The Fast Evolving, Tremendous and Blue Superoutburst in ASASSN-21au Reveals a Dichotomy in the Outbursts of Long-period AM CVns

TL;DR

This study of ASASSN-21au reveals a distinct dichotomy in outburst behaviors among long-period AM CVn systems, highlighting different underlying mechanisms such as disk instabilities versus enhanced mass transfer.

Contribution

It provides the first multiwavelength observation of a superoutburst in ASASSN-21au, demonstrating a dichotomy in outburst mechanisms among similar systems.

Findings

Identification of a 19-day superoutburst with a 7.5 mag amplitude.

Correlation between X-ray, UV, and optical emissions during outburst.

Evidence supporting disk instability as the cause of the superoutburst.

Abstract

ASASSN-21au is an ultracompact accreting white dwarf binary (AM CVn) with a period of approx. 58 min. Using multiwavelength observations of the system, we discovered a dichotomy in the behavior of outbursts in AM CVns. The binary showed an initial brightness increase which lasted for at least 82 days, followed by an additional increase which lasted 2 weeks. Afterwards ASASSN-21au went into superoutburst with a total duration of 19 days, showing an amplitude with respect to quiescence of approx. 7.5 mags in g, with a precursor and an echo outburst. A correlation between X-rays, UV and optical was identified for the first time in an AM CVn during this stage. The color evolution of ASASSN-21au indicates that during the superoutburst the dominant component was the accretion disk. The short duration, large amplitude and color evolution of the superoutburst agree with expectations from the disk instability model. These characteristics are opposite to the ones observed in SDSS J080710+485259 and SDSS J113732+405458, which have periods of approx. 53 min and 60 min, respectively. The initially slow brightness increase in the light curve of ASASSN-21au and the behavior after the superoutburst favors a scenario in which changes in the mass-transfer rate led to disk instabilities, while the outburst mechanism of SDSS J080710+485259 and SDSS J113732+405458 has been attributed to enhanced mass-transfer alone. Further observations are needed to understand the origin of this dichotomy.