2011A&A...531A..22P

In the central part of M 31, a high number of optical novae can be targeted within the field of view of the XMM-Newton EPIC and Chandra HRC-I X-ray detectors. A special monitoring program of the area has allowed us to investigate supersoft emission of individual novae in detail and perform a statistical analysis of the sample. For the He/N nova M31N 2007-12b, we aimed to constrain the time of appearance of a supersoft source (SSS) and the duration of the SSS state as well as determine the spectral and time variability while the source was bright. We analyzed XMM-Newton EPIC and Chandra HRC-I observations of our monitoring program performed at intervals of ten days and added results of a XMM-Newton target of opportunity observation and Swift XRT observations. We performed source detection, determined long-term time and spectral variation of M31N 2007-12b, and searched for shorter-term time variability in the individual observations when the source was bright, using fast Fourier and folding techniques to analyze periodicities. The SSS emission started between 21 and 30d after the optical outburst and ended between 60 and 120d after outburst, making M31N 2007-12b one of the few novae with the shortest SSS phase known. The X-ray spectrum was supersoft and can be fitted with a white dwarf (WD) atmosphere model with solar abundances absorbed by the Galactic foreground. The temperature of the WD atmosphere seems to increase at the beginning of the SSS phase from ∼70 to ∼80eV. The luminosity of M31N 2007-12b during maximum was at the Eddington limit of a massive WD and dropped by ∼30% in the observation 60d after outburst. The radius of the emission region is ∼6x10⁸cm. In the four bright state observations, we detected a stable 1110s pulsation, which we interpret as the WD rotation period. In addition, we detect dips in three observations that might represent a 4.9h or 9.8h binary period of the system. Nova envelope models with ≲50% mixing between solar-like accreted material and the degenerate core of the WD can be used to describe the data. We derive a WD mass of 1.2M_☉, as well as an ejected and burned mass of 2.0x10^–6M_☉ and 0.2x10^–6M_☉, respectively. The observed periodicities indicate that nova M31N 2007-12b erupted in an intermediate polar (IP) system. The WD photospheric radius seems to be larger than expected for a non-magnetic WD but in the range for magnetic WDs in an IP system.