SU Uma stars: Rebrightenings after superoutburst

TL;DR

This paper explains rebrightenings in SU UMa stars after superoutbursts as reflections of transition waves in the accretion disk, linked to increased viscosity and magnetic field dynamics, with implications for disk thermal stability.

Contribution

It introduces a model of rebrightenings as wave reflections caused by increased viscosity and magnetic fields, providing a new understanding of post-superoutburst behavior in SU UMa stars.

Findings

Rebrightenings are caused by wave reflections in the accretion disk.

Increased viscosity is linked to magnetic field formation during superoutburst.

Mini-rebrightenings are explained as a limit cycle from thermal equilibrium wiggles.

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 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.