A remarkable recurrent nova in M31: Discovery and optical/UV observations of the predicted 2014 eruption^⋆,⋆⋆

¹ Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, Liverpool L3 5RF, UK
e-mail: M.J.Darnley@ljmu.ac.uk
² European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ Instituto de Astrofísica de Canarias, Vía Láctea, s/n, La Laguna, 38205 Santa Cruz de Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna, La Laguna, 38206 Santa Cruz de Tenerife, Spain
⁶ Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
⁷ Department of Earth Science and Astronomy, College of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, 153-8902 Tokyo, Japan
⁸ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, Fac. Ciències, 08193 Bellaterra, Spain
⁹ Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic
¹⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹¹ Department of Astronomy, Keio University, Hiyoshi, Yokohama 223-8521, Japan
¹² Variable Stars Observers League in Japan (VSOLJ), 7-1 Kitahatsutomi, 273-0126 Kamagaya, Japan
¹³ Astronomical Institute of the Charles University, Faculty of Mathemathics and Physics, V Holešovičkách 2, 180 00 Praha 8, Czech Republic
¹⁴ Okayama Astrophysical Observatory, NAOJ, NINS, 3037-5 Honjo, Kamogata, Asakuchi, 719-0232 Okayama, Japan
¹⁵ Departament de Física i Enginyeria Nuclear, EUETIB, Universitat Politècnica de Catalunya, c/ Compte d’Urgell 187, 08036 Barcelona, Spain
¹⁶ Institut d’Estudis Espacials de Catalunya, c/ Gran Capità 2-4, Ed. Nexus-201, 08034 Barcelona, Spain
¹⁷ Isaac Newton Group of Telescopes, Apartado de correos 321, 38700 Santa Cruz de La Palma, Spain

Received: 5 March 2015
Accepted: 13 June 2015

Abstract

The Andromeda Galaxy recurrent nova M31N 2008-12a had been caught in eruption eight times. The inter-eruption period of M31N 2008-12a is ~1 yr, making it the most rapidly recurring system known, and a strong single-degenerate Type Ia supernova progenitor candidate. Following the 2013 eruption, a campaign was initiated to detect the predicted 2014 eruption and to then perform high cadence optical photometric and spectroscopic monitoring using ground-based telescopes, along with rapid UV and X-ray follow-up with the Swift satellite. Here we report the results of a high cadence multi-colour optical monitoring campaign, the spectroscopic evolution, and the UV photometry. We also discuss tantalising evidence of a potentially related, vastly-extended, nebulosity. The 2014 eruption was discovered, before optical maximum, on October 2, 2014. We find that the optical properties of M31N 2008-12a evolve faster than all Galactic recurrent novae known, and all its eruptions show remarkable similarity both photometrically and spectroscopically. Optical spectra were obtained as early as 0.26 days post maximum, and again confirm the nova nature of the eruption. A significant deceleration of the inferred ejecta expansion velocity is observed which may be caused by interaction of the ejecta with surrounding material,possibly a red giant wind. We find a low ejected mass and low ejection velocity, which are consistent with high mass-accretion rate, high mass white dwarf, and short recurrence time models of novae. We encourage additional observations, especially around the predicted time of the next eruption, towards the end of 2015.