Discovery of a luminous galactic supersoft X-ray source in the ROSAT all-sky survey

Details Discovery of a luminous galactic supersoft X-ray source in the ROSAT all-sky survey Discovery of a luminous galactic supersoft X-ray source in the ROSAT all-sky survey

Apr 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...284..827m&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 284, no. 3, p. 827-838

Astronomy and Astrophysics

Astronomy

44

Milky Way Galaxy, Spatial Distribution, Stellar Spectra, X Ray Astronomy, X Ray Stars, Astronomical Photometry, Charge Coupled Devices, Position Sensing, Proportional Counters, Rosat Mission, Sky Surveys (Astronomy), Spectrometers, Telescopes

Scientific paper

We report in this paper the discovery of an X-ray luminous supersoft source in the Galaxy. The source, RXJ0925.7-4758 was selected from the ROSAT all sky survey on the basis of its peculiar X-ray hardness ratios. The optical counterpart is a V approximately equals 17.15 mag star displaying the high excitation emission lines of He II lambda 4686, N III-C III lambda lambda 4640-60 and H alpha. The strength of the interstellar absorption lines and the highly reddened optical continuum indicate E(B-V) = 2.1 or N H approximately equals 1.3 x 1022 H atom cm-2. Pointed Position Sensitive Proportional Counter (PSPC) observations show that the soft X-ray energy distribution is fully consistent with that of a blackbody with Tbb = 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 x 1038 erg s-1 implies a maximum distance of approximately equals 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-1. 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-1, 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. Tbb greater than or equal to 40 eV; Lbol approximately equals 2 x 1038 erg s-1) 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 solar mass 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 Tbb greater than or equal to 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 (Tbb less than or equal to 20-30 eV).

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