Astron. Astrophys. 338, L13-L16 (1998)

4. Discussion and conclusions

RX J0925.7-4758 is the third supersoft source in which collimated outflows are observed demonstrating that jets are common phenomena in this class of high mass transfer rate accreting binaries.

However, compared to other supersoft sources, the jet of RX J0925.7-4758 appears to be a rather rare and rapidly variable phenomenon. The jet was detected during only one among 23 nights of observation performed since the identification of the source in 1992, and it disappeared in less than 24 h. For comparison, the jet of RX J0513.9-6951 is almost constantly seen at about the same velocity. Crampton et al. (1996) report non-detection during only one or two nights. On the other hand, the jet of RX J0019.8+2156 is transient on time scales of months (Becker et al. 1998).

The observation of a jet with a projected velocity of 5,200 km s^-1 confirms that RX J0925.7-4758 belongs to the class of supersoft sources in spite of its unusual X-ray spectrum. In particular, if the jet velocity is of the order of the escape velocity from the central object (see e.g., Livio 1997) then the M/R ratio of the source is similar to that of a white dwarf. Wind velocities of the order of 6,000 km s^-1 are indeed observed in some cataclysmic variables (Drew 1997)

Fits of NLTE model atmospheres to ASCA data (Ebisawa et al. 1996) and ROSAT PSPC data (Hartmann & Heise 1997) indicate high effective temperatures close to 70 eV and amazingly small source radii in the range of 160-370 (d/1 kpc) km. The reduced emitting area has been sometimes considered as evidence that the source was in fact a neutron star with an extended atmosphere (Hartmann & Heise 1997, Kylafis & Xilouris 1993). If the jet originates from the close surrounding of the X-ray emitting surface then the source radius is of the order of 10⁹ cm for solar masses objects, independently of the actual nature of the central engine, white-dwarf or shrouded neutron star. From this point of view, RX J0925.7-4758 does not look different from other supersoft sources and its distance should be at least 10 kpc in order for the X-ray emitting region to reach a radius similar to that of the jet producing region. At 10 kpc, the bolometric luminosity is 5 10³⁷ erg s^-1 and the radius of the source is at most 3,700 km suggesting a very massive white dwarf. However, as the bulk of the energy distribution of RX J0925.7-4758 is masked by photoelectric absorption, it is still possible that spectral fits give somewhat biased source parameters.

Optical data do not rule out such large distances. The interstellar absorption towards the source ( 1.3 10²² cm^-2, Motch et al. 1994) is similar to the integrated galactic value while a large part of the reddening probably takes place rather locally in the Vela sheet molecular cloud located at 425 pc. At 10 kpc, the intrinsic V magnitude of RX J0925.7-4758 is = -4, two magnitudes brighter than the brightest of the Magellanic supersoft sources, RX J0513.9-6951. This high optical luminosity could reflect the long orbital period and large accretion disc of RX J0925.7-4758. In supersoft sources, the white dwarf luminosity due to nuclear burning is much larger than the total accretion luminosity of the disc and X-ray reprocessing in the disc and on the secondary atmosphere should play an important role (see e.g. Popham & DiStefano 1996). If as for low-mass X-ray binaries scales as (van Paradijs and McClintock 1994), then the larger orbital period and accretion disc in RX J0925.7-4758 may already explain a 1.2 magnitude difference. Different disc rim structures (Meyer-Hofmeister et al. 1997) and uncertainties on could account for the rest of the difference in absolute magnitude between RX J0925.7-4758 and RX J0513.9-6951. In addition, a large visual flux emission from the X-ray heated structures of the binary would explain the absence of detectable late type features in the optical spectrum. In a 3.8 d orbit, the Roche lobe filling evolved star is expected to have (Motch 1996).

For stable shell burning, the nuclear luminosity mainly depends on the mass of the burning envelope and is thus insensitive to short time scale changes in mass accretion rate (Fujimoto 1982). Only for the most massive white dwarfs which undergo high accretion rates and retain light envelopes can the nuclear luminosity vary significantly on a time scale of a week. If the appearance of the jet is due to a sudden increase of the mass accretion rate onto the white dwarf, only the much weaker accretion luminosity may vary on short time scales. Therefore, no large and fast change in bolometric nor optical luminosity is expected to accompany the jet, in agreement with the V band photometry.

Jet inclinations larger than seem unlikely as they would imply outflow velocities in excess of the escape velocity of the most massive white dwarfs ( 11,000 km s^-1). On the other hand, if the velocity dispersion is mainly of geometric origin then the shape of the H blue component profile implies . Since the jet is likely to be emitted perpendicularly to the plane of the accretion disc (Becker et al. 1998) it is probable that RX J0925.7-4758 is seen at low inclination angles, consistent with the lack of detected X-ray eclipses. As for RX J0019.8+2156 (Becker et al. 1998), such low inclinations may be incompatible with the relatively large amplitude of the photometric light curve. However, possible jet precession in RX J0925.7-4758 does not allow to draw definite conclusions.

As a whole, the high effective X-ray temperature, small source radius and large jet velocity hint at a massive white dwarf, which may be close to the Chandrasekhar limit.

© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998
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