Observation of a partial X-ray eclipse in the long-period low-mass X-ray binary 2S 0921- 630

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Abstract
A partial eclipse of the X-ray flux from 2S 0921– 630 has been recorded for the first time, using the EXOSAT satellite. The time of the X-ray eclipse agrees with an extrapolation of the ephemeris of optical dips in 2S 0921– 630, strongly supporting the hypothesis that 2S 0921– 630 is an eclipsing binary with a period of just over 9 day. The X-ray eclipse lasts for over 1 day and has a smooth ingress and egress. At mid-eclipse the source flux drops to ~ 30 per cent of its pre-eclipse value. The results of another, shorter, X-ray observation, also made at the predicted time of eclipse centre, suggest that the X-ray eclipse is a stable feature. The eclipse depth and shape are not measurably dependent on energy. The observed spectrum does become slightly softer during the eclipse, but this is most likely the result of contamination of the signal by nearby field sources. The partial nature of the X-ray eclipse leads to the hypothesis that the binary companion in 2S 0921– 630 obscures an extended medium which is scattering X-rays from a central, compact source into the line-of-sight. Variability of the X-ray flux before eclipse may be caused by obscuration of the scattering region by structure on the edge of an accretion disc. The X-ray emitting star is probably hidden from direct view by the accretion disc, which is consistent with the very low ratio of X-ray to optical flux of 2S 0921– 630. A simple model for the X-ray eclipse combined with optical radial velocity data suggests that the companion star in 2S 0921– 630 is undermassive for its size compared to a normal, isolated star.

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