THE EVOLUTION OF ACCRETION DISKS WITH CORONAE - A MODEL FOR THE LOW-FREQUENCY QUASI-PERIODIC OSCILLATIONS IN X-RAY BINARIES

The global nonlinear time-dependent evolution of accretion disk-corona systems in X-ray binary sources has been investigated to provide an u nderstanding of the low-frequency (similar to 0.04 Hz) quasi-periodic oscillations (QPOs) observed recently in the rapid burster MXB 1730-33 5 and in some black hole candidate sources (Cyg X-1 and GRO J0422+32). We consider oc-viscosity models in which the viscous stress is propor tional to the total pressure. In contrast to previous time-dependent s tudies, it is assumed that all mass accretion and angular momentum tra nsport take place in an optically thick disk, but that a fraction of t he gravitational energy that is released is dissipated in a corona. It is found that the coronal energy dissipation can effectively reduce t he amplitudes of the mass flow variations generated from the thermal a nd viscous instabilities (in comparison with models without a corona). Provided that the disk is close to a marginally stable state, mild os cillatory nonsteady behavior results. These oscillations are globally coherent in the unstable regions of the disk. A model for the high and low states of black hole candidate systems is also proposed. It is su ggested that the low state, which is characterized by a hard X-ray spe ctrum, corresponds to a disk configuration in which the inner disk is in an advection-dominated, hot, optically thin state, whereas the high state corresponds to a configuration in which the inner disk is in an optically thick state surrounded by a corona. In this model, the mass accretion rate in the system is higher in the low state than in the h igh state. The hard X-ray spectrum of QPOs observed in the low state c an be naturally explained by such a model.