141 research outputs found
Stabilization of radiation pressure dominated accretion disks by viscous fluctuations
The standard thin accretion disk model has been successfully used to explain
the soft X-ray spectra of Galactic black hole systems and perhaps the UV
emission of Active Galactic Nuclei. However, radiation pressure dominated disks
are known to be viscously unstable and should produce large amplitude
oscillations that are typically not observed. Instead, these sources exhibit
stochastic variability which may naturally arise due to viscous fluctuations in
a turbulent disk. Here we investigate whether these aperiodic viscous
fluctuations can stabilize the inner radiation pressure dominated disks and
hence maybe the answer to a forty year old problem in accretion disk theory.Comment: 6 pages, 6 figures; accepted to Astronomy and Astrophysic
Intermittent activity of radio sources. Accretion instabilities and jet precession
We consider the radiation pressure instability operating on short timescales
10^3 - 10^6 years in the accretion disk around a supermassive black hole as the
origin of the intermittent activity of radio sources. We test whether this
instability can be responsible for short ages (<10^4 years) of Compact Steep
Spectrum sources measured by hot spots propagation velocities in VLBI
observations and statistical overabundance of Gigahertz Peaked Spectrum
sources.The implied timescales are consistent with the observed ages of the
sources. We aslo discuss possible implications of the intermittent activity on
the complex morphology of radio sources, such as the quasar 1045+352, dominated
by a knotty jet showing several bends. It is possible that we are whitnessing
an ongoing jet precession in this source due to internal instabilities within
the jet flow.Comment: 2 pages, 1 figure, Proceedings of the 275 IAU Symposium "Jets at all
scales", Buenos Aires, 13-17.09.2010; eds. G. Romero, R. Sunyaev, T. Bellon
Disk/corona model: The transition to ADAF
We propose a model of the accretion flow onto a black hole consisting of the
accretion disk with an accreting two-temperature corona. The model is based on
assumptions about the radiative and conductive energy exchange between the two
phases and the pressure equilibrium. The complete model is determined by the
mass, the accretion rate, and the viscosity parameter. We present the radial
dependencies of parameters of such a two-phase flow, with advection in the
corona and the disk/corona mass exchange due to evaporation/condensation
included, and we determine the transition radius from a two-phase disk/corona
accretion to a single-phase optically thin flow (ADAF) in the innermost part of
the disk as a function of accretion rate. We identify the NLS1 galaxies with
objects accreting at a rate close to the Eddington accretion rate. The strong
variability of these objects may be related to the limit cycle behaviour
expected in this luminosity range, as the disk, unstable due to the dominance
by the radiation pressure, oscillates between the two stable branches: the
advection-dominated optically thick branch and the evaporation branch.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Accreting corona model of the X-ray variability in soft state X-ray binaries and active galactic nuclei
We develop a two-flow model of accretion onto a black hole which incorporates
the effect of the local magneto-rotational instability. The flow consists of an
accretion disk and an accreting corona, and the local dynamo affects the
disk/corona mass exchange. The model is aimed to explain the power spectrum
density of the sources in their soft, disk-dominated states. The local
perturbations of the magnetic field in the disk are described as in King et al.
(2004) and Mayer and Pringle (2005), but the time-dependent local magnetic
field is assumed to affect the local supply of the material to the corona.
Since the viscous timescale in the corona is much shorter than in the disk, the
local perturbations are not smeared in the corona. Simple analytical estimates
and full time-dependent computations of the disk-corona system are performed.
The accreting corona model can reproduce the broad power spectra of Soft State
X-ray binaries and AGN. The model, however, predicts that (i) sources
undergoing radiation pressure instability (GRS 1915+105) should have
systematically steeper power spectra than other sources, (ii) AGN should have
systematically steeper power spectra than GBH, even if their disks are
described using viscosity proportional to the gas pressure. More precise
measurements of power spectra of Soft State sources are clearly needed.Comment: 14 pages; 15 figures. Accepted for Astronomy & Astrophysic
Time delays between the soft and hard X-ray bands in GRS 1915+105
The hard X-ray lightcurves exhibit delays of s with respect to the
soft X-ray lightcurves when the microquasar GRS 1915+105 is in the state of
frequent, regular outbursts (states and of Belloni et al.
2000). Such outbursts are supposed to be driven by the radiation pressure
instability of the inner disc parts. The hard X-ray delays are then caused by
the time needed for the adjustment of the corona to changing conditions in the
underlying disc. We support this claim by the computation of the time evolution
of the disc, including a non-stationary evaporation of the disc and mass
exchange with the corona.Comment: 13 pages, 15 figures; MNRAS accepte
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