SU UMa stars: Rebrightenings after superoutburst

Abstract

Abstract SU UMa stars after their long superoutbursts often show single or multiple rebrightenings. We show how this phenomenon can be understood as repeated reflections of transition waves which mediate changes between the hot and the cool state of the accretion disk and travel back and forth in the outer disk region, leaving an inner part permanently hot. This points to a temporarily increased viscosity, possibly related to the formation of large-scale and longer persisting magnetic fields by the dynamo operation during the long superoutburst. The “mini-rebrightenings” in the early post-outburst light curve of V585 Lyr discovered by Kato and Osaki (2013, PASJ, 65, 97) in Kepler observations seem to be understandable as a small limit cycle of low-luminosity changes originating from a “wiggle” feature in the thermal equilibrium curve of the cool, optically thick disk.