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 days. 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 & 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 day e-folding timescale for the decay and the secondary maximum with its rapid rise time of ~1-3 days which is seen in the X-ray nova light curves can be produced by adjusting the strength of the evaporation.