The transient jet of the galactic supersoft X - ray source RX J 0925 . 7 - 4758 ?

We report the discovery of a transient jet in the supersoft X-ray source RX J0925.7-4758. The jet was observed in the Hα line during a single night in June 1997 and had disappeared one day later. RX J0925.7-4758 is the third supersoft source to exhibit collimated outflows. The peak jet velocity of 5,200 km s−1, strongly argues in favour of a white dwarf in RX J0925.7-4758. Simple modelling of the jet profile suggests half opening angles in the range of 17◦ to 41◦ although the outflow may be narrower if part of the observed spread in velocity is intrinsic to the jet. X-ray spectral modelling (Ebisawa et al. 1996; Hartmann & Heise 1997) indicates distances of the order of 10 kpc or more with the consequence that RX J0925.7-4758 may be the most optically luminous supersoft source known. The overall observational picture points to a massive white dwarf which may be close to the Chandrasekhar limit.