1,626 research outputs found
Convective-Dynamical Instability in Radiation-Supported Accretion Disks
We study radiation-hydrodynamical normal modes of radiation-supported
accretion disks in the WKB limit. It has long been known that in the large
optical depth limit the standard equilibrium is unstable to convection. We
study how the growth rate depends on location within the disk, optical depth,
disk rotation, and the way in which the local dissipation rate depends on
density and pressure. The greatest growth rates are found near the disk
surface. Rotation stabilizes vertical wavevectors, so that growing modes tend
to have nearly-horizontal wavevectors. Over the likely range of optical depths,
the linear growth rate for convective instability has only a weak dependence on
disk opacity. Perturbations to the dissipation have little effect on convective
mode growth rates, but can cause growth of radiation sound waves.Comment: 20 pages, AAS LaTe
Vertical Structure of Gas Pressure-Dominated Accretion Disks with Local Dissipation of Turbulence and Radiative Transport
(shortened) We calculate the vertical structure of a local patch of an
accretion disk in which heating by dissipation of MRI-driven MHD turbulence is
balanced by radiative cooling. Heating, radiative transport, and cooling are
computed self-consistently with the structure by solving the equations of
radiation MHD in the shearing-box approximation. Using a fully 3-d and
energy-conserving code, we compute the structure of this disk segment over a
span of more than five cooling times. After a brief relaxation period, a
statistically steady-state develops. Measuring height above the midplane in
units of the scale-height H predicted by a Shakura-Sunyaev model, we find that
the disk atmosphere stretches upward, with the photosphere rising to about 7H,
in contrast to the approximately 3H predicted by conventional analytic models.
This more extended structure, as well as fluctuations in the height of the
photosphere, may lead to departures from Planckian form in the emergent
spectra. Dissipation is distributed across the region within roughly 3H of the
midplane, but is very weak at greater altitudes. Because fluctuations in the
dissipation are particularly strong away from the midplane, the emergent
radiation flux can track dissipation fluctuations with a lag that is only
0.1--0.2 times the mean cooling time of the disk. Long timescale asymmetries in
the dissipation distribution can also cause significant asymmetry in the flux
emerging from the top and bottom surfaces of the disk. Radiative diffusion
dominates Poynting flux in the vertical energy flow throughout the disk.Comment: accepted by Ap
Thermodynamics of an Accretion Disk Annulus with Comparable Radiation and Gas Pressure
We explore the thermodynamic and global structural properties of a local
patch of an accretion disk whose parameters were chosen so that radiation
pressure and gas pressure would be comparable in magnitude. Heating, radiative
transport, and cooling are computed self-consistently with the structure by
solving the equations of radiation MHD in the shearing-box approximation. Using
a fully 3-d and energy-conserving code, we compute the structure and energy
balance of this disk segment over a span of more than forty cooling times. As
is also true when gas pressure dominates, the disk's upper atmosphere is
magnetically-supported. However, unlike the gas-dominated case, no steady-state
is reached; instead, the total (i.e., radiation plus gas) energy content
fluctuates by factors of 3--4 over timescales of several tens of orbits, with
no secular trend. Because the radiation pressure varies much more than the gas
pressure, the ratio of radiation pressure to gas pressure varies over the
approximate range 0.5--2. The volume-integrated dissipation rate generally
increases with increasing total energy, but the mean trend is somewhat slower
than linear, and the instantaneous dissipation rate is often a factor of two
larger or smaller than the mean for that total energy level. Locally, the
dissipation rate per unit volume scales approximately in proportion to the
current density; the time-average dissipation rate per unit mass is
proportional to m^{-1/2}, where m is the horizontally-averaged mass column
density to the nearer of the top or bottom surface. As in our earlier study of
a gas-dominated shearing-box, we find that energy transport is completely
dominated by radiative diffusion, with Poynting flux carrying less than 1% of
the energy lost from the box.Comment: ApJ, in pres
The Inverse Compton Thermostat in Hot Plasmas Near Accreting Black Holes
The hard X-ray spectra of accreting black holes systems are generally
well-fit by thermal Comptonization models with temperatures keV. We
demonstrate why, over many orders of magnitude in heating rate and seed photon
supply, hot plasmas radiate primarily by inverse Compton scattering, and find
equilibrium temperatures within a factor of a few of 100 keV. We also determine
quantitatively the (wide) bounds on heating rate and seed photon supply for
which this statement is true.
Plasmas in thermal balance in this regime obey two simple scaling laws, one
relating the product of temperature and optical depth to the ratio of seed
photon luminosity to plasma heating rate , the other relating the
spectral index of the output power-law to . Because is almost
independent of everything but , the observed power law index may be
used to estimate . In both AGN and stellar black holes, the mean value
estimated this way is . As a corollary, must
be -- 0.2, depending on plasma geometry.Comment: 26 pages, AASLaTeX, to appear in July 10 Ap.J. Figures available in
uuencoded form at ftp://jhufos.pha.jhu.edu/pub/put/jhk/comptfigs.u
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