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A Candidate Recurrent Nova in M31: The Progenitor System?

ATel #5611; S. C. Williams (LJMU), M. J. Darnley (LJMU), M. F. Bode (LJMU), A. W. Shafter (SDSU)
on 29 Nov 2013; 18:32 UT
Credential Certification: Matt Darnley (M.J.Darnley@ljmu.ac.uk)

Subjects: Optical, Nova

Referred to by ATel #: 5627, 6527

The iPTF collaboration recently reported what appears to be the fourth outburst in five years of a very short period recurrent nova in M31 (ATEL #5607). Previous (detected) outbursts are M31N 2008-12a, 2011-10e and 2012-10a (ATEL #3725, #3727, #3728, #4503). Spectra taken following the 2012 outburst confirmed the He/N nova nature of the event (ATEL #4503). Following the most recent outburst, a photometric observation program of this nova was initiated on the 2m robotic Liverpool Telescope (Steele et al. 2004, SPIE, 5489, 679), La Palma. In addition to the photometry, these observations were to be used to accurately determine the outburst position of the nova to aid the search for a progenitor system.

Here we report the first epoch of LT photometry taken through B, V and i' filters using the IO:O CCD camera.

JD=2456625.357: B = 19.51 ± 0.01
JD=2456625.425: B = 19.61 ± 0.01
JD=2456625.430: V = 19.65 ± 0.02
JD=2456625.435: i' = 19.29 ± 0.02

Following the procedure described in Bode et al. (2009), ApJ, 705, 1056, Williams et al. (2013), arXiv:1303.1980 and Williams et al. (in prep) we conducted a search for the progenitor system of this nova using archival Hubble Space Telescope (HST) ACS/WFC data taken on 2010 August 7 (from proposal ID: 12056). This HST epoch was approximately 20 months following the 2008 outburst and 14 months before the 2011 outburst.

There is a resolved object 0.556 ACS/WFC pixels from the 2013 outburst position of the nova in HST data. This represents a separation of 28 milli-arcseconds or 0.9 sigma. The probability of an alignment this close happening by chance, given the local resolved stellar density, is only 2.5%. Hence, we are very confident that this object is related to the nova outburst. The photometry of this object in the ACS/WFC data is, F814W = 23.90 ± 0.02 and F475W = 24.07 ± 0.01. Converting these to the UBVRI system gives, I = 23.90 ± 0.03 and B = 24.12 ± 0.02, (B-I) = 0.22 ± 0.03.

At the distance of M31, the photometry of this object is consistent with particularly blue Galactic quiescent novae harboring red giant secondaries (RG-novae, Darnley et al. 2012, ApJ, 746, 61); such as the suspected recurrent KT Eri and the recurrent V2487 Oph. Despite containing red giant secondaries these Galactic systems are particularly blue due to an expected high mass accretion rate. However, this photometry is also consistent with Galactic quiescent novae with sub-giant secondaries (SG-novae); similar to the recurrent nova U Sco.

Whilst the archival HST data suggest that we may have uncovered the progenitor system of this recurrent nova; a system with an evolved secondary and potentially high accretion rate, caution is urged. Given the apparent short outburst interval of this nova it is possible that we are simply observing part of the late decline following the 2008 outburst.

Continued follow-up observations of this system at all possible wavelengths are strongly encouraged.