VARIABILITY OF ACCRETION DISKS SURROUNDING BLACK-HOLES - THE ROLE OF INERTIAL-ACOUSTIC MODE-INSTABILITIES

The global nonlinear time-dependent evolution of the inertial-acoustic mode instability in accretion disks surrounding black holes has been investigated. The viscous stress is assumed to be proportional to the gas pressure only, i.e., tau = - alphap(g). It is found that an oscillatory nonsteady behavior exists in the inner regions of disks (r < 10r(g), where r(g) is the Schwarzschild radius) for sufficiently large alpha (greater-than-or-similar-to 0.2) and for mass accretion rates less than about 0.3 times the Eddington value. The variations of the integrated bolometric luminosity from the disk, DELTAL/L, are less than 3%. A power spectrum analysis of these variations reveals a power spectrum which can be fitted to a power-law function of the frequency P infinity f-(gamma), with index gamma = 1.4-2.3 and a low-frequency feature at about 4 Hz in one case. In addition, a narrow peak centered at a frequency corresponding to the maximum epicyclic frequency of the disk at approximately 100-130 Hz and its first harmonic is also seen. The low-frequency modulations are remarkably similar to those observed in black hole candidate systems. The possible existence of a scattering corona in the inner region of the disk and/or other processes contributing to the power at high frequencies in the inner region of the accretion disk may make the detection of the high-frequency component difficult.