  The 1991 ROSAT PSPC M31 X-ray point source catalog has been screened in order
to set up a sample of candidate supersoft sources in this galaxy, additional to
the 16 supersoft sources of Supper et al. (1997). 26 candidates have been
found. Absorbing hydrogen column densities, effective temperatures and white
dwarf masses (assuming the sources are on the stability line of surface nuclear
burning) are derived. An observed white dwarf mass distribution is derived
which indicates that the masses are constrained to values above 0.90 solar
masses. The entire population of supersoft sources in M31 is estimated taking a
theoretical white dwarf mass distribution into account, assuming that the
observationally derived sample is restricted to white dwarf masses above 0.90
solar, and taking into account that the gas and the source population have a
different scale height. A total number of at least 200-500 and at most
6,000-15,000 sources is deduced depending on the used galaxy N_H model, making
use of the population synthesis calculation of Yungelson (1996). The spatial
distribution favors a disk (or spiral-arm) dominated young stellar population
with a ratio of 1/(4-7) of bulge/disk systems. Assuming that all supersoft
sources with masses in excess of 0.5 solar masses are progenitors of supernovae
of type Ia, a SN Ia rate of (0.8-7) 10^-3 yr^-1 is derived for M31 based on
these progenitors. Supersoft sources might account for 20-100% of the total SN
Ia rate in a galaxy like M31.

Astrophysics