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Astron. Astrophys. 344, 459-471 (1999)

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1. Introduction

Supersoft sources constitute an interesting new class of X-ray binary sources (cf. van den Heuvel et al. 1992). This is due to the fact that their spectra are extremely soft (effective temperatures of a few [FORMULA]) and their luminosities are substantial ([FORMULA]). They thus can not only be studied in the Milky Way and the near-by Magellanic Clouds (LMC and SMC) but also in more distant galaxies (the Andromeda galaxy M31 and the spiral galaxy NGC 55). For a review see Hasinger (1994), Kahabka & Trümper (1996) and Kahabka & van den Heuvel (1997), see also Greiner (1996). They are considered to be at least one class of progenitors of type Ia supernovae (cf. Branch et al.1995, Livio 1996, Li & van den Heuvel 1997, Yungelson & Livio 1998, and Branch 1998).

In this article the observed sample of supersoft sources in the Andromeda galaxy (M31) is studied. A distance of 700 kpc is adopted (slightly different distances of 690 and 725 kpc are used in other literature). The inclination of the galaxy is 77.5o. The inner "apparent bulge" has a radius of 3 kpc and the bulge is truncated at a radius of 6.4 kpc. The disk extends to a radius of [FORMULA]20 kpc (Hatano et al. 1997). First a number of candidate supersoft sources in M31 is derived from the 1991 X-ray point source catalog (retrieved from CDS via anonymous ftp making use of the hardness ratios ("X-ray colors") HR1, HR2 and the count rate information.

For the 26 candidates a hydrogen column ([FORMULA]) distribution and a white dwarf mass distribution is derived assuming non-LTE white dwarf atmosphere spectra. The observationally derived white dwarf mass distribution is compared with the mass distribution predicted from population synthesis calculations and the number of the population is corrected accordingly. The observationally derived [FORMULA] distribution is compared with a galaxy scale height [FORMULA] distribution and the population is corrected accordingly. This allows to constrain the whole active population distributed over the whole galaxy. Assuming objects with masses [FORMULA] (and [FORMULA] respectively) contribute as progenitors of type Ia SNe and explode after an evolutionary time scale of [FORMULA] a SN Ia rate is derived for M31 from this population. This rate is compared with the total M31 SN Ia rate.

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999