WZ Sge-type dwarf novae: an extreme laboratory in cataclysmic variables

TL;DR

WZ Sge-type dwarf novae are a highly evolved class of cataclysmic variables exhibiting energetic outbursts, with recent studies revealing diverse behaviors, possible magnetic white dwarfs, and their connections to other compact binary systems.

Contribution

This paper reviews recent observational and theoretical advances in understanding WZ Sge stars, highlighting their outburst mechanisms, diversity, and evolutionary links.

Findings

Absence of hotspot emission during early outburst rise questions enhanced mass transfer.

Some WZ Sge stars may contain magnetic white dwarfs causing inner disk truncation.

Diverse behaviors observed, including superoutbursts with/without early superhumps and spectral effects from disk winds.

Abstract

WZ Sge-type dwarf novae form one of the most intriguing classes of compact accreting binaries. They are recognized as the most evolved population of hydrogen-rich cataclysmic variables. Yet they exhibit energetic disk-powered outbursts with an amplitude of 6-9 mag, duration of a month, and decade-long outburst cycles. Despite the dramatic increase in the number of WZ Sge stars over the last decades, there are many unresolved questions, both in terms of their binary evolution and outburst mechanism. In this proceeding paper, I review the recent studies on WZ Sge stars, focusing on their outbursts. Recent observations, both photometrically and spectroscopically, have found the absence of enhanced emission from the hotspot during the early outburst rise in WZ Sge stars, casting doubt on the occurrence of the enhanced mass transfer. Meanwhile, several WZ Sge stars are newly suggested to harbor a magnetic white dwarf, inferring an inner disk truncation in quiescence. Diversity among systems has been discovered: a WZ Sge star with superoutbursts both accompanying and lacking an early superhump phase, one possibly harboring an ONe and massive white dwarf, and one showing optical spectra strongly affected by disk winds. I introduce the connections of WZ Sge stars to period bouncers, helium-rich AM CVn stars, and low-mass X-ray binaries. Finally, prospects of WZ Sge stars in the upcoming time-domain surveys, such as the Rubin Observatory LSST, are presented.