Astron. Astrophys. 355, 1041-1048 (2000)

1. Introduction

Supersoft X-ray sources (SSS) are persistent or transient sources whose radiation is almost completely emitted in the energy band below 0.5 keV and whose bolometric luminosity is erg s^-1 (close to the Eddington limit for a 1 star). After the Einstein discovery of CAL 83 and CAL 87 (Long et al. 1981, Crampton et al. 1987, Pakull et al. 1987, 1988) this class of objects was established by many ROSAT discoveries (see Orio 1995, Greiner 1996 and references therein). Several different observational facts suggest that all these very luminous sources contain white dwarfs and that many of these are burning hydrogen in a shell, with a very thin and hot atmosphere on top. The white dwarf might be single (hot PG 1159 stars or planetary nebulae nuclei, e.g. Wang 1991, Motch et al. 1993) or in a binary system. The binary supersoft X-ray sources also belong to different types with orbital periods ranging from 1 year for symbiotic stars down to 4 hours for SMC 13 type sources (Kahabka and Ergma 1997 and references therein). The subgroup with short orbital periods (3 days) now encompasses 9 sources in the Magellanic Clouds and the Milky Way. The widely accepted scenario for these so-called close-binary supersoft sources (CBSS) is based on a companion more massive than the white dwarf, so that mass transfer is unstable and can occur at the high rates which are required to burn the hydrogen steadily on the white dwarf surface (van den Heuvel et al. 1992, Rappaport et al. 1994). While a recent review can be found in van den Heuvel and Kahabka (1997), we remind here that supersoft X-ray sources are of great astrophysical interest also because of their possible nature as progenitors of type Ia supernovae or leading to a neutron star by accretion induced collapse.

The discovery of the supersoft X-ray source RX J0537.7-7034 was first announced by Orio & Ögelman (1993). Later Orio et al. (1997) showed that the source was transient in X-rays and appeared "on" for 1 year. Also, the optical identification with a blue variable star at mag was proposed. The optical spectrum shows the HeII 4686 line in emission and the two Balmer lines H and H in absorption. These spectral features appear redshifted by almost 5 Å, as is expected for a LMC membership. Finally, Orio et al. (1997) also showed that the photometric data for RX J0537.7-7034 might be consistent with a possible orbital period of only 2.5-3 hours.

Here we present new photometric and spectroscopic data of RX J0537.7-7034 that indeed prove it to have a very short period.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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