Spectroscopic observations and orbit of LMC X-4

Spectroscopic data are presented for the eclipsing X-ray source LMC X-4. The period is refined to be 1.40830 + or - 0.0005 days. The primary is found to be an O7 star of about 25 solar masses which nearly fills its critical potential lobe, while the secondary (X-ray source) appears to lie in the mass range between 2 and 4 solar masses. Thus the mass of the X-ray source is near the upper limits proposed for the masses of neutron stars. A number of complications are discussed which lend uncertainty to this interpretation. A variety of observational data suggest there is a disturbance, or wake, trailing the X-ray source. Of the massive X-ray binaries investigated to date, the primary may be the least evolved, least optically luminous, and least undermassive for its temperature and luminosity. The rotation of the primary appears to be considerably slower than synchronous.