Nonlinear Oscillations of Thermally Unstable Slim Accretion Disks around a Neutron Star or a Black Hole

Numerical simulations were performed in order to examine the time evolution of thermally unstable accretion disks which surround a black hole or a weakly magnetized neutron star. Optically thick, slim transonic accretion disk models are used. The radial-azimuthal component of a viscous stress tensor is assumed to have the form -alpha beta (q) P, where beta is the ratio of the gas pressure to the total pressure, P, with constants alpha and q. Three cases (q = 0, 0.25, and 0.5) are examined. For q = 0, the disks exhibit burst-like oscillations. For q = 0.25, in comparison with the case of q = 0, the amplitude of the oscillations is much smaller and their period much shorter. For q = 0.5, the disks are stable.