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
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>10^36 erg/s. Their
combined unabsorbed luminosity is of the order of ~10^39 erg/s. Their
luminosity distribution shows significant steepening around log(L_x)~37.7-38
and becomes zero at L_x~2x10^38 erg/s, 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~1.5x10^6 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 exceed 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 ~10^21
cm^-2, will remove the discrepancy.
