Disc instability models for X-ray transients: evidence for evaporation and low alpha-viscosity ?

Abstract

We construct time-dependent models of accretion discs around black holes and neutron stars. We investigate the effect of evaporating the disc inner regions during quiescence on the predictions of the Disc Instability Model (DIM) for these systems. We do not include irradiation of the disc in the models. Removing the inner, most unstable parts of the accretion disc increases the predicted recurrence times. However, DIMs with values of the viscosity parameter alpha_hot ~ 0.1 and alpha_cold ~ 0.02 (typical of applications of the DIM to standard dwarf nova outbursts) fail to reproduce the long recurrence times of SXTs (unless we resort to fine-tuning of the parameters) independent of the evaporation strength. We show that models with evaporation and a smaller value of alpha_cold (~ 0.005) do reproduce the long recurrence times and the accretion rates at the level of the Eddington rate observed in outburst. The large difference between the values of alpha_hot and alpha_cold, if confirmed when disc irradiation is included, suggests that several viscosity mechanisms operate in these accretion discs. For some choices of parameters our models predict reflares during the decline from outburst. They are a physical property of the model and result from a heating front forming in the wake of an initial cooling front and subsequent, multiple front reflections. The reflares disappear in low-alpha models where front reflection can not occur.