An extremely fast fading population II dwarf nova candidate: caught spectroscopically on the rise

TL;DR

This paper reports the discovery and detailed spectroscopic observation of AT2022kak, an extremely fast and faint dwarf nova candidate in the Galactic thick disk, potentially representing a rare Population II dwarf nova system.

Contribution

It provides the first detailed spectroscopic analysis of a rapidly evolving, faint dwarf nova candidate, highlighting its unique properties and potential classification as a Population II system.

Findings

AT2022kak is one of the fastest and faintest dwarf novae observed.

The object is located in the Galactic thick disk at ~6.6 kpc.

Spectroscopic data support its classification as a dwarf nova.

Abstract

We present AT2022kak, a rapidly evolving optical transient discovered by the KiloNova and Transients Program (KNTraP). This interesting burst exhibited extremely fast evolution, with a large amplitude blue outburst of m > 3.3 in a single night, and a rapid fade back to quiescence in the following two nights. We deployed a multi-wavelength follow-up campaign, monitoring the object for the next two months, but saw no recurrent burst. Three years later, while observing to get spectroscopy of the object in quiescence, there was a new outburst, enabling the collection of time-resolved spectra of the rise and fade of the outburst. The light curve properties of the first burst and spectra of the second burst are consistent with a dwarf nova. Its fast evolving behaviour makes it one of the fastest and faintest dwarf novae observed. The estimated distance of AT2022kak from the Galactic centre is ~6.6 kpc, with a scale height of ~2 kpc. This scale height places it in the Galactic thick disk, where only very few dwarf novae have been found, and is therefore a potential Population II dwarf novae system.