Population of post-nova supersoft X-ray sources

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 L_x ≥ 10³⁶ erg s^-1. Their combined unabsorbed luminosity is of the order of ∼10³⁹ erg s^-1. Their luminosity distribution shows significant steepening around log (L_x) ∼ 37.7-38 and becomes zero at L_x ≈ 2 × 10³⁸ erg s^-1, the maximum L_x 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 T_eff ≈ 1.5 × 10⁶ 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 cm^-2, will remove the discrepancy.