Title: M31N 2008-12a - The Remarkable Recurrent Nova in M31 Author: M. J. Darnley
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 solar masses) and high mass accretion rate (~1.6 x 10^-7 solar masses 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.
Title: A remarkable recurrent nova in M 31: The 2010 eruption recovered and evidence of a six-month period Author: M. Henze, M. J. Darnley, F. Kabashima, K. Nishiyama, K. Itagaki, X. Gao
The Andromeda Galaxy recurrent nova M31N 2008-12a has been caught in eruption nine times. Six observed eruptions in the seven years from 2008 to 2014 suggested a duty cycle of ~1 year, which makes this the most rapidly recurring system known and the leading single-degenerate Type Ia Supernova progenitor candidate; but no 2010 eruption has been found so far. Here we present evidence supporting the recovery of the 2010 eruption, based on archival images taken at and around the time. We detect the 2010 eruption in a pair of images at 2010 Nov 20.52 UT, with a magnitude of m_R = 17.84 ± 0.19. The sequence of seven eruptions shows significant indications of a duty cycle slightly shorter than one year, which makes successive eruptions occur progressively earlier in the year. We compared three archival X-ray detections with the well observed multi-wavelength light curve of the 2014 eruption to accurately constrain the time of their optical peaks. The results imply that M31N 2008-12a might have in fact a recurrence period of ~6 months (175 ± 11 days), making it even more exceptional. If this is the case, then we predict that soon two eruptions per year will be observable. Furthermore, we predict the next eruption will occur around late Sep 2015. We encourage additional observations.