The dwarf nova EX Draconis: a short review

TL;DR

This review of EX Draconis demonstrates that its outburst behavior aligns with mass transfer outburst models and challenges the traditional thermal-viscous disk instability theory, highlighting high-viscosity disk responses.

Contribution

The paper provides critical tests favoring MTO over DI models and suggests higher disk viscosity values than traditionally assumed.

Findings

Four tests contradict DI predictions

Outburst behavior matches high-viscosity MTO models

Disk viscosity estimates are higher than alpha=1

Abstract

EX Draconis (EX Dra) is a long period dwarf nova showing ~2 mag outburst which lasts for ~7 d and recur on a timescale of (20-30) d. Its deep eclipses allows one to trace the changes in surface brightness and radius of its accretion disk along the outburst cycle and to perform critical tests of the predictions of the thermal-viscous disk instability (DI) and the mass transfer outburst (MTO) models proposed to explain dwarf nova outbursts. The results of four critical tests are in clear contradiction with DI while in good agreement with MTO expectations. Furthermore, the observed variations in brightness and outer disk radius throughout EX Dra outbursts are well described by the response of a high-viscosity (alpha = 3-4) accretion disk to events in which the mass transfer rate increases by factors of ~30 for ~7 d, in line with MTO expectations. We further argue that the old expectation of accretion disk theory, alpha <= 1, seems unjustified and contradicts the values derived from dwarf nova outburst decline timescales if they are driven by MTO.