Accretion disk thermal instability in galactic nuclei

The non-linear evolution and spatial propagation of the thermal instability in accretion disks in galactic nuclei are investigated. For relevant black hole masses, M, and mass accretion rates, M^dot^_0_, parts of the disk are thermally unstable. The behavior of the unstable disks depends on the detailed prescription for the viscosity parameter α. For constant α, the thermal instability cannot propagate through a large radial extent, so the disk structure is only modified locally. We call this a "purr" type disk. On the other hand, if we prescribe α is proportional to (h/r)^n^ with n = 1 or 2, where h is the scale height of the disk, more extended regions in the disk suffer the thermal instability. This leads to dramatic outbursts of rapid accretion onto the hole, separated by longer intervals of quiescence. We call this a "roar" type disk. The rise time of a roar-type outburst is short, but the decay occurs more gradually on a viscous time scale. The repetition period is