M31N 2008-12a - The Remarkable Recurrent Nova in M31: Panchromatic Observations of the 2015 Eruption.
Ness, J. -U.; Boumis, P.; Henze, M.; Hernanz, M.; Sala, G.; Hartmann, D. H.; Quimby, R.; Gehrels, N.; Kawai, N.; Steele, I. A.; Shafter, A. W.; Sion, E. M.; Kennea, J. A.; Kučáková, H.; Osborne, J. P.; Ribeiro, V. A. R. M.; Page, K. L.; Bode, M. F.; Maehara, H.; Arcavi, I.; Rodríguez-Gil, P.; Kaur, A.; Fabrika, S.; Hosseinzadeh, G.; Xu, Z.; O'Brien, T. J.; Watanabe, M.; Williams, B. F.; Darnley, M. J.; Godon, P.; Kato, M.; Hachisu, I.; Hornoch, K.; Williams, S. C.; Hounsell, R.; Shara, M. M.; Valeev, A. F.; Goranskij, V. P.; Barsukova, E. A.; Nakajima, K.; Itagaki, K.; Harman, D. J.; Sano, Y.; Kabashima, F.; Nishiyama, K.; Arai, A.; Chen, T.; Figueira, J.; Gao, X.; Horst, J. Chuck; José, J.; Kiyota, S.; Lau, K. M.; Naito, H.; Watanabe, F.
United Kingdom, Spain, Japan, Czech Republic, United States, Netherlands, Russia, Greece, China
Abstract
The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption 10 times, including yearly eruptions from 2008 to 2014. With a measured recurrence period of {P}{rec}=351+/- 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 August 28.28 ± 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 ∼13,000 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 supersoft 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 Henze et al. (2015a), the updated recurrence period based on all known eruptions is {P}{rec}=174+/- 10 days, and we expect the next eruption of M31N 2008-12a to occur around 2016 mid-September.