A remarkable recurrent nova in M 31: The optical observations

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 Astronomy, San Diego State University, San Diego CA 92182, USA
⁴ Astronomical Institute, Academy of Sciences, 251 65 Ondřejov, Czech Republic

Received: 13 January 2014
Accepted: 10 February 2014

Abstract

Context. In late Nov. 2013 the fifth eruption in five years of the M 31 recurrent nova M 31N 2008-12a was announced.

Aims. In this Letter we address the optical lightcurve and progenitor system of M 31N 2008-12a.

Methods. Optical imaging data of the 2013 eruption from the Liverpool Telescope, La Palma; Danish 1.54 m Telescope, La Silla; and archival Hubble Space Telescope near-IR, optical, and near-UV data are astrometrically and photometrically analysed.

Results. Photometry of the 2013 eruption, combined with three previous eruptions, enabled construction of a template lightcurve of a very fast nova (${\mathit{t}}_{\mathrm{2}}{}^{\mathrm{\right(}}\mathit{V}^{\mathrm{\left)}}\mathrm{\simeq }\mathrm{4}$ days). The archival data allowed recovery of the progenitor system in optical and near-UV data, indicating a red-giant secondary with bright accretion disk, or alternatively a system with a sub-giant secondary but dominated by a disk.

Conclusions. The eruptions of M 31N 2008-12a, and a number of historic X-ray detections, indicate a unique system with a recurrence timescale of ~1 yr. This implies the presence of a very high-mass white dwarf and a high accretion rate. The recovered progenitor system is consistent with such an elevated rate of accretion. We encourage additional observations, especially towards the end of 2014.