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  • A remarkable recurrent nova in M31

    Liverpool Telescope IO:O image showing the 2013 outburst of M31N 2008-12a taken on 28th Nov 2013. The white circle shows the position of the RN. The white boxes indicate the position of one of the coincident HST fields. Inset: HST image of the region surrounding M31N 2008-12a. The green ellipses indicate the search region for the progenitor, and the red cross indicates the position of the progenitor candidate.

    Towards the end of November 2013, another outburst of the M31 recurrent nova (RN) M31N 2008-12a was announced by the intermediate Palomar Transient Factory (iPTF). This followed recorded outbursts in late 2008, 2009, 2011 and 2012; it is likely that a further outburst at the end of 2010 was missed. Such a short recurrence timescale (~1 year) is unprecedented; previously the most frequent RN was the Galactic system U Scorpii with a ~10 year inter-outburst time.

    This particular nova is classed as "very-fast", taking just four days to decline by two magnitudes. As such, knowledge of its lightcurve is limited even after five outbursts. A spectrum taken following the 2012 outburst indicated that the system was a nova of the He/N type (as is common for recurrent novae) and that the line-of-sight velocity of the ejecta was ~2,250 km/s.

    Following the 2013 outburst, Dr Matt Darnley (Astrophysics Research Institute, Liverpool John Moores University) and Dr Martin Henze (European Space Astronomy Centre, Spain) formed a collaboration to investigate ground-based optical and Swift X-ray and UV observations of the outburst, and Hubble Space Telescope (HST) IR, optical and UV observations of the progenitor system. This collaboration also included LJMU’s Steven Williams and Prof. Mike Bode.

    Liverpool Telescope observations of the 2013 outburst were used to precisely determine the position of the nova to enable the search for any resolvable progenitor. The HST archival observations (taken well between all detected outbursts) indicated the presence of a system coincident with the outburst position. This progenitor was consistent with a system containing an evolved secondary star (sub-giant or red giant) and a luminous accretion disk. Such a bright disk, which indicates a high accretion rate, is a requirement of a system with such a short recurrence timescale.

    An X-ray/UV monitoring campaign undertaken on the orbiting NASA Swift observatory soon after the 2013 outburst clearly detected a bright supersoft X-ray source (SSS) only six days after the optical discovery. The SSS phase emission arises from directly viewing the nuclear burning on the surface of the white dwarf; the rapid unveiling of this source indicates a particularly low ejected mass from the system for the ejection velocities observed. The SSS persisted for two weeks before “turning-off” indicating the end of the nuclear burning phase. From this we infer the presence of a very high mass white dwarf. Additional, archival, X-ray observations indicate that this system also had detected outbursts in 1992, 1993 and 2001.

    Such a system, containing an evolved secondary star, a high mass white dwarf (close to the Chandrasekhar limit), and a luminous accretion disk, coupled with such a short recurrence time is a prime candidate for a Type Ia supernova progenitor. With the expected one-year outburst timescale, we shall be monitoring this system eagerly towards the end of 2014.

    A pair of Letters describing the optical work and the X-ray observations has been submitted to Astronomy & Astrophysics.