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

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.

Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and NASA.