Astronomy & Astrophysics
Context. Classical novae (CNe) represent the major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. Aims. We performed a dedicated monitoring of the M 31 central region, which aimed to detect SSS counterparts of CNe, with XMM-Newton and Chandra between Nov. and Mar. of the years 2009/10, 2010/11, and 2011/12. Methods. We systematically searched our data for X-ray counterparts of CNe and determined their X-ray light curves and also their spectral properties in the case of XMM-Newton data. Additionally, we determined luminosity upper limits for all previously known X-ray emitting novae, which are not detected anymore, and for all CNe in our ﬁeld of view with recent optical outbursts. Results. In total, we detected 24 novae in X-rays. Seven of these sources were known from previous observations, including the M 31 nova with the longest SSS phase, M31N 1996-08b, which was found to fade below our X-ray detection limit 13.8 yr after outburst. Of the new discoveries, several novae exhibit signiﬁcant variability in their short-term X-ray light curves with one object showing a suspected period of about 1.3 h. We studied the SSS state of the most recent outburst of a recurrent nova, which had previously shown the shortest time ever observed between two outbursts (∼5 yr). The total number of M 31 novae with X-ray counterpart was increased to 79, and we subjected this extended catalogue to detailed statistical studies. Four previously indicated correlations between optical and X-ray parameters could be conﬁrmed and improved. Furthermore, we found indications that the multi-dimensional parameter space of nova properties might be dominated by a single physical parameter, and we provide interpretations and suggest implications. We studied various outliers from the established correlations and discuss evidence of a diﬀerent X-ray behaviour of novae in the M 31 bulge and disk. Conclusions. Exploration of the multi-wavelength parameter space of optical and X-ray measurements is shown to be a powerful tool for examining properties of extragalactic nova populations. While there are hints that the diﬀerent stellar populations of M 31 (bulge vs. disk) produce dissimilar nova outbursts, there is also growing evidence that the overall behaviour of an average nova might be understood in surprisingly simple terms.
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