Gemini/GMOS Spectroscopy of EXO 0748-676 (=UY Vol) in Outburst

TL;DR

This study provides a detailed phase-resolved optical spectroscopic analysis of the eclipsing low-mass X-ray binary EXO 0748-676 during its outburst, revealing emission line structures, donor star motion, and constraints on system masses.

Contribution

It offers the most detailed blue spectroscopic data during outburst for this source, identifying emission components and constraining the neutron star and donor star masses.

Findings

Strong emission downstream of stream-impact point varies nightly.

We measure donor star velocity Kem = 329+/-26 km/s.

System likely contains a ~1.5Msun neutron star and a very low mass donor.

Abstract

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 favor a neutron star mass close to canonical (M1~1.5Msun) and a very low mass donor (M2~0.1$Msun). 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 2Msun and substantially sub-Keplerian disk emission.