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.\,, $\tau = - \alpha p_g$. It is found that an oscillatory instability exists in the inner regions of disks ($r < 10 r_g$, where $r_g$ is the Schwarzschild radius) for sufficiently large $\alpha$ ($\gapprox 0.2$), and for mass accretion rates less than about 0.3 times the Eddington value. The variations of the integrated bolometric luminosity from the disk, $\Delta L/L$, are less than 3\%. A power spectrum analysis of these variations reveals a power spectrum which can be fit to a power law function of the frequency $P \propto f^{-\gamma}$, with index $\gamma \sim 1.4-2.3$ 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 $\sim 100-130$ 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 to 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. 
Xingming Chen and Ronald E. Taam (Northwestern University)