M31N 2008-12a - The Remarkable Recurrent Nova in M31

Abstract: M31N 2008-12a is a remarkable recurrent nova within the Andromeda Galaxy. With eleven eruptions now identified, including eight in the past eight years, the system exhibits a recurrence period of one year, and possibly just six months. This short inter eruption period is driven by the combination of a high mass white dwarf ($1.38\,\mathrm{M}\odot$) and high mass accretion rate ($\sim1.6\times10^{-7}\,\mathrm{M}\odot\,\mathrm{yr}^{-1}$). Such a high accretion rate appears to be provided by the stellar wind of a red giant companion. Deep H$\alpha$ observations have revealed the presence of a vastly extended nebula around the system, which could be the `super-remnant' of many thousands of past eruptions. With a prediction of the white dwarf reaching the Chandrasekhar mass in less than a mega-year, M31N 2008-12a has become the leading pre-explosion type Ia supernova candidate. The 2016 eruption - to be the twelfth detected eruption - is expected imminently, and a vast array of follow-up observations are already planned.