A remarkable recurrent nova in M 31: The 2010 eruption recovered and evidence of a six-month period

¹ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, C/Can Magrans s/n, 08193 Cerdanyola del Valles, Spain
e-mail: henze@ice.cat
² Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, Liverpool, L3 5RF, UK
e-mail: M.J.Darnley@ljmu.ac.uk
³ Miyaki-Argenteus Observatory, Miyaki, 990-2492 Saga-ken, Japan
⁴ Itagaki Astronomical Observatory, Teppo, 990-2492 Yamagata, Japan
⁵ Xingming Observatory, Mt. Nanshan, Urumqi, 830011 Xinjiang, PR China

Received: 11 August 2015
Accepted: 9 September 2015

Abstract

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 yr, 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 of eruption. 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 actually have a recurrence period of ~6 months (175 ± 11 days), making it even more exceptional. If this is the case, then we predict that two eruptions per year will be observable soon. Furthermore, we predict that the next eruption will occur in late Sep. 2015. We encourage additional observations.