A Close Look at the Population of Supersoft and Quasi-soft X-Ray Sources Observed in M31 with XMM-Newton

The deepest X-ray images of M31, obtained with XMM-Newton, are examined to derive spectral and statistical properties of the population of the very soft X-ray sources. When classifying supersoft X-ray sources (SSSs) with criteria based on the same hardness ratios defined for recent Chandra observations, one-quarter of the selected SSSs turn out to be supernova remnants (SNRs). Another quarter of the SSSs are spatially coincident with recent classical novae, although they are less than 10% of the nova population observed in the last 25 yr. Only 3 out of the 15 non-SNR SSSs show clear variability, with X-ray flux variation of more than 1 order of magnitude within a few months. Two of these sources display additional, smaller amplitude variability on timescales of several minutes. Their broadband spectra and those of the novae are approximately fitted with a blackbody or white dwarf atmospheric model at near-Eddington luminosity for the distance of M31. Two SSSs appear to reach very large, perhaps super-Eddington luminosities for part of the time and probably eject material in a wind until the luminosity decreases again after a few months. Most quasi-soft sources (QSSs) are repeatedly detected. I discuss the possibility that most QSSs in M31 may be SNRs or foreground neutron stars. Two X-ray sources with both a soft and hard component are in the positions of a recurrent nova and a possible symbiotic nova, but they are probably black hole transients.