The UV Spectrum of the Ultra-compact X-ray Binary- X1627-673

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Abstract
We have obtained Hubble Space Telescope/STIS low-resolution ultraviolet spectroscopy of the X-ray pulsar X1627-673; X1627-673 is unusual even among X-ray pulsars due to its ultra-short binary period (P=41.4 min) and remarkably low mass-function (<1.3e-6 Msun). The far-UV spectrum was exposed for a total of 32ks and has sufficient signal-to-noise to reveal numerous broad emission and prominent narrower absorption lines. Most of the absorption lines are consistent in strength with a purely interstellar origin. However, there is evidence that both CI and CIV require additional absorbing gas local to the system. In emission, the usual prominent lines of NV and HeII are absent, whilst both OIV and OV are relatively strong; and we further identify a rarely seen feature at ~1660A as the OIII] multiplet. Our ultraviolet spectra provide independent support for, and extend, the suggestion of Schulz et al. 2001 (from high-resolution X-ray spectroscopy) that the mass donor is the chemically fractionated core of either a C-O-Ne or O-Ne-Mg white dwarf. The velocity profiles of the ultraviolet lines are in all cases broad and/or flat-topped, or perhaps even double-peaked for the highest ionization cases of O; in either case the ultraviolet line profiles are in broad agreement with the Doppler pairs found in the X-ray spectra. Both the X-ray and far-UV lines are plausibly formed in (or in an corona just above) a Keplerian accretion disc; the combination of ultraviolet and X-ray spectral data may provide a rich data set for follow-on detailed models of the disk dynamics and ionization structure in this highly unusual low-mass X-ray pulsar system.

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