Determination of the mass of the neutron star in SMC X-1, LMC X-4, and Cen X-3 with VLT/UVES

We present the results of a spectroscopic monitoring campaign of the OB-star companions to the eclipsing X-ray pulsars SMC X-1, LMC X-4 and CenX-3. High-resolution optical spectra obtained with UVES on the ESO Very Large Telescope are used to determine the radial-velocity orbit of the OB ( super) giants with high precision. The excellent quality of the spectra provides the opportunity to measure the radial-velocity curve based on individual lines, and to study the effect of possible distortions of the line profiles due to e. g. X-ray heating on the derived radial-velocity amplitude. Several spectral lines show intrinsic variations with orbital phase. The magnitude of these variations depends on line strength, and thus provides a criterion to select lines that do not suffer from distortions. The undistorted lines show a larger radial-velocity amplitude than the distorted lines, consistent with model predictions. Application of our line-selection criteria results in a mean radial-velocity amplitude K-opt of 20.2 +/- 1.1, 35.1 +/- 1.5, and 27.5 +/- 2.3 kms(-1) (1 sigma errors), for the OB companion to SMC X-1, LMC X-4 and CenX-3, respectively. Adding information on the projected rotational velocity of the OB companion ( derived from our spectra), the duration of X-ray eclipse and orbital parameters of the X-ray pulsar ( obtained from literature), we arrive at a neutron star mass of 1.06(-0.10)(+0.11),1.25(-0.10)(+0.11) and 1.34(-0.14)(+0.16) M-circle dot for SMC X-1, LMC X-4 and CenX-3, respectively. The mass of SMC X-1 is near the minimum mass (similar to 1 M-circle dot) expected for a neutron star produced in a supernova. We discuss the implications of the measured mass distribution on the neutron-star formation mechanism, in relation to the evolutionary history of the massive binaries.