M31N 2008-05d: a M 31 disk nova with a dipping supersoft X-ray light curve^⋆

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr. 85741 Garching, Germany
e-mail: mhenze@mpe.mpg.de
² Institut de Ciències de l’Espai (CSIC-IEEC), Campus UAB, Fac. Ciències, 08193 Bellaterra, Spain
³ Departament de Física i Enginyeria Nuclear, EUETIB, Universitat Politècnica de Catalunya, c/ Comte d’Urgell 187, 08036 Barcelona, Spain
⁴ Institut d’Estudis Espacials de Catalunya, c/Gran Capità 2-4, Ed. Nexus-201, 08034, Barcelona, Spain
⁵ INAF – Napoli, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁶ International Centre for Relativistic Astrophysics, Piazzale della Repubblica 2, 65122 Pescara, Italy
⁷ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy

Received: 26 April 2012
Accepted: 24 June 2012

Abstract

Context. Classical novae (CNe) represent a major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. Significantly different SSS properties of CNe in the M 31 bulge and disk were indicated by recent X-ray population studies, which however considered only a small number of disk novae.

Aims. We initiated a target of opportunity (ToO) program with XMM-Newton to observe the SSS phases of CNe in the disk of M 31 and improve the database for further population studies.

Methods. We analysed two XMM-Newton ToO observations triggered in Aug. 2011 and Jan. 2012, respectively, and extracted X-ray spectra and light curves.

Results. We report the discovery of an X-ray counterpart to the M 31 disk nova M31N 2008-05d. The X-ray spectrum of the object allows us to classify it as a SSS parametrised by a blackbody temperature of 32 ± 6 eV. More than three years after the nova outburst, the X-ray light curve of the SSS exhibits irregular, broad dip features. These dips affect primarily the very soft part of the X-ray spectrum, which might indicate absorption effects.

Conclusions. Dipping SSS light curves are rarely observed in M 31 novae. As well as providing an unparalleled statistical sample, the M 31 population of novae with SSS counterparts produces frequent discoveries of unusual objects, thereby underlining the importance of regular monitoring.