Spectroscopy of AT 2017gay, another outburst of PT And/M31N 1957-10b

ATel #10647; S. C. Williams (Lancaster), M. J. Darnley (LJMU)
on 17 Aug 2017; 10:43 UT
Credential Certification: Steven Williams (scw@astro.ljmu.ac.uk)

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AT 2017gay was discovered by E. Conseil on 2017 Aug 15.00 (see TNS). Comparing the position to previous M31 nova candidates, we find that it is very close to the position of M31N 2010-12a (as noted by E. Conseil), which was another outburst of PT And/M31N 1957-10b, and thus AT 2017gay is very likely to be another outburst of that system. Outbursts of PT And have been observed in 1957, 1983, 1986, 1988, 1998 and 2010 (see Alksnis & Zharova 2000 and Shafter et al. 2015). Cao et al. 2012 concluded it was most likely to be an M31 recurrent nova, whereas Shafter et al. 2015 concluded it was most likely to be a Galactic dwarf nova. If the object was a recurrent nova in M31, it would be the second fastest recurring nova ever observed (after M31N 2008-12a; see Darnley et al. 2016) and extremely important to study.

We therefore obtained a spectrum of the new outburst of PT And with the SPRAT spectrograph (resolution R ~ 350; Piascik et al. 2014) on the 2-m Liverpool Telescope (Steele et al. 2004) on 2017 Aug 17.05 UT. The spectrum shows clear absorption lines from Hβ, Hγ and Hδ, with no indication of any obvious blueshift to the absorption minima. There is also weak Na I (D) absorption, and we tentatively identify He I 4471 Å absorption. We interpret this spectrum as indicating that the object is probably a Galactic dwarf nova.

Previous outbursts indicate the object will fade by two magnitudes in around 10 to 20 days. During the 2010 outburst, likely superhumps were observed ([vsnet-alert 12484], [vsnet-alert 12497], [vsnet-alert 12527]; see also Kato 2015), consistent with the object being a dwarf nova. We will obtain a further spectrum as the object declines (when a classical/recurrent nova would be expected to show several strong emission lines) and encourage further photometric observations to confirm the superhump period. Reporting of photometric measurements made during the outburst to determine the stage of decline of the object would also be useful to help interpret future spectra (and to decide when to take such spectra).