2016MNRAS.455..668S

Novae undergo a supersoft X-ray phase of varying duration after the optical outburst. Such transient post-nova supersoft X-ray sources (SSSs) are the majority of the observed SSSs in M31. In this paper, we use the post-nova evolutionary models of Wolf et al. to compute the expected population of post-nova SSSs in M31. We predict that depending on the assumptions about the white dwarf (WD) mass distribution in novae, at any instant there are about 250-600 post-nova SSSs in M31 with (unabsorbed) 0.2-1.0 keV luminosity Lx ≥ 10³⁶ erg/s. Their combined unabsorbed luminosity is of the order of ∼ 10³⁹ erg/s. Their luminosity distribution shows significant steepening around log(Lx) ∼ 37.7-38 and becomes zero at Lx ~ 2x10³⁸ erg/s, the maximum Lx achieved in the post-nova evolutionary tracks. Their effective temperature distribution has a roughly power-law shape with differential slope of ~4-6 up to the maximum temperature of Teff ~ 1.5x10⁶ K. We compare our predictions with the results of the XMM-Newton monitoring of the central field of M31 between 2006 and 2009. The predicted number of post-nova SSSs exceeds the observed number by a factor of ~2-5, depending on the assumed WD mass distribution in novae. This is good agreement, considering the number and magnitude of uncertainties involved in calculations of the post-nova evolutionary models and their X-ray output. Furthermore, only a moderate circumstellar absorption, with hydrogen column density of the order of ∼ 10²¹/cm2, will remove the discrepancy.