On the Role of Irradiation and Evaporation in Strongly Irradiated Accretion Disks in the Black Hole X-ray Binaries: Toward an Understanding of FREDs and Secondary Maxima

We examine a new paradigm to account for the exponential decay seen in the light curves of some of the bright X-ray novae. These systems show an exponential decay in soft X-rays with an e-folding time constant of ~30 d. We investigate a scenario in which evaporation of matter into a corona is the dominant mass removal mechanism from the accretion disk. We utilize the thermal evaporative instability discovered by Shaviv and Wehrse. First we parametrize local mass loss rates from the disk (fitted to vertical structure computations of the optically thin structure using the photoionization code CLOUDY), and then we utilize the scalings in our numerical time dependent model for the decay. Both the ~30 d e-folding time scale for the decay and the secondary maximum with its rapid rise time ~1-3 d which is seen in the X-ray nova light curves can be produced by adjusting the strength of the evaporation.