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

Abstract

The deepest X-ray images of M31, obtained with XMM-Newton, are examined in this paper to derive spectral and statistical properties of the population of the softest X-ray sources. Classifying supersoft X-ray sources (SSS) with criteria based on the same hardness ratios defined for recent Chandra observations, a quarter of the selected SSS turn out to be supernova remnants (SNR). Another quarter of SSS are spatially coincident with recent classical novae (but they are less than 10% of the nova population observed in the last 25 years). Only 3 among 15 non-SNR SSS show clear time variability, with X-ray flux variation of more than one order of magnitude within few months. Two of these sources display additional, smaller amplitude variability on time scales of several minutes. Their broad band spectra and those of the novae are approximately fit with a blackbody or white dwarf atmospheric model at near-Eddington luminosity for the distance of M31. Two SSS 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. One of the two objects has some characteristics in common with Ultra Luminous X-ray Sources observed outside the Local Group. Most Quasi-Soft Sources (QSS) are repeatedly detected. I discuss the possibilities that most QSS may be SNR in M31, or foreground neutron stars. Two X-ray sources with both a soft and hard component are in the positions of a recurrent nova and another object that was tentatively classified as a symbiotic nova. These two sources may be black hole transients.