THE EFFECT OF AN OPTICALLY THIN REGION ON STABILITY OF AN ACCRETION DISK AROUND A SUPERMASSIVE BLACK-HOLE

We analyse conditions of the innermost portion of an accretion disk an d establish a set of equations for this region. A stable innermost reg ion may exist, which can probably explain the observed UV and X-ray sp ectra, avoiding the unstable emission. We then discuss the detailed ra dial structure of a disk around a black hole for typical AGN parameter s and obtain different kinds of M-Sigma relationships for different re gions of a disk. On the basis of this, we discuss the stability. A new type of cycle is present, which we call a double 5 shaped cycle. In t his cycle, the extent of accretion rate variability is much larger tha n that in dwarf nova cycles. This probably solves the problem of viole nt variability of AGN. In the meantime, the very high accretion rate a t the hottest state in limit cycles in the unstable region may provide continuous injection of matter to the jet and power the relativistic motion of the jet.