Optical studies of the X-ray transient XTE J2123-058 - II. Phase-resolved spectroscopy

Abstract

We present time-resolved spectroscopy of the soft X-ray transient XTE J2123-058 in outburst. A useful spectral coverage of 3700-6700A was achieved spanning two orbits of the binary, with single epoch coverage extending to \~9000A. The optical spectrum approximates a steep blue power-law, consistent with emission on the Rayleigh-Jeans tail of a hot black body spectrum. The strongest spectral lines are HeII 4686A and CIII/NIII 4640A (Bowen blend) in emission. Their relative strengths suggest that XTE J2123-058 was formed in the Galactic plane, not in the halo. Other weak emission lines of HeII and CIV are present and Balmer lines show a complex structure, blended with HeII. HeII 4686A profiles show a complex multiple S-wave structure with the strongest component appearing at low velocities in the lower-left quadrant of a Doppler tomogram. H alpha shows transient absorption between phases 0.35-0.55. Both of these effects appear to be analogous to similar behaviour in SW Sex type cataclysmic variables. We therefore consider whether the spectral line behaviour of XTE J2123-058 can be explained by the same models invoked for those systems.