4,736 research outputs found
Ya. B. Zeldovich and foundation of the accretion theory
This short review is dedicated to academician Yakov Borisovich Zeldovich, the
science of his epoch and the creation of modern accretion theory.Comment: 11 pages, 25 figures, accepted to special volume of Astronomy Report
Viscous Stability of Relativistic Keplerian Accretion Disks
We investigate the viscous stability of thin, Keplerian accretion disks in
regions where general relativistic (GR) effects are essential. For gas pressure
dominated (GPD) disks, we show that the Newtonian conclusion that such disks
are viscously stable is reversed by GR modifications in the behaviors of
viscous stress and surface density over a significantly large annular region
not far from the innermost stable orbit at r=\rms. For slowly-rotating
central objects, this region spans a range of radii 14\lo r\lo 19 in units of
the central object's mass . For radiation pressure dominated (RPD) disks,
the Newtonian conclusion that they are viscously unstable remains valid after
including the above GR modifications, except in a very small annulus around
, which has a negligible influence. Inclusion of the stabilizing
effect of the mass-inflow through the disk's inner edge via a GR analogue of
Roche-lobe overflow adds a small, stable region around \rms~for RPD disks, but
leaves GPD disks unchanged. We mention possible astrophysical relevance of
these results, particularly to the high-frequency X-ray variabilities observed
by the .Comment: 18 pages, 3 figures, accepted by The Astrophysical Journal Letter
Comment on Viscous Stability of Relativistic Keplerian Accretion Disks
Recently Ghosh (1998) reported a new regime of instability in Keplerian
accretion disks which is caused by relativistic effects. This instability
appears in the gas pressure dominated region when all relativistic corrections
to the disk structure equations are taken into account. We show that he uses
the stability criterion in completely wrong way leading to inappropriate
conclusions. We perform a standard stability analysis to show that no unstable
region can be found when the relativistic disk is gas pressure dominated.Comment: 9 pages, 4 figures, uses aasms4.sty, submitted for ApJ Letter
Photon Bubbles and the Vertical Structure of Accretion Disks
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 object, while remaining geometrically
thin. However, photon bubble-dominated disks with alpha-viscosity are subject
to the same thermal and viscous instabilities that plague standard radiation
pressure-dominated disks, suggesting that they may be intrinsically unsteady.
Photon bubbles can lead to a "core-halo" vertical disk structure. In
super-Eddington disks the halo forms the base of a wind, which carries away
substantial energy and mass, but not enough to prevent the luminosity from
exceeding L_E. Photon bubble-dominated disks may have smaller color corrections
than standard accretion disks of the same luminosity. They remain viable
contenders for some ultraluminous X-ray sources and may play a role in the
rapid growth of supermassive black holes at high redshift.Comment: 38 pages, 2 figures, accepted for publication in The Astrophysical
Journa
Counter-rotating Accretion Disks
We consider accretion disks consisting of counter-rotating gaseous components
with an intervening shear layer. Configurations of this type may arise from the
accretion of newly supplied counter-rotating gas onto an existing co-rotating
gas disk. For simplicity we consider the case where the gas well above the disk
midplane is rotating with angular rate and that well below has the
same properties but is rotating with rate . Using the Shakura-Sunyaev
alpha turbulence model, we find self-similar solutions where a thin (relative
to the full disk thickness) equatorial layer accretes very rapidly, essentially
at free-fall speed. As a result the accretion speed is much larger than it
would be for an alpha disk rotating in one direction. Counter-rotating
accretion disks may be a transient stage in the formation of counter-rotating
galaxies and in the accretion of matter onto compact objects.Comment: 7 pages, 3 figures, aas2pp4.sty, submitted to Ap
Accretion and Outflow in Active Galaxies
I review accretion and outflow in active galactic nuclei. Accretion appears
to occur in a series of very small--scale, chaotic events, whose gas flows have
no correlation with the large--scale structure of the galaxy or with each
other. The accreting gas has extremely low specific angular momentum and
probably represents only a small fraction of the gas involved in a galaxy
merger, which may be the underlying driver.
Eddington accretion episodes in AGN must be common in order for the
supermassive black holes to grow. I show that they produce winds with
velocities and ionization parameters implying the presence of
resonance lines of helium-- and hydrogenlike iron. The wind creates a strong
cooling shock as it interacts with the interstellar medium of the host galaxy,
and this cooling region may be observable in an inverse Compton continuum and
lower--excitation emission lines associated with lower velocities. The shell of
matter swept up by the shocked wind stalls unless the black hole mass has
reached the value implied by the relation. Once this
mass is reached, further black hole growth is prevented. If the shocked gas did
not cool as asserted above, the resulting (`energy-driven') outflow would imply
a far smaller SMBH mass than actually observed. Minor accretion events with
small gas fractions can produce galaxy-wide outflows, including fossil outflows
in galaxies where there is little current AGN activity.Comment: invited review, IAU Symposium 267, Co-Evolution of Central Black
Holes and Galaxies, B.M. Peterson, R.S. Somerville, and T. Storchi-Bergmann,
eds typos in eq (2.2) correcte
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