A remarkable recurrent nova in M 31: The predicted 2014 outburst in X-rays with Swift ^⋆

¹ European Space Astronomy Centre, PO Box 78, 28692 Villanueva de la Cañada, Madrid, Spain
current e-mail: henze@ice.cat
² Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, Liverpool, L3 5RF, UK
³ Department of Astronomy, San Diego State University, San Diego, CA 92182, USA
⁴ Departament de Física i Enginyeria Nuclear, EUETIB, Universitat Politècnica de Catalunya, c/ Comte d’Urgell 187, 08036 Barcelona, Spain
⁵ Institut d’Estudis Espacials de Catalunya, c/Gran Capità 2-4, Ed. Nexus-201, 08034 Barcelona, Spain
⁶ Department of Astronomy, Keio University, Hiyoshi, 223-8521 Yokohama, Japan
⁷ Department of Earth Science and Astronomy, College of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, 153-8902 Tokyo, Japan
⁸ Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, C/Can Magrans s/n, 08193 Cerdanyola del Valles, Spain

Received: 5 March 2015
Accepted: 23 April 2015

Abstract

Context. The M 31 nova M31N 2008-12a was recently found to be a recurrent nova (RN) with a recurrence time of about one year. This is by far the fastest recurrence time scale of any known RN.

Aims. Our optical monitoring programme detected the predicted 2014 outburst of M31N 2008-12a in early October. We immediately initiated an X-ray/UV monitoring campaign with Swift to study the multiwavelength evolution of the outburst.

Methods. We monitored M31N 2008-12a with daily Swift observations for 20 days after discovery, covering the entire supersoft X-ray source (SSS) phase.

Results. We detected SSS emission around day six after outburst. The SSS state lasted for approximately two weeks until about day 19. M31N 2008-12a was a bright X-ray source with a high blackbody temperature.

Conclusions. The X-ray properties of this outburst are very similar to the 2013 eruption. Combined X-ray spectra show a fast rise and decline of the effective blackbody temperature. The short-term X-ray light curve showed strong, aperiodic variability which decreased significantly after about day 14. Overall, the X-ray properties of M31N 2008-12a are consistent with the average population properties of M 31 novae. The optical and X-ray light curves can be scaled uniformly to show similar time scales to those of the Galactic RNe U Sco or RS Oph. The SSS evolution time scales and effective temperatures are consistent with a high-mass WD. We predict the next outburst of M31N 2008-12a to occur in Oct.–Dec. 2015.