A year long superoutburst from an ultracompact white dwarf binary reveals the importance of donor star irradiation

TL;DR

This paper reports the longest superoutburst observed in an ultracompact white dwarf binary, revealing the significant role of donor star irradiation in its extended duration and post-outburst behavior.

Contribution

It presents the first detailed observation of an exceptionally long superoutburst in an ultracompact white dwarf binary and highlights the importance of donor star irradiation in such phenomena.

Findings

Superoutburst lasted for a year, exceeding typical durations by a factor of five.

The superoutburst duration did not follow established empirical or theoretical relations.

Post-outburst, the system remained brighter and showed variable X-ray activity, indicating ongoing accretion changes.

Abstract

SDSS J080710+485259 is the longest period outbursting ultracompact white dwarf binary. Its first ever detected superoutburst started in November of 2018 and lasted for a year, the longest detected so far for any short orbital period accreting white dwarf. Here we show that the superoutburst duration of SDSS J080710+485259 exceeds the 2 months viscous time of its accretion disk by a factor of about 5. Consequently it follows neither the empirical relation nor the theoretical relation between the orbital period and the superoutburst duration for AM CVn systems. Six months after the end of the superoutburst the binary remained 0.4 mag brighter than its quiescent level before the superoutburst. We detect a variable X-ray behavior during the post-outburst cooling phase, demonstrating changes in the mass accretion rate. We discuss how irradiation of the donor star, a scenario poorly explored so far and which ultimately can have important consequences for AM CVns as gravitational wave sources, might explain the unusual observed features of the superoutburst.