M31N 2008-12a - The REMARKABLE RECURRENT NOVA in M31: PANCHROMATIC OBSERVATIONS of the 2015 ERUPTION
journal contributionposted on 20.01.2017, 10:37 by M. J. Darnley, M. Henze, M. F. Bode, I. Hachisu, M. Hernanz, K. Hornoch, R. Hounsell, M. Kato, J. U. Ness, J. P. Osborne, K. L. Page, V. A. R. M. Ribeiro, P. Rodríguez-Gil, A. W. Shafter, M. M. Shara, I. A. Steele, S. C. Williams, A. Arai, I. Arcavi, E. A. Barsukova, P. Boumis, T. Chen, S. Fabrika, J. Figueira, X. Gao, N. Gehrels, P. Godon, V. P. Goranskij, D. J. Harman, D. H. Hartmann, G. Hosseinzadeh, J. C. Horst, K. Itagaki, J. José, F. Kabashima, A. Kaur, N. Kawai, J. A. Kennea, S. Kiyota, H. Kučáková, K. M. Lau, H. Maehara, H. Naito, K. Nakajima, K. Nishiyama, T. J. O'Brien, R. Quimby, G. Sala, Y. Sano, E. M. Sion, A. F. Valeev, F. Watanabe, M. Watanabe, B. F. Williams, Z. Xu
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 Prec = 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 Prec = 174 ± 10 days, and we expect the next eruption of M31N 2008-12a to occur around 2016 mid-September.