GEMINI MULTI-OBJECT SPECTROGRAPH SPECTROSCOPY OF EXO 0748―676 (=UY Vol) IN OUTBURST

We present a phase-resolved, optical, spectroscopic study of the eclipsing low-mass X-ray binary, EXO 0748-676 = UY Vol. The sensitivity of Gemini, combined with our complete phase coverage, makes for the most detailed blue spectroscopic study of this source obtained during its extended 24 year period of activity. We identify 12 optical emission lines and present trailed spectra, tomograms, and the first modulation maps of this source in outburst. The strongest line emission originates downstream of the stream-impact point, and this component is quite variable from night to night. Underlying this is weaker, more stable axisymmetric emission from the accretion disk. We identify weak, sharp emission components moving in phase with the donor star, from which we measure K sub(em) = 329 + or - 26 km s super(-1). Combining all the available dynamical constraints on the motion of the donor star with our observed accretion disk velocities, we favor a neutron star mass close to canonical (M sub(1) [Asymptotically = to] 1.5 M sub([middot in circle])) and a very low mass donor (M sub(2) [Asymptotically = to] 0.1 M sub([middot in circle])). We note, however, that there is no evidence for CNO processing, which is often associated with undermassive donor stars. A main-sequence donor would require both a neutron star more massive than 2 M sub([middot in circle]), and substantially sub-Keplerian disk emission.