698 research outputs found
Dynamics of warped accretion discs
Accretion discs are present around both stellar-mass black holes in X-ray
binaries and supermassive black holes in active galactic nuclei. A wide variety
of circumstantial evidence implies that many of these discs are warped. The
standard Bardeen--Petterson model attributes the shape of the warp to the
competition between Lense--Thirring torque from the central black hole and
viscous angular-momentum transport within the disc. We show that this
description is incomplete, and that torques from the companion star (for X-ray
binaries) or the self-gravity of the disc (for active galactic nuclei) can play
a major role in determining the properties of the warped disc. Including these
effects leads to a rich set of new phenomena. For example, (i) when a companion
star is present and the warp arises from a misalignment between the companion's
orbital axis and the black hole's spin axis, there is no steady-state solution
of the Pringle--Ogilvie equations for a thin warped disc when the viscosity
falls below a critical value; (ii) in AGN accretion discs, the warp can excite
short-wavelength bending waves that propagate inward with growing amplitude
until they are damped by the disc viscosity. We show that both phenomena can
occur for plausible values of the black hole and disc parameters, and briefly
discuss their observational implications.Comment: 28 pages, 11 figure
A Grid of Relativistic, non-LTE Accretion Disk Models for Spectral Fitting of Black Hole Binaries
Self-consistent vertical structure models together with non-LTE radiative
transfer should produce spectra from accretion disks around black holes which
differ from multitemperature blackbodies at levels which may be observed. High
resolution, high signal-to-noise observations warrant spectral modeling which
both accounts for relativistic effects, and treats the physics of radiative
transfer in detail. In Davis et al. (2005) we presented spectral models which
accounted for non-LTE effects, Compton scattering, and the opacities due to
ions of abundant metals. Using a modification of this method, we have tabulated
spectra for black hole masses typical of Galactic binaries. We make them
publicly available for spectral fitting as an Xspec model. These models
represent the most complete realization of standard accretion disk theory to
date. Thus, they are well suited for both testing the theory's applicability to
observed systems and for constraining properties of the black holes, including
their spins.Comment: 7 pages, emulate ApJ, accepted to Ap
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