Two‐dimensional Radiation‐Hydrodynamic Model for Limit‐Cycle Oscillations of Luminous Accretion Disks

We investigate the time evolution of luminous accretion disks around blackholes, conducting the two-dimensional radiation-hydrodynamic simulations. Weadopt the alpha prescription for the viscosity. The radial-azimuthal componentof viscous stress tensor is assumed to be proportional to the total pressure inthe optically thick region, while the gas pressure in the optically thinregime. The viscosity parameter, alpha, is taken to be 0.1. We find thelimit-cycle variation in luminosity between high and low states. When we setthe mass input rate from the outer disk boundary to be 100 L_E/c^2, theluminosity suddenly rises from 0.3L_E to 2L_E, where L_E is the Eddingtonluminosity. It decays after retaining high value for about 40 s. Our numericalresults can explain the variation amplitude and duration of the recurrentoutbursts observed in microquasar, GRS 1915+105. We show that themulti-dimensional effects play an important role in the high-luminosity state.In this state, the outflow is driven by the strong radiation force, and somepart of radiation energy dissipated inside the disk is swallowed by the blackhole due to the photon-trapping effects. This trapped luminosity is comparableto the disk luminosity. We also calculate two more cases: one with a muchlarger accretion rate than the critical value for the instability and the otherwith the viscous stress tensor being proportional to the gas pressure only evenwhen the radiation pressure is dominant. We find no quasi-periodic lightvariations in these cases. This confirms that the limit-cycle behavior found inthe simulations is caused by the disk instability.Comment: 6 pages, 4 figures, accepted for publication in ApJ (ApJ 01 April 2006, v640, 2 issue