X-ray monitoring of classical novae in the central region of M 31

Abstract

Context. Classical novae (CNe) represent the major class of supersoft X-ray sources (SSSs) in the central region of the galaxy M 31. Aims. We performed a dedicated monitoring of the M 31 central region with XMM-Newton and Chandra between respectively, to find SSS counterparts of CNe, determine the duration of their SSS phase, and derive physical outburst parameters. Methods. We systematically searched our data for X-ray counterparts of CNe, determined their X-ray light curves and characterised their spectra using blackbody fits and white dwarf (WD) atmosphere models. Additionally, we determined luminosity upper limits for all previously known X-ray emitting novae that are not detected any more and for all CNe in our field of view with optical outbursts between one year before the start of the X-ray monitoring (Oct. 2006) and its end (Feb. 2009). Results. We detected 17 X-ray counterparts of CNe in M 31, only four of which were previously known. These latter sources are still active 12.5, 11.0, 7.4 and 4.8 years after the optical outburst. In addition, we detected three known SSSs without a nova counterpart. Four novae displayed short SSS phases (<100 d). Based on these results and previous studies we compiled a catalogue of all novae with SSS counterparts in M 31 known so far. We used this catalogue to derive correlations between the following X-ray and optical nova parameters: turn-on time, turn-off time, blackbody temperature (X-ray), t 2 decay time and expansion velocity of the ejected envelope (optical). Temperatures derived from blackbody fits and WD atmosphere models were found to characterise the effective SSS temperatures almost equally well. Furthermore, we found a first hint for the existence of a difference between SSS parameters of novae associated with the stellar populations of the M 31 bulge and disk. Additionally, we conducted a Monte Carlo Markov chain simulation on the intrinsic fraction of novae with SSS phase. This simulation showed that the high fraction of novae without detected SSS emission might be explained by the inevitably incomplete coverage with X-ray observations in combination with a large part of novae with short SSS states, as expected from the WD mass distribution. Conclusions. Our results confirm that novae are the major class of SSSs in the central region of M 31. The catalogue of novae with X-ray counterpart, mainly based on our X-ray monitoring, contains valuable insight into the physics of the nova process. In order to verify our results with an increased sample, more monitoring observations are needed.