* Astronomy

Title: Gemini/GMOS Spectroscopy of EXO 0748-676 (=UY Vol) in Outburst
Authors: Valerie J. Mikles, Robert I. Hynes (Louisiana State Univ.)

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 twenty-four 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 Kem = 329±26 km/s. Combining all the available dynamical constraints on the motion of the donor star with our observed accretion disk velocities we favour a neutron star mass close to canonical (M1~1.5 solar masses) and a very low mass donor (M2~0.1 solar masses). We note that there is no evidence for CNO processing that is often associated with undermassive donor stars, however. A main sequence donor would require both a neutron star more massive than 2 solar masses and substantially sub-Keplerian disk emission.

Read more (365kb, PDF)