4U 1916-053. We simulated the accretion disc of Bo 158 using cutting-edge 3D smoothed particle hydrodynamics (SPH), and using the observed parameters. Our results show disc variability on two time-scales. The disc precesses in a prograde direction on a p...

  19. Composition of Airy disc

    Guoquan Zhou; Xiuxiang Chu; Jun Zheng

    2008-01-01

    The description of a plane wave diffracted by a circular aperture is directly started from the Maxwell's equations. Based on the vector angular spectrum representation of Maxwell's equations, the diffracted plane wave is decomposed into the TE and TM terms. The analytical TE and TM terms in the far field are presented by stationary phase. As the TE and TM terms are orthogonal to each other in the far field, their sum constitutes the so-called Airy disc pattern. Therefore, this research reveals the composition of Airy disc, which is beneficial to deepen and enhance the recognition of the classical diffraction problem.

  20. A New Parameter In Accretion Disk Model

    Yuan, Feng(Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA)

    2000-01-01

    Taking optically thin accretion flows as an example, we investigate the dynamics and the emergent spectra of accretion flows with different outer boundary conditions (OBCs) and find that OBC plays an important role in accretion disk model. This is because the accretion equations describing the behavior of accretion flows are a set of {\\em differential} equations, therefore, accretion is intrinsically an initial-value problem. We argue that optically thick accretion flow should also show OBC-d...

  1. The comparative analysis of rocks' resistance to forward-slanting disc cutters and traditionally installed disc cutters

    Zhang, Zhao-Huang; Fei, Sun; Liang, Meng

    2016-08-01

    At present, disc cutters of a full face rock tunnel boring machine are mostly mounted in the traditional way. Practical use in engineering projects reveals that this installation method not only heavily affects the operation life of disc cutters, but also increases the energy consumption of a full face rock tunnel boring machine. To straighten out this issue, therefore, a rock-breaking model is developed for disc cutters' movement after the research on the rock breaking of forward-slanting disc cutters. Equations of its displacement are established based on the analysis of velocity vector of a disc cutter's rock-breaking point. The functional relations then are brought forward between the displacement parameters of a rock-breaking point and its coordinate through the analysis of micro displacement of a rock-breaking point. Thus, the geometric equations of rock deformation are derived for the forward-slanting installation of disc cutters. With a linear relationship remaining between the acting force and its deformation either before or after the leap breaking, the constitutive relation of rock deformation can be expressed in the form of generalized Hooke law, hence the comparative analysis of the variation in the resistance of rock to the disc cutters mounted in the forward-slanting way with that in the traditional way. It is discovered that with the same penetration, strain of the rock in contact with forward-slanting disc cutters is apparently on the decline, in other words, the resistance of rock to disc cutters is reduced. Thus wear of disc cutters resulted from friction is lowered and energy consumption is correspondingly decreased. It will be useful for the development of installation and design theory of disc cutters, and significant for the breakthrough in the design of full face rock tunnel boring machine.

  2. Spontaneously disappearing lumbar disc protrusion

    Ushewokunze, Shungu; Abbas, Naeem; Dardis, Ronan; Killeen, Ian

    2008-01-01

    Spontaneous disappearance of a herniated lumbar disc is known to occur. This case study describes a 45-year-old patient whose symptoms of lumbar radiculopathy resolved and follow-up imaging showed complete disappearance of the disc prolapse. This phenomenon strengthens the role of conservative treatment in the management of lumbar disc protrusions.

  3. Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM-Newton and INTEGRAL

    Pintore, Fabio; Bozzo, Enrico; Sanna, Andrea; Burderi, Luciano; D'Aì, Antonino; Riggio, Alessandro; Scarano, Fabiana; Iaria, Rosario

    2015-01-01

    Broad emission features of abundant chemical elements, such as Iron, are commonly seen in the X-ray spectra of accreting compact objects and their studies can provide useful information about the geometry of the accretion processes. In this work, we focus our attention on GX 3+1, a bright, persistent accreting low mass X-ray binary, classified as an atoll source. Its spectrum is well described by an accretion disc plus a stable comptonizing, optically thick corona which dominates the X-ray emission in the 0.3-20 keV energy band. In addition, four broad emission lines are found and we associate them with reflection of hard photons from the inner regions of the accretion disc where doppler and relativistic effects are important. We used self-consistent reflection models to fit the spectra of the 2010 XMM-Newton observation and the stacking of the whole datasets of 2010 INTEGRAL observations. We conclude that the spectra are consistent with reflection produced at ~10 gravitational radii by an accretion disc with...

  4. Study of the reflection spectrum of the accreting neutron star GX 3+1 using XMM-Newton and INTEGRAL

    Pintore, F.; Di Salvo, T.; Bozzo, E.; Sanna, A.; Burderi, L.; D'Aì, A.; Riggio, A.; Scarano, F.; Iaria, R.

    2015-06-01

    Broad emission features of abundant chemical elements, such as iron, are commonly seen in the X-ray spectra of accreting compact objects and their studies can provide useful information about the geometry of the accretion processes. In this work, we focus our attention on GX 3+1, a bright, persistent accreting low-mass X-ray binary, classified as an atoll source. Its spectrum is well described by an accretion disc plus a stable Comptonizing, optically thick corona which dominates the X-ray emission in the 0.3-20 keV energy band. In addition, four broad emission lines are found and we associate them with reflection of hard photons from the inner regions of the accretion disc, where Doppler and relativistic effects are important. We used self-consistent reflection models to fit the spectra of the 2010 XMM-Newton observation and the stacking of the whole data sets of 2010 INTEGRAL observations. We conclude that the spectra are consistent with reflection produced at ˜10 gravitational radii by an accretion disc with an ionization parameter of ξ ˜ 600 erg cm s-1 and viewed under an inclination angle of the system of ˜35°. Furthermore, we detected for the first time for GX 3+1, the presence of a power-law component dominant at energies higher than 20 keV, possibly associated with an optically thin component of non-thermal electrons.

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

    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...

  6. Launching jets from accretion belts

    Schreier, Ron

    2016-01-01

    We propose that sub-Keplerian accretion belts around stars might launch jets. The sub-Keplerian inflow does not form a rotationally supported accretion disk, but it rather reaches the accreting object from a wide solid angle. The basic ingredients of the flow are a turbulent region where the accretion belt interacts with the accreting object via a shear layer, and two avoidance regions on the poles where the accretion rate is very low. A dynamo that is developed in the shear layer amplifies magnetic fields to high values. It is likely that the amplified magnetic fields form polar outflows from the avoidance regions. Our speculative belt-launched jets model has implications to a rich variety of astrophysical objects, from the removal of common envelopes to the explosion of core collapse supernovae by jittering jets.

  7. Tracing Planets in Circumstellar Discs

    Uribe Ana L.

    2013-04-01

    Full Text Available Planets are assumed to form in circumstellar discs around young stellar objects. The additional gravitational potential of a planet perturbs the disc and leads to characteristic structures, i.e. spiral waves and gaps, in the disc density profile. We perform a large-scale parameter study on the observability of these planet-induced structures in circumstellar discs in the (submm wavelength range for the Atacama Large (SubMillimeter Array (ALMA. On the basis of hydrodynamical and magneto-hydrodynamical simulations of star-disc-planet models we calculate the disc temperature structure and (submm images of these systems. These are used to derive simulated ALMA maps. Because appropriate objects are frequent in the Taurus-Auriga region, we focus on a distance of 140 pc and a declination of ≈ 20°. The explored range of star-disc-planet configurations consists of six hydrodynamical simulations (including magnetic fields and different planet masses, nine disc sizes with outer radii ranging from 9 AU to 225 AU, 15 total disc masses in the range between 2.67·10-7 M⊙ and 4.10·10-2 M⊙, six different central stars and two different grain size distributions, resulting in 10 000 disc models. At almost all scales and in particular down to a scale of a few AU, ALMA is able to trace disc structures induced by planet-disc interaction or the influence of magnetic fields in the wavelength range between 0.4...2.0 mm. In most cases, the optimum angular resolution is limited by the sensitivity of ALMA. However, within the range of typical masses of protoplane tary discs (0.1 M⊙...0.001 M⊙ the disc mass has a minor impact on the observability. At the distance of 140 pc it is possible to resolve discs down to 2.67·10-6 M⊙ and trace gaps in discs with 2.67·10-4 M⊙ with a signal-to-noise ratio greater than three. In general, it is more likely to trace planet-induced gaps in magneto-hydrodynamical disc models, because gaps are wider in the presence of

  8. Active states and structure transformations in accreting white dwarfs

    Boneva, Daniela; Kaygorodov, Pavel

    2016-07-01

    Active states in white dwarfs are usually associated with light curve's effects that concern to the bursts, flickering or flare-up occurrences. It is common that a gas-dynamics source exists for each of these processes there. We consider the white dwarf binary stars with accretion disc around the primary. We suggest a flow transformation modeling of the mechanisms that are responsible for ability to cause some flow instability and bring the white dwarfs system to the outburst's development. The processes that cause the accretion rate to sufficiently increase are discussed. Then the transition from a quiescent to an active state is realized. We analyze a quasi-periodic variability in the luminosity of white dwarf binary stars systems. The results are supported with an observational data.

  9. Bryan total disc arthroplasty: a replacement disc for cervical disc disease

    Wenger, Markus

    2010-01-01

    Markus Wenger1, Thomas-Marc Markwalder21Neurosurgery, Klinik Beau-Site and Salem-Spital, Berne, Switzerland; 2Attending Neurosurgeon FMH, Private Practice Spine Surgery, Berne-Muri, SwitzerlandAbstract: Total disc arthroplasty is a new option in the treatment of cervical degenerative disc disease. Several types of cervical disc prostheses currently challenge the gold-standard discectomy and fusion procedures. This review describes the Bryan Cervical Disc System and presents the Bryan prosthes...

  10. Accretion Stream Mapping

    Vrielmann, S; Vrielmann, Sonja; Schwope, Axel D.

    1998-01-01

    We present a new mapping algorithm, the Accretion Stream Mapping, which uses the complete emission-line light curve to derive spatially resolved intensity distributions along the stream on a surface created as a duodecadon shaped tube. We successfully test this method on artificial data and then applied it to emission line light curves in Hbeta, Hgamma and HeII 4686 of the magnetic CV HU Aqr. We find Balmer emission near the threading point in the stream facing the white dwarf and Helium emission all over the magnetic part of the stream.

  11. Disc outflows and high-luminosity true type 2 AGN

    Elitzur, Moshe; Netzer, Hagai

    2016-06-01

    The absence of intrinsic broad-line emission has been reported in a number of active galactic nuclei (AGN), including some with high Eddington ratios. Such `true type 2 AGN' are inherent to the disc-wind scenario for the broad-line region: broad-line emission requires a minimal column density, implying a minimal outflow rate and thus a minimal accretion rate. Here we perform a detailed analysis of the consequences of mass conservation in the process of accretion through a central disc. The resulting constraints on luminosity are consistent with all the cases where claimed detections of true type 2 AGN pass stringent criteria, and predict that intrinsic broad-line emission can disappear at luminosities as high as ˜4 × 1046 erg s-1 and any Eddington ratio, though more detections can be expected at Eddington ratios below ˜1 per cent. Our results are applicable to every disc outflow model, whatever its details and whether clumpy or smooth, irrespective of the wind structure and its underlying dynamics. While other factors, such as changes in spectral energy distribution or covering factor, can affect the intensities of broad emission lines, within this scenario they can only produce true type 2 AGN of higher luminosity then those prescribed by mass conservation.

  12. Warps and waves in fully cosmological models of galactic discs

    Gómez, Facundo A; Grand, Robert J J; Marinacci, Federico; Springel, Volker; Pakmor, Rüdiger

    2016-01-01

    Recent studies have revealed an oscillating asymmetry in the vertical structure of the Milky Way's disc. Here we analyze 16 high-resolution, fully cosmological simulations of the evolution of individual Milky Way-sized galaxies, carried out with the MHD code AREPO. At redshift zero, about $70\\%$ of our galactic discs show strong vertical patterns, with amplitudes that can exceed 2 kpc. Half of these are typical `integral sign' warps. The rest are oscillations similar to those observed in the Milky Way. Such structures are thus expected to be common. The associated mean vertical motions can be as large as 30 km/s. Cold disc gas typically follows the vertical patterns seen in the stars. These perturbations have a variety of causes: close encounters with satellites, distant flybys of massive objects, accretion of misaligned cold gas from halo infall or from mergers. Tidally induced vertical patterns can be identified in both young and old stellar populations, whereas those originating from cold gas accretion are...

  13. Why stellar feedback promotes disc formation in simulated galaxies

    Übler, Hannah; Oser, Ludwig; Aumer, Michael; Sales, Laura V; White, Simon

    2014-01-01

    We study how feedback influences baryon infall onto galaxies using cosmological, zoom-in simulations of haloes with present mass $\\mathrm{M}_{\\mathrm{vir}}=6.9\\times10^{11} \\mathrm{M}_{\\odot}$ to $1.7\\times10^{12} \\mathrm{M}_{\\odot}$. Starting at $z=4$ from identical initial conditions, implementations of weak and strong stellar feedback produce bulge- and disc-dominated galaxies, respectively. Strong feedback favours disc formation: (1) because conversion of gas into stars is suppressed at early times, as required by abundance matching arguments, resulting in flat star formation histories and higher gas fractions; (2) because $50\\%$ of the stars form ${\\it in}$ ${\\it situ}$ from recycled disc gas with angular momentum only weakly related to that of the $z=0$ dark halo; (3) because late-time gas accretion is typically an order of magnitude stronger and has higher specific angular momentum, with recycled gas dominating over primordial infall; (4) because $25-30\\%$ of the total accreted gas is ejected entirely ...

  14. Giant planet formation in radially structured protoplanetary discs

    Coleman, Gavin A L

    2016-01-01

    Our recent N-body simulations of planetary system formation, incorporating models for the main physical processes thought to be important during the building of planets (i.e. gas disc evolution, migration, planetesimal/boulder accretion, gas accretion onto cores, etc.), have been successful in reproducing some of the broad features of the observed exoplanet population (e.g. compact systems of low mass planets, hot Jupiters), but fail completely to form any surviving cold Jupiters. The primary reason for this failure is rapid inward migration of growing protoplanets during the gas accretion phase, resulting in the delivery of these bodies onto orbits close to the star. Here, we present the results of simulations that examine the formation of gas giant planets in protoplanetary discs that are radially structured due to spatial and temporal variations in the effective viscous stresses, and show that such a model results in the formation of a population of cold gas giants. Furthermore, when combined with models f...

  15. A toy model for magnetized neutrino-dominated accretion flows

    2010-01-01

    In this paper,we present a simplified model for a magnetized neutrino-dominated accretion flow(NDAF) in which the effect of black hole(BH) spin is taken into account by adopting a set of relativistic correction factors,and the magnetic field is parameterized as β,the ratio of the magnetic pressure to the total pressure.It is found that the disc properties are sensitive to the values of the BH spin and β,and more energy can be extracted from NDAFs by using a faster spin and lower β.

  16. Accretion mode of the Ultra-Luminous X-ray source M82 X-2

    Karino, S

    2016-01-01

    Periodic pulsations have been found in emission from the ultra-luminous X-ray source (ULX) M82 X-2, strongly suggesting that the emitter is a rotating neutron star rather than a black hole. However, the radiation mechanisms and accretion mode involved have not yet been clearly established. In this paper, we examine the applicability to this object of standard accretion modes for high mass X-ray binaries (HMXBs). We find that spherical wind accretion, which drives OB-type HMXBs, cannot apply here but that there is a natural explanation in terms of an extension of the picture for standard Be-type HMXBs. We show that a neutron star with a moderately strong magnetic field, accreting from a disc-shaped wind emitted by a Be-companion, could be compatible with the observed relation between spin and orbital period. A Roche lobe overflow picture is also possible under certain conditions.

  17. Angular Momentum Transport in Quasi-Keplerian Accretion Disks

    Prasad Subramanian; B. S. Pujari; Peter A. Becker

    2004-03-01

    We reexamine arguments advanced by Hayashi & Matsuda (2001), who claim that several simple, physically motivated derivations based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the nature of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expression for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The derivation is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.

  18. Discs in misaligned binary systems

    Rawiraswattana, Krisada; Goodwin, Simon P

    2016-01-01

    We perform SPH simulations to study precession and changes in alignment between the circumprimary disc and the binary orbit in misaligned binary systems. We find that the precession process can be described by the rigid-disc approximation, where the disc is considered as a rigid body interacting with the binary companion only gravitationally. Precession also causes change in alignment between the rotational axis of the disc and the spin axis of the primary star. This type of alignment is of great important for explaining the origin of spin-orbit misaligned planetary systems. However, we find that the rigid-disc approximation fails to describe changes in alignment between the disc and the binary orbit. This is because the alignment process is a consequence of interactions that involve the fluidity of the disc, such as the tidal interaction and the encounter interaction. Furthermore, simulation results show that there are not only alignment processes, which bring the components towards alignment, but also anti-...

  19. Grand Unification of AGN and the Accretion and Spin Paradigms

    Meier, D L

    1999-01-01

    While attempts to unify certain classes of AGN using orientation and environmental effects have been successful, it is widely recognized that intrinsic properties of the accreting black hole system also must play a role in determining the appearance of such an object. In addition to mass and accretion rate, the angular momentum (or spin) of the black hole can play a crucial role in determining the power of a relativistic jet that is generated by magnetohydrodynamic acceleration near the hole. In this paper a scenario is presented, based on accretion theory and recent models of MHD jet production, in which the primary (although not only) parameter differentiating between radio loud and quiet objects is the black hole spin, and that determining quasar vs. radio galaxy is the accretion rate. A surprising number of desirable features result from these simple concepts and the accompanying equations. In addition, there are several testable predictions that can determine whether this grand unification scheme has fur...

  20. Photon Bubbles and the Vertical Structure of Accretion Disks

    Begelman, M C

    2006-01-01

    We consider the effects of "photon bubble" shock trains on the vertical structure of radiation pressure-dominated accretion disks. These density inhomogeneities are expected to develop spontaneously in radiation-dominated accretion disks where magnetic pressure exceeds gas pressure, even in the presence of magnetorotational instability. They increase the rate at which radiation escapes from the disk, and may allow disks to exceed the Eddington limit by a substantial factor. We first generalize the theory of photon bubbles to include the effects of finite optical depths and radiation damping. Modifications to the diffusion law at low optical depth tend to fill in the low-density regions of photon bubbles, while radiation damping inhibits the formation of photon bubbles at large radii, small accretion rates, and small heights above the equatorial plane. Accretion disks dominated by photon bubble transport may reach luminosities of 10 to >100 times the Eddington limit (L_E), depending on the mass of the central ...

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

    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...

  2. Influence of the water content in protoplanetary discs on planet migration and formation

    Bitsch, Bertram; Johansen, Anders

    2016-05-01

    The temperature and density profiles of protoplanetary discs depend crucially on the mass fraction of micrometre-sized dust grains and on their chemical composition. A larger abundance of micrometre-sized grains leads to an overall heating of the disc, so that the water ice line moves further away from the star. An increase in the water fraction inside the disc, maintaining a fixed dust abundance, increases the temperature in the icy regions of the disc and lowers the temperature in the inner regions. Discs with a larger silicate fraction have the opposite effect. Here we explore the consequence of the dust composition and abundance for the formation and migration of planets. We find that discs with low water content can only sustain outwards migration for planets up to 4 Earth masses, while outwards migration in discs with a larger water content persists up to 8 Earth masses in the late stages of the disc evolution. Icy planetary cores that do not reach run-away gas accretion can thus migrate to orbits close to the host star if the water abundance is low. Our results imply that hot and warm super-Earths found in exoplanet surveys could have formed beyond the ice line and thus contain a significant fraction in water. These water-rich super-Earths should orbit primarily around stars with a low oxygen abundance, where a low oxygen abundance is caused by either a low water-to-silicate ratio or by overall low metallicity.

  3. Influence of the water content in protoplanetary discs on planet migration and formation

    Bitsch, Bertram

    2016-01-01

    The temperature and density profiles of protoplanetary discs depend crucially on the mass fraction of micrometre-sized dust grains and on their chemical composition. A larger abundance of micrometre-sized grains leads to an overall heating of the disc, so that the water ice line moves further away from the star. An increase in the water fraction inside the disc, maintaining a fixed dust abundance, increases the temperature in the icy regions of the disc and lowers the temperature in the inner regions. Discs with a larger silicate fraction have the opposite effect. Here we explore the consequence of the dust composition and abundance for the formation and migration of planets. We find that discs with low water content can only sustain outwards migration for planets up to 4 Earth masses, while outwards migration in discs with a larger water content persists up to 8 Earth masses in the late stages of the disc evolution. Icy planetary cores that do not reach run-away gas accretion can thus migrate to orbits close...

  4. Electron thermodynamics in GRMHD simulations of low-luminosity black hole accretion

    Ressler, S. M.; Tchekhovskoy, A.; Quataert, E.; Chandra, M.; Gammie, C. F.

    2015-12-01

    Simple assumptions made regarding electron thermodynamics often limit the extent to which general relativistic magnetohydrodynamic (GRMHD) simulations can be applied to observations of low-luminosity accreting black holes. We present, implement, and test a model that self-consistently evolves an entropy equation for the electrons and takes into account the effects of spatially varying electron heating and relativistic anisotropic thermal conduction along magnetic field lines. We neglect the backreaction of electron pressure on the dynamics of the accretion flow. Our model is appropriate for systems accreting at ≪10-5 of the Eddington accretion rate, so radiative cooling by electrons can be neglected. It can be extended to higher accretion rates in the future by including electron cooling and proton-electron Coulomb collisions. We present a suite of tests showing that our method recovers the correct solution for electron heating under a range of circumstances, including strong shocks and driven turbulence. Our initial applications to axisymmetric simulations of accreting black holes show that (1) physically motivated electron heating rates that depend on the local magnetic field strength yield electron temperature distributions significantly different from the constant electron-to-proton temperature ratios assumed in previous work, with higher electron temperatures concentrated in the coronal region between the disc and the jet; (2) electron thermal conduction significantly modifies the electron temperature in the inner regions of black hole accretion flows if the effective electron mean free path is larger than the local scaleheight of the disc (at least for the initial conditions and magnetic field configurations we study). The methods developed in this work are important for producing more realistic predictions for the emission from accreting black holes such as Sagittarius A* and M87; these applications will be explored in future work.

  5. Disc-mass distribution in star-disc encounters

    Steinhausen, M; Pfalzner, S

    2011-01-01

    Investigations of stellar encounters in cluster environments have demonstrated their potential influence on the mass and angular momentum of protoplanetary discs around young stars. In this study it is investigated in how far the initial surface density in the disc surrounding a young star influences the outcome of an encounter. Based on a power-law ansatz for the surface density, $\\Sigma(r) \\propto r^{-p}$, a parameter study of star-disc encounters with different initial disc-mass distributions has been performed using N-body simulations. It is demonstrated that the shape of the disc-mass distribution has a significant impact on the quantity of the disc-mass and angular momentum losses in star-disc encounters. Most sensitive are the results where the outer parts of the disc are perturbed by high-mass stars. By contrast, disc-penetrating encounters lead more or less independently of the disc-mass distribution always to large losses. However, maximum losses are generally obtained for initially flat distributed...

  6. Thermodynamics of Phantom Energy Accreting onto a Black Hole in Einstein-Power-Maxwell Gravity

    Abbas, G.

    2013-01-01

    In this paper, we investigate the phantom energy accretion onto 3D black hole formulated in Einstein-Power-Maxwell theory. We have presented the conditions for critical accretion of phantom energy onto black hole. Further, we discuss the thermodynamics of phantom energy accreting onto black hole and found that first law of thermodynamics is easily satisfied while second law and generalized second law of thermodynamics remain invalid and conditionally valid, respectively. The results for BTZ b...

  7. Total disc replacement.

    Vital, J-M; Boissière, L

    2014-02-01

    Total disc replacement (TDR) (partial disc replacement will not be described) has been used in the lumbar spine since the 1980s, and more recently in the cervical spine. Although the biomechanical concepts are the same and both are inserted through an anterior approach, lumbar TDR is conventionally indicated for chronic low back pain, whereas cervical TDR is used for soft discal hernia resulting in cervicobrachial neuralgia. The insertion technique must be rigorous, with precise centering in the disc space, taking account of vascular anatomy, which is more complex in the lumbar region, particularly proximally to L5-S1. All of the numerous studies, including prospective randomized comparative trials, have demonstrated non-inferiority to fusion, or even short-term superiority regarding speed of improvement. The main implant-related complication is bridging heterotopic ossification with resulting loss of range of motion and increased rates of adjacent segment degeneration, although with an incidence lower than after arthrodesis. A sufficiently long follow-up, which has not yet been reached, will be necessary to establish definitively an advantage for TDR, particularly in the cervical spine. PMID:24412045

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

    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...

  9. Winds and Accretion in Young Stars

    Edwards, Suzan

    2008-01-01

    Establishing the origin of accretion powered winds from forming stars is critical for understanding angular momentum evolution in the star-disk interaction region. Here, the high velocity component of accretion powered winds is launched and accreting stars are spun down, in defiance of the expected spin-up during magnetospheric accretion. T Tauri stars in the final stage of disk accretion offer a unique opportunity to study the connection between accretion and winds and their relation to stel...

  10. Evolution of Massive Protostars via Disk Accretion

    Hosokawa, Takashi; Yorke, Harold W.; Omukai, Kazuyuki

    2010-01-01

    Mass accretion onto (proto-)stars at high accretion rates > 10^-4 M_sun/yr is expected in massive star formation. We study the evolution of massive protostars at such high rates by numerically solving the stellar structure equations. In this paper we examine the evolution via disk accretion. We consider a limiting case of "cold" disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles ont...

  11. Bimodal gas accretion in the MareNostrum galaxy formation simulation

    Ocvirk, P; Teyssier, R

    2008-01-01

    The physics of diffuse gas accretion and the properties of the cold and hot modes of accretion onto proto-galaxies between z=2 and z=5.4 are investigated using the large cosmological simulation performed with the RAMSES code on the MareNostrum supercomputing facility. Galactic winds, chemical enrichment, UV background heating and radiative cooling are taken into account in this very high resolution simulation. Using {\\it accretion--weighted temperature histograms}, we have perfomed two different measurements of the thermal state of the gas accreted towards the central galaxy. The first measurement, performed using accretion--weighted histograms on a spherical surface of radius $0.2 \\Rv$ centred on {the densest gas structure in the vicinity of the halo centre of mass}, is a good indicator of the presence of an accretion shock in the vicinity of the galactic disc. We define the hot shock mass, $\\Msh$, as the typical halo mass separating cold dominated from hot dominated accretion in the vicinity of the galaxy. ...

  12. Characterisation of global flow and local fluctuations in 3D SPH simulations of protoplanetary discs

    Arena, Serena

    2013-01-01

    A complete and detailed knowledge of the structure of the gaseous component in protoplanetary discs is essential to the study of dust evolution during the early phases of pre-planetesimal formation. The aim of this paper is to determine if three-dimensional accretion discs simulated by the Smoothed Particle Hydrodynamics (SPH) method can reproduce the observational data now available and the expected turbulent nature of protoplanetary discs. The investigation is carried out by setting up a suite of diagnostic tools specifically designed to characterise both the global flow and the fluctuations of the gaseous disc. The main result concerns the role of the artificial viscosity implementation in the SPH method: in addition to the already known ability of SPH artificial viscosity to mimic a physical-like viscosity under specific conditions, we show how the same artificial viscosity prescription behaves like an implicit turbulence model. In fact, we identify a threshold for the parameters in the standard artificia...

  13. Disc-planet interactions in sub-keplerian discs

    Paardekooper, S.-J.

    2009-01-01

    One class of protoplanetary disc models, the X-wind model, predicts strongly subkeplerian orbital gas velocities, a configuration that can be sustained by magnetic tension. We investigate disc-planet interactions in these subkeplerian discs, focusing on orbital migration for low-mass planets and gap formation for high-mass planets. We use linear calculations and nonlinear hydrodynamical simulations to measure the torque and look at gap formation. In both cases, the subkeplerian nature of the ...

  14. Agitating mass transfer with a warped disc's shadow

    Cambier, H.

    2015-10-01

    For compact objects fed by Roche lobe overflow, accretion-generated X-rays irradiating the donor star can alter gas flow towards the Lagrange point thus varying mass transfer. The latest work specific to this topic consists of simple yet insightful two-dimensional hydrodynamics simulations stressing the role of global flow. To explore how a time-varying disc shadow affects mass transfer, I generalize the geometry, employ a robust hydrodynamics solver, and use phase space analysis near the nozzle to include coriolis lift there. Without even exposing the nozzle, a warped disc's shadow can drive mass transfer cycles by shifting the equatorial edges of the irradiation patches in turns: drawing in denser ambient gas before sweeping it into the nozzle. Other important effects remain missing in two-dimensional models, which I discuss along with prospects for more detailed yet efficient models.

  15. Predictions for the reverberating spectral line signal from a newly formed black hole accretion disk: case of tidal disruption flares

    Zhang, W.; Yu, W.; Karas, Vladimír; Dovčiak, Michal

    2015-01-01

    Roč. 807, č. 89 (2015), s. 1-12. ISSN 0004-637X R&D Projects: GA MŠk(CZ) LH14049; GA ČR(CZ) GC13-00070J Institutional support: RVO:67985815 Keywords : black holes * accretion discs Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.993, year: 2014

  16. Vertical structure of the accreting two-temperature corona and the transition to an ADAF

    Różańska, A.; Czerny, B.

    2000-01-01

    We investigate the model of the disc/corona accretion flow around the black hole. The model, parameterized by the total (i.e. disk plus corona) accretion rate, $\\dot m$, mass of the black hole, $M$ and the viscosity parameter, $\\alpha$, uniquely determines the fraction of energy released in the corona as a function of radius and, in particular, the transition radius to the single-phase ADAF flow. Self-consistent solutions with the mass exchange between phases display radial dependence of the ...

  17. Seeing to the Event Horizon: Probing Accretion Physics with X-ray Reflection

    Wilkins, Dan

    2015-09-01

    Accretion onto supermassive black holes in active galactic nuclei is known to power some of the most luminous objects we see in the Universe, which through their vast energy outputs must have played an important role in shaping the large scale structure of the Universe we see today. Much remains unknown, however, about the fine details of this process; exactly how energy is liberated from accretion flows onto black holes, how the 'corona' that produces the intense X-ray continuum is formed and what governs this process over time. I will outline how the detection of X-rays reflected from the discs of accreting material around black holes by the present generation of large X-ray observatories, shifted in energy and blurred by relativistic effects in the strong gravitational field close to the black hole, has enabled measurements of the inner regions of the accretion flow in unprecedented detail. In particular, exploiting the shift in energy of atomic emission lines by relativistic effects as a function of location on the disc has enabled the measurement of the illumination pattern of the accretion flow by the X-ray continuum from which the geometry of the emitting region can be inferred and how the detection of time lags between the primary and reflected X-rays owing to the additional path the reflected rays must travel between the corona and the disc places further constraints on the nature of the emitting corona. These techniques allow the evolution of the corona that accompanies transitions from high to low X-ray flux to be studied, giving clues to the physical process that forms and powers the intense X-ray source and uncovering evidence for the potential launching of jets. I will discuss the great steps forward in understanding accretion physics that can be made with the Athena X-ray observatory, combining detailed analysis of observations with predictions and models from general relativistic ray tracing simulations. In particular, I will discuss how high

  18. Numerical test of the method for revealing traces of deterministic chaos in the accreting black holes

    Suková, Petra; Janiuk, Agnieszka

    2016-01-01

    The high energy radiation emitted by black hole X-ray binaries originates in an accretion disk, hence the variability of the lightcurves mirrors the dynamics of the disc. We study the time evolution of the emitted flux in order to find evidences, that low dimensional non-linear equations govern the accretion flow. Here we test the capabilities of our novel method to find chaotic behaviour on the two numerical time series describing the motion of a test particle around a black hole surrounded ...

  19. The Origin of UV-optical Variability in AGN and Test of Disc Models: XMM-Newton and ground based observations of NGC4395

    McHardy, Ian; Peterson, Brad; Bieryla, Allyson; Chand, Hum; Elvis, Martin; Emmanoulopoulos, Dimitrios; Falco, Emilio; Gandhi, Poshak; Kaspi, Shai; Latham, David; Lira, Paulina; McCully, Curtis; Netzer, Hagai; Uemura, Makoto

    2016-01-01

    The origin of short timescale (weeks/months) variability of AGN, whether due to intrinsic disc variations or reprocessing of X-ray emission by a surrounding accretion disc, has been a puzzle for many years. However recently a number of observational programmes, particularly of NGC5548 with Swift, have shown that the UV/optical variations lag behind the X-ray variations in a manner strongly supportive of X-ray reprocessing. Somewhat surprisingly the implied size of the accretion disc is ~3x greater than expected from a standard, smooth, Shakura-Sunyaev thin disc model. Although the difference may be explained by a clumpy accretion disc, it is not clear whether the difference will occur in all AGN or whether it may change as, eg, a function of black hole mass, accretion rate or disc temperature. Measurements of interband lags for most AGN require long timescale monitoring, which is hard to arrange. However for low mass (<1 million solar mass) AGN, the combination of XMM-Newton EPIC (X-rays) with the optical ...

  20. Edge-on thick discs

    Kasparova, A.; Katkov, I.; Chilingarian, I.; Silchenko, O.; Moiseev, A.; Borisov, S.

    2016-06-01

    Although thick stellar discs are detected in nearly all edge-on disc galaxies, their formation scenarios still remain a matter of debate. Due to observational difficulties, there is a lack of information about their stellar populations. Using the Russian 6-m telescope BTA we collected deep spectra of thick discs in three edge-on early-type disc galaxies located in different environments: NGC4111 in a dense group, NGC4710 in the Virgo cluster, and NGC5422 in a sparse group. We see intermediate age (4 ‑ 5 Gyr) metal rich ([Fe/H] ~ ‑0.2 ‑ 0.0 dex) stellar populations in NGC4111 and NGC4710. On the other hand, NGC5422 does not harbour young stars, its only disc is thick and old (10 Gyr) and its α-element abundance suggests a long formation epoch implying its formation at high redshift. Our results prove the diversity of thick disc formation scenarios.

  1. CT discography for cervical soft disc hernia

    In this study the effectiveness of computed tomographic discography (CTD) in diagnosing cervical soft disc hernia was evaluated. Twenty-five interververtebral discs of 15 cases with cervical soft disc hernia were examined with a discography and then a CT scan. Results of the CT scan were as follows: three discs were protruded, 12 discs were prolapsed, 6 discs were extruded, and 4 discs were sequestrated. The findings were helpful in determining the location of soft disc hernians between the median and posterolateral discs. They were also valuable in classifying types of hernians and surgical aproaches. (author)

  2. Interlaced dynamics of density waves and vortices in self-gravitating Discs

    Mamatsashvili G. R.

    2013-01-01

    Latest developments in the dynamics of density waves and vortices in selfgravitating protoplanetary discs is reviewed. It is well established by now that in discs, vortices are dynamically coupled with density waves due to the disc’s differential rotation, or shear. On the other hand, density waves play a central role in the theory of self-gravitating discs and recently revealed their coupling with vortices implies that the latter can also be subject to self-gravity effects, thus taking ...

  3. On the convective overstability in protoplanetary discs

    Latter, Henrik

    2015-01-01

    This paper explores the driving of low-level hydrodynamical activity in protoplanetary-disc dead zones. A small adverse radial entropy gradient, ordinarily stabilised by rotation, excites oscillatory convection (`convective overstability') when thermal diffusion, or cooling, is neither too strong nor too weak. I revisit the linear theory of the instability, discuss its prevalence in protoplanetary discs, and show that unstable modes are exact nonlinear solutions in the local Boussinesq limit. Overstable modes cannot grow indefinitely, however, as they are subject to a secondary parametric instability that limits their amplitudes to relatively low levels. If parasites set the saturation level of the ensuing turbulence then the convective overstability is probably too weak to drive significant angular momentum transport or to generate vortices. But I also discuss an alternative, and far more vigorous, saturation route that generates radial `layers' or `zonal flows' (witnessed also in semiconvection). Numerical ...

  4. Tilted Accretion Disk Models of Sgr A* Flares

    Dexter, Jason; Fragile, P. C.

    2013-01-01

    Sagittarius A* (Sgr A*), the Galactic center massive black hole candidate, is an unparalleled laboratory for low-luminosity accretion theory. First discovered as a compact radio source, Sgr A* has since been observed to undergo rapid, large amplitude NIR/X-ray flares. The many proposed phenomenological models cannot simultaneously explain both the flaring emission and the peak of the SED in the submillimeter. I will describe flares seen in numerical simulations of black hole accretion flows where the disk angular momentum is misaligned from that of the black hole. Eccentric fluid orbits driven by gravitational torques converge and form strong shocks, which can lead to significant particle heating. The resulting NIR emission can reproduce the observations, and is completely unrelated to the submillimeter emission, which is included in these models and is also in excellent agreement with observations. I will describe the prospects for testing accretion theory and constraining the properties of Sgr A* with exciting ongoing multi-wavelength observations.

  5. Local and global dynamics of warped astrophysical discs

    Ogilvie, Gordon I.; Latter, Henrik N.

    2013-08-01

    Astrophysical discs are warped whenever a misalignment is present in the system, or when a flat disc is made unstable by external forces. The evolution of the shape and mass distribution of a warped disc is driven not only by external influences but also by an internal torque, which transports angular momentum through the disc. This torque depends on internal flows driven by the oscillating pressure gradient associated with the warp, and on physical processes operating on smaller scales, which may include instability and turbulence. We introduce a local model for the detailed study of warped discs. Starting from the shearing sheet of Goldreich and Lynden-Bell, we impose the oscillating geometry of the orbital plane by means of a coordinate transformation. This warped shearing sheet (or box) is suitable for analytical and computational treatments of fluid dynamics, magnetohydrodynamics, etc., and it can be used to compute the internal torque that drives the large-scale evolution of the disc. The simplest hydrodynamic states in the local model are horizontally uniform laminar flows that oscillate at the orbital frequency. These correspond to the non-linear solutions for warped discs found in previous work by Ogilvie, and we present an alternative derivation and generalization of that theory. In a companion paper, we show that these laminar flows are often linearly unstable, especially if the disc is nearly Keplerian and of low viscosity. The local model can be used in future work to determine the non-linear outcome of the hydrodynamic instability of warped discs, and its interaction with others such as the magnetorotational instability.

  6. Misaligned gas discs around eccentric black-hole binaries and implications for the final-parsec problem

    Aly, Hossam; Nixon, Chris; King, Andrew

    2015-01-01

    We investigate the evolution of low mass (Md /Mb = 0.005) misaligned gaseous discs around eccentric supermassive black hole (SMBH) binaries. These are expected to form from randomly oriented accretion events onto a SMBH binary formed in a galaxy merger. When expanding the interaction terms between the binary and a circular ring to quadrupole order and averaging over the binary orbit, we expect four non-precessing disc orientations: aligned or counter-aligned with the binary, or polar orbits around the binary eccentricity vector with either sense of rotation. All other orientations precess around either of these, with the polar precession dominating for high eccentricity. These expectations are borne out by smoothed particle hydrodynamics simulations of initially misaligned viscous circumbinary discs, resulting in the formation of polar rings around highly eccentric binaries in contrast to the co-planar discs around circular binaries. Moreover, we observe disc tearing and violent interactions between different...

  7. The physical properties of z>2 Lyman limit systems: new constraints for feedback and accretion models

    Fumagalli, Michele; Prochaska, J Xavier

    2015-01-01

    We study the physical properties of a homogeneous sample of 157 optically-thick absorption line systems at redshifts ~1.8-4.4, selected from a high-dispersion spectroscopic survey of Lyman limit systems (LLSs). By means of multiple ionisation models and Bayesian techniques, we derive the posterior probability distribution functions for the density, metallicity, temperature, and dust content of the absorbing gas. We find that z>2 LLSs are highly ionised with ionisation parameters between -32 are characterised by a broad unimodal distribution over >4 orders of magnitude, with a peak at log Z/Zsun~-2. LLSs are metal poor, significantly less enriched than DLAs, with ~70% of the metallicity PDF below log Z/Zsun19 rapidly evolves with redshift, with a ten-fold increase between z~2.1-3.6 (~1.5 Gyr). Based on this sample, we find that LLSs at z=2.5-3.5 account for ~15% of all the metals produced by UV-selected galaxies. The implications for theories of cold gas accretion and metal ejection from galaxies are also disc...

  8. No Correlation Between Disc Scale-Height and Jet Power in GRMHD Simulations

    Fragile, P Chris; Rodriguez, Marco

    2012-01-01

    It is now well established that changes in the X-ray spectral state of black hole low-mass X-ray binaries are correlated with changes in the radio properties of those systems. Assuming radio power is a proxy for jet power, we can say that the jet is continuously present in the hard state and undetectable (and therefore weaker) in the soft state. Since the different accretion states are also generally assumed to be associated with different disc geometries -- the hard state with a hot, thick flow, and the soft state with a cold, thin disc -- we investigate the possibility that these two phenomena are linked; i.e., that the difference in disc geometry is the cause of the difference in observed jet power. We do this by comparing various measures of jet power in numerical simulations of accretion discs of differing temperatures and thicknesses. We perform these simulations using the general relativistic magnetohydrodynamic code Cosmos++ and a newly added cooling function, which allows us to regulate the disc scal...

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

    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...

  10. A photo-evaporative gap in the closest planet forming disc

    Ercolano, Barbara; Picogna, Giovanni; Testi, Leonardo

    2016-01-01

    The dispersal of the circumstellar discs of dust and gas surrounding young low- mass stars has important implications for the formation of planetary systems. Photo- evaporation from energetic radiation from the central object is thought to drive the dispersal in the majority of discs, by creating a gap which disconnects the outer from the inner regions of the disc and then disperses the outer disc from the inside-out, while the inner disc keeps draining viscously onto the star. In this Letter we show that the disc around TW Hya, the closest protoplanetary disc to Earth, may be the first object where a photoevaporative gap has been imaged around the time at which it is being created. Indeed the detected gap in the ALMA images is consistent with the expectations of X-ray photoevaporation models, thus not requiring the presence of a planet. The photoevaporation model is also consistent with a broad range of properties of the TW Hya system, e.g. accretion rate and the location of the gap at the onset of dispersal...

  11. Accretion onto Stars with Octupole Magnetic Fields: Matter Flow, Hot Spots and Phase Shifts

    Long, Min; Lamb, Frederick K

    2009-01-01

    Recent measurements of the surface magnetic fields of classical T Tauri stars (CTTSs) and magnetic cataclysmic variables show that their magnetic fields have a complex structure. The magnetic field associated with the octupole moment may dominate the magnetic field associated with other moments in some stars, such as the CTTS V2129 Oph. Previously, we studied disc accretion onto stars with magnetic fields described by a superposition of aligned or misaligned dipole and quadrupole moments. In this paper, we present results of the first simulations of disc accretion onto stars with an \\textit {octupole} field. As examples, we consider stars with a superposition of octupole and dipole fields of different strengths and investigate matter flow around them, the shapes of hot spots on their surfaces, and the light curves produced by their rotation. We investigate two possible mechanisms for producing phase shifts in the light curves of stars with complex fields: (1) change of the star's intrinsic magnetic field and ...

  12. Two-dimensional models of layered protoplanetary discs. II. - The effect of a residual viscosity in the dead zone

    Wünsch, Richard; Gawryszczak, A.; Klahr, H.; Rózycka, M.

    2006-01-01

    Roč. 367, č. 2 (2006), s. 773-780. ISSN 0035-8711 R&D Projects: GA AV ČR IAB3003106 Institutional research plan: CEZ:AV0Z10030501 Keywords : accretion discs * hydrodynamics * instabilities Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics Impact factor: 5.057, year: 2006

  13. Quasi-spherical accretion in X-ray pulsars

    Postnov, K; Kochetkova, A; Hjalmarsdotter, L

    2011-01-01

    Quasi-spherical accretion in wind-fed X-ray pulsars is discussed. At X-ray luminosities <4 10^{36} erg/s, a hot convective shell is formed around the neutron star magnetosphere, and subsonic settling accretion regime sets in. In this regime, accretion rate onto neutron star is determined by the ability of plasma to enter magnetosphere via Rayleigh-Taylor instability. A gas-dynamic theory of settling accretion is constructed taking into account anisotropic turbulence. The angular momentum can be transferred through the quasi-static shell via large-scale convective motions initiating turbulence cascade. The angular velocity distribution in the shell is found depending on the turbulent viscosity prescription. Comparison with observations of long-period X-ray wind-fed pulsars shows that an almost iso-angular-momentum distribution is most likely realized in their shells. The theory explains long-term spin-down in wind- fed accreting pulsars (e.g. GX 1+4) and properties of short-term torque-luminosity correlatio...

  14. Percutaneous laser disc decompression for cervical disc herniation

    Objective: To discuss the clinical application of percutaneous laser disc decompression (PLDD) in the treatment of cervical disc herniation. Methods: The region between anterolateral cervical visceral sheath and carotid sheath was used as puncture access. Under C-arm fluoroscopic monitoring, the puncture needle was inserted to the cervical intervertebral space center, then, the photofiber was implanted in PLDD was performed in 32 patients of cervical disc herniation by using SLT30 semiconductor laser device with the laser output of 500-1 000 J. Results: The effective rate was 87.5%. No infection or serious complications occurred. Conclusion: Percutaneous laser disc decompression is a safe, effective and less invasive treatment for cervical disc herniation. (authors)

  15. Dynamics of continental accretion.

    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. PMID:24670638

  16. Artificial Cervical Disc Replacement Improves Mobility

    Full Text Available ... over, you know, fusion. People wonder, does it cause problems at the next level up, so if ... a concern. Will the fusion at this disc cause problems at the disc above or the disc ...

  17. 46 CFR 64.61 - Rupture disc.

    2010-10-01

    ... SYSTEMS Pressure Relief Devices and Vacuum Relief Devices for MPTs § 64.61 Rupture disc. If a rupture disc is the only pressure relief device on the tank, the rupture disc must— (a) Rupture at a pressure...

  18. Role of the emission angular directionality in the spin determination of accreting black holes

    Svoboda, Jiří; Dovčiak, Michal; Goosmann, René; Karas, Vladimír

    Melville : American Institute of Physics, 2010 - (Zatloukal, M.), s. 515-516 ISBN 978-0-7354-0795-4. ISSN 0094-243X. - (AIP Conference Proceedings. 1248). [X-ray astronomy 2009. Bologna (IT), 07.09.2009-11.09.2009] Institutional research plan: CEZ:AV0Z10030501 Keywords : black hole * accretion disc * X-ray spectroscopy Subject RIV: BN - Astronomy, Celestial Mechanics, Astrophysics

  19. Limit-Cycle Behaviour of Thermally-Unstable Accretion Flows onto Black Holes

    Szuszkiewicz, Ewa; Miller, John C.

    1998-01-01

    Nonlinear time-dependent calculations are being carried out in order to study the evolution of vertically-integrated models of non-selfgravitating, transonic accretion discs around black holes. In this paper we present results from a new calculation for a high-alpha model similar to one studied previously by Honma, Matsumoto and Kato who found evidence for limit-cycle behaviour connected with thermal instability. Our results are in substantial agreement with theirs but, in our calculation, th...

  20. ACCRETING CIRCUMPLANETARY DISKS: OBSERVATIONAL SIGNATURES

    Zhu, Zhaohuan, E-mail: zhzhu@astro.princeton.edu [Department of Astrophysical Sciences, 4 Ivy Lane, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States)

    2015-01-20

    I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10{sup –10} M {sub ☉} yr{sup –1} around a 1 M{sub J} planet can be brighter than the planet itself. A moderately accreting circumplanetary disk ( M-dot ∼10{sup −8} M{sub ⊙} yr{sup −1}; enough to form a 10 M{sub J} planet within 1 Myr) around a 1 M{sub J} planet has a maximum temperature of ∼2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 M{sub J} planet with a ''hot start''. To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 M{sub J} ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L', M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks ( M-dot ≲10{sup −9} M{sub ⊙} yr{sup −1}) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.