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

In the first observational run of the Kilonova and Transients Programme (KNTraP), a rapidly evolving optical transient, KNTraP2022-02a (TNS name: AT2022kak), was discovered. 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 multiwavelength follow-up campaign, monitoring the object for the next two months, but saw no recurrent burst. Three years later, while obtaining 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 rapid evolution makes it one of the fastest and faintest dwarf novae known. The estimated distance of KNTraP2022-02a from the Galactic centre is ~6.6 kpc, with a scale height of ~2 kpc. This scale height places it in the Galactic thick disc, where only very few dwarf novae have been found, and is therefore a potential Population II dwarf novae system.