Eclipse mapping the flickering sources in the dwarf nova HT Cassiopeia

Abstract

We report results of the eclipse mapping analysis of an ensemble of light curves of HT Cas. The fast response of the white dwarf to the increase in mass transfer rate, the expansion rate of the accretion disc at the same time, and the relative amplitude of the high-frequency flickering indicate that the quiescent disc of HT Has has high viscosity, alpha ~ 0.3-0.7. This is in marked disagreement with the disc-instability model and implies that the outbursts of HT Cas are caused by bursts of enhanced mass-transfer rate from its donor star.