Photometry of the classical nova ASASSN-19xw = AT 2019rki

ATel #13180; Christopher Lloyd (University of Sussex), Tonny Vanmunster (CBA Belgium Observatory)
on 11 Oct 2019; 11:17 UT
Credential Certification: Christopher Lloyd (c.lloyd@sussex.ac.uk)

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We report seven nights of time-series photometry of the large amplitude optical transient AT 2019rki = ASASSN-19xw which has the light curve of a classical nova with t₂ = 16 days.

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). One day earlier it had been seen at g = 16.4 and a day before that it was below the detection limit of g = 16.8. In the original report it was suggested that the object was caught on the rise but subsequent ASAS-SN data on the days after maximum (see the light curve and Kochanek et al., 2017) show the object is fading so the maximum occurred within a narrow range of one day from 2019-09-28.2 to 29.2 UT (JD = 2458754.7 to ...55.7).

New unfiltered time series runs were made with the 0.40-m telescope of the CBA Extremadura Observatory (Spain) for four hours each on seven consecutive nights from 2019-09-29.8 (JD = 2458756.3) which interleave with the ASAS-SN data. A small offset was made to align the two magnitude systems. The time-series data do not show any periodic variation with an amplitude > 0.02 magnitudes down to 50 cycles/day. All the runs do have slow declines which are broadly consistent with the general trend of 0.16 magnitudes/day from all the data.

There are no precursor candidates in VizieR and the closest faint stars with catalogued magnitudes have g = 21.5 and G = 21.5, but fainter stars are visible on the Pan-STARRS images. There is nothing visible at the position of the transient so it is certainly fainter than g = 22 and probably below g = 23. The outburst amplitude is then at least 8 magnitudes making it a likely WZ Sge-type CV or a classical nova, as has already been suggested by Denisenko, 2019. WZ Sge stars in outburst generally show an orbital modulation in the early stages and then a progression of SU UMa-like superhump periods as the outburst unfolds. The major part of outburst itself usually lasts for some days, perhaps 10 or more (the plateau phase) during which time the magnitude declines only slowly. Given the lack of photometric modulation and the shape of the decline it is very unlikely to be a WZ Sge system so despite the lack of spectroscopic confirmation the transient is most probably a classical nova. The rate of decline implies t₂ = 16 days.