The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption ten times , including yearly eruptions from 2008–2014 .
With a measured recurrence period of P _ { \mathrm { rec } } = 351 \pm 13 days ( we believe the true value to be half of this ) and a white dwarf very close to the Chandrasekhar limit , M31N 2008-12a has become the leading pre-explosion supernova type Ia progenitor candidate .
Following multi-wavelength follow-up observations of the 2013 and 2014 eruptions , we initiated a campaign to ensure early detection of the predicted 2015 eruption , which triggered ambitious ground and space-based follow-up programs .
In this paper we present the 2015 detection ; visible to near-infrared photometry and visible spectroscopy ; and ultraviolet and X-ray observations from the Swift observatory .
The LCOGT 2 m ( Hawaii ) discovered the 2015 eruption , estimated to have commenced at Aug . 28.28 \pm 0.12 UT .
The 2013–2015 eruptions are remarkably similar at all wavelengths .
New early spectroscopic observations reveal short-lived emission from material with velocities \sim 13000 km s ^ { -1 } , possibly collimated outflows .
Photometric and spectroscopic observations of the eruption provide strong evidence supporting a red giant donor .
An apparently stochastic variability during the early super-soft X-ray phase was comparable in amplitude and duration to past eruptions , but the 2013 and 2015 eruptions show evidence of a brief flux dip during this phase .
The multi-eruption Swift /XRT spectra show tentative evidence of high-ionization emission lines above a high-temperature continuum .
Following \citet 2015A Align A…582L…8H , the updated recurrence period based on all known eruptions is P _ { \mathrm { rec } } = 174 \pm 10 d , and we expect the next eruption of M31N 2008-12a to occur around mid-Sep. 2016 .