Accreting corona model of the X-ray variability in soft state X-ray binaries and active galactic nuclei

Abstract

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.