Structure and evolution of irradiated accretion disks. II. Dynamical evolution of a thermally unstable torus

The nonlinear evolution of the thermal instability in nonirradiated and irradiated accretion disk annuli is studied using a one-dimensional dynamical code. First we calculate nonirradiated models. It is shown that the thermal instability propagates in the vertical direction, keeping δF_z/F_z constant, while δT/T is variable, where δ represents the time variation and F_z is the vertical energy flux. The dynamical calculations confirm that the disk-instability model can well account for the UV delay and the presence of a Balmer jump detected in some dwarf-nova outbursts. Effects of irradiation on the thermal instability of accretion disks are investigated according to the following three cases. In the first case, the irradiation flux is fixed. Moderately strong irradiation with T_irr ∼ 6000 K, where the irradiation flux is defined as F_irr = σT_irr⁴, is shown to reduce the amplitude and the quiescent times and to increase the outburst duration of the resultant light curves. Such light curves are observed in some X-ray binaries, including Cyg X-1. In the second case, in which F_irr varies in proportion to the mass accretion rate at the inner edge of the disk, Ṁ_a, we succeed in reproducing light curves with a plateau in the decay from outbursts by introducing a time delay in the response of F_irr to the modulation in Ṁ_a. Such a light curve is very reminiscent of the light curves of black-hole candidate A0620-00 and of some soft X-ray transients which contain neutron stars. In the third case, in which irradiation is suddenly switched on, it is shown that the surface temperature of the disk rises even faster than the central temperature, leading to a temperature inversion. This would cause wind mass loss and the formation of an accretion corona.