Constraints from the UV delay in dwarf nova outbursts.

Abstract

Observations of outbursts of the dwarf nova system VW Hydri show a delay of the rise of the UV flux with respect to that of the optical flux. The difficulties in modeling this feature in the context of the accretion disk instability are discussed and a modified limit cycle based on the same value of the frictional parameter alpha for the cool disk before transition and the hot disk afterwards is proposed. This is in contrast with the idea that alpha must be lower on the cool than on the hot branch. For the modeling of the disk evolution, a continuing depletion of the disk after outburst is assumed, which results in a delay of the changeover to the hot state during the following outburst.