SIMBAD references

We report in this paper the discovery of an X-ray luminous supersoft source in the Galaxy. The source, RX J0925.7-4758 was selected from the ROSAT all sky survey on the basis of its peculiar X-ray hardness ratios. The optical counterpart is a V ≃ 17.15 mag star displaying the high excitation emission lines of HeII λ4686, NIII-CIII λλ4640-60 and Hα. The strength of the interstellar absorption lines and the highly reddened optical continuum indicate E(B-V) = 2.1 or NH ≃ 1.3 10²² H atom cm^–2. Pointed PSPC observations show that the soft X-ray energy distribution is fully consistent with that of a blackbody with T_bb = 40-55 eV undergoing the high amount of interstellar absorption detected in the optical. This large column density indicates that the source is located within or behind the nearby (d = 425 pc) Vela Sheet molecular cloud whereas an assumed bolometric blackbody luminosity of 2 10³⁸ erg/s implies a maximum distance of ≃ 2 kpc. Survey and pointed ROSAT observations show no evidence for large X-ray intensity variations around a mean PSPC intensity of 1 cnt/s. Optical photometric monitoring of the counterpart reveals the presence of a 0.3 mag full amplitude modulation at a possible 3.5 day period. The overall optical and X-ray characteristics of this galactic source are similar to those of the three well studied Magellanic supersoft sources and more particularly to Cal87. From the absence in the ROSAT survey of other candidate brighter than 0.27 cnt/s, we find that the spatial distribution of the class of supersoft emitters with X-ray characteristics similar to those of the new source reported here (i.e. T_bb≥ 40 eV; L_bol ≃ 2x10³⁸ erg/s) may be more concentrated in the galactic plane than that of the low-mass X-ray binaries. We argue that the model proposed by van den Heuvel et al. (1992) consisting of an accreting white dwarf plus a 1.5-2.5 M_☉ main sequence companion could account for the characteristics of the new source and for those of a relatively numerous (but unseen because deeply buried into the galactic plane) population of luminous supersoft sources with T_bb≥ 40 eV. We speculate that this new source could be the progenitor of a type Ia supernova. However, we stress that we do not constrain at all the properties of a population of luminous supersoft sources with significantly lower temperature (T_bb≤ 20-30 eV).