2012A&A...544A..44H

Classical novae (CNe) represent a major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M31. Significantly different SSS properties of CNe in the M31 bulge and disk were indicated by recent X-ray population studies, which however considered only a small number of disk novae. We initiated a target of opportunity (ToO) program with XMM-Newton to observe the SSS phases of CNe in the disk of M31 and improve the database for further population studies. We analysed two XMM-Newton ToO observations triggered in Aug. 2011 and Jan. 2012, respectively, and extracted X-ray spectra and light curves. 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±6eV. 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. Dipping SSS light curves are rarely observed in M31 novae. As well as providing an unparalleled statistical sample, the M31 population of novae with SSS counterparts produces frequent discoveries of unusual objects, thereby underlining the importance of regular monitoring.