ASASSN-19xw is a large amplitude dwarf nova now showing superhumps

ATel #13225; Christopher Lloyd (University of Sussex), Tonny Vanmunster (CBA Belgium Observatory)
on 24 Oct 2019; 10:23 UT
Credential Certification: Christopher Lloyd (c.lloyd@sussex.ac.uk)

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We report further photometry of the large-amplitude optical transient AT 2019rki = ASASSN-19xw which now shows a modulation with a full amplitude of 0.^m28 and a period of 0.^d057.

The transient was first reported by Stanek and Kochanek, 2019 on behalf of the ASAS-SN team (see Shappee et al., 2014) at Sloan-g magnitude 14.9 on 2019-09-29.2 UT (JD=2458755.7). For the following 14 days until 2019-10-13 it declined smoothly in the manner of a classical nova with no significant modulation (see Lloyd and Vanmunster, 2019, ATel #13180).

New unfiltered time series runs were made on three nights with the 0.40-m telescopes of the CBA Belgium and Extremadura (Spain) Observatories on 2019-10-17/18, 18/19 and 20/21. The new time-series data show that the slow decline seen previously has become more erratic. They also show a clear modulation with a full amplitude of 0.^m28 and a period of 0.^d057±0.02, which are interpreted as superhumps.

The outburst amplitude is at least 8 magnitudes (ATel #13180) and a recent spectrum shows that the system is not a nova but a dwarf nova in outburst (Kawash, 2019). With the photometric modulation the object can now be confirmed as a new large-amplitude SU UMa or WZ Sge-type system.

Further observations are encouraged particularly monitoring for possible rebrightenings. As of 2019-10-21 the system has g ~ 17.0.