INTERMITTENT ACTIVITY IN AGN

摘要

There is a growing evidence that the AGN activity could be intermittent. It remains an open question if this behavior is caused by changes in the fuel supply to the supermassive black hole from the large distances, or rather by a processes intrinsic to the active nucleus. We consider the possibility that accretion onto a supermassive black hole is controlled by an accretion disk which is subject to the hydrogen ionization instability. This drives the observed on-off activity cycle, since periodically the accretion flow becomes inefficient and the disk goes to quiescence. We consider effects of the MHD turbulence on the viscosity during the evolution of a standard α - disk. We perform a self-consistency check of the α description of the angular momentum transfer. Having shown that the viscosity parameter is constant throughout the whole instability cycle, as implied by the strength of the MHD turbulence, we calculate the time evolution of the disk under the influence of the ionization instability. We demonstrate that if the accretion onto a supermassive black hole proceeds through an outer standard accretion disk and inner, radiatively inefficient and advection dominated flow, the modelled amplitudes of disk luminosity variations are sufficiently high to account for the observations.