Supersoft X-ray sources are believed to be accreting white dwarfs burning material on their surface (van den Heuvel et al. 1992) in a more or less steady fashion. Quasi-stable H burning requires high mass transfer rates ( 10-7 yr-1, Iben 1982). Such high transfer rates which are at least a factor 10 higher than those occurring in cataclysmic variables may be achieved by thermally unstable Roche lobe overflow when the mass donor star is more massive than the accreting object. Steady thermonuclear burning allows the white dwarf to increase its mass since processed material can remain at the surface of the accreting object. A large fraction of SNIa could originate in such systems when the white dwarf mass approaches the Chandrasekhar limit. A recent review of these systems can be found in Kahabka & van den Heuvel (1997).
RX J0925.7-4758 and RX J0019.8+2156 (Beuermann et al. 1995) are the only two luminous and steady supersoft X-ray sources known so far in the Galaxy. RX J0925.7-4758 was discovered in the ROSAT all-sky survey and optically identified with a V17 heavily reddened object (Motch et al. 1994). Because of the large interstellar absorption, only the very high energy part (E 0.5 keV) of the intrinsically soft energy distribution is offered to observation. The resulting X-ray spectrum peaks at 0.8 keV and is unique among supersoft sources (Hartmann & Heise 1997). Optical photometric and spectroscopic observations suggest an orbital period of 3.8 d (Motch 1996).
The existence of bipolar jets with velocities in the range of 1,000-4,000 km s-1 has been recently reported in two supersoft sources, (RX J0513.9-6951: Crampton et al. 1996; Southwell et al. 1996 and RX J0019.8+2156: Tomov et al. 1998; Quaintrell & Fender 1998; Becker et al. 1998). In this paper we report on the discovery of a well formed bipolar outflow with a projected velocity of 5,200 km s-1 in RX J0925.7-4758.
© European Southern Observatory (ESO) 1998
Online publication: September 8, 1998