Gaia22ayj: outburst from a deeply eclipsing 9.36-min binary?

TL;DR

Gaia22ayj is a potentially unique ultrashort-period eclipsing binary exhibiting a dwarf nova outburst, possibly representing the shortest known orbital period in such systems, with implications for understanding binary evolution.

Contribution

This paper reports the discovery of Gaia22ayj with a 9.36-minute orbital period, potentially the shortest in eclipsing accreting binaries, and suggests it may be in a rare evolutionary phase.

Findings

Gaia22ayj shows a 9.36-minute period with eclipsing binary features.

The system experienced a dwarf nova-type outburst lasting at least one day.

If confirmed, Gaia22ayj would be the shortest-period eclipsing accreting binary.

Abstract

Gaia22ayj (=ZTF19aagmvuk) was detected as a transient on 2022 March 3 both by the Gaia satellite and the Zwicky Transient Facility (ZTF). I analyzed the past public ZTF data and found that Gaia22ayj showed coherent large-amplitude double-wave variations with a period of 0.00649910257(13) d = 9.36 min. The period can be either from the spin period of the white dwarf or the orbital period of the binary. I consider the latter possibility more likely based on the light curve resembling an eclipsing binary and on the stability of the profile. The presence of an outburst lasting at least ~1 d suggests that this system has an accretion disk. If Gaia22ayj is indeed an eclipsing binary with a period of 9.36 min, this is the shortest orbital period ever measured in eclipsing accreting binaries and is the shortest record of a system with a dwarf nova-type outburst. The presence of an outburst is unusual for an AM CVn system with this orbital period and this object might be in the turn-on phase of the mass-transfer. If the ultrashort orbital period of Gaia22ayj is confirmed, this object would an ideal target to detect period variations within several years to confirm its evolutionary state and to identify the future evolutionary consequence.