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 $\sim 1$ s with respect to the soft X-ray lightcurves when the microquasar GRS 1915+105 is in the state of frequent, regular outbursts (states $\rho$ and $\kappa$ 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