REFINED NEUTRON STAR MASS DETERMINATIONS FOR SIX ECLIPSING X-RAY PULSAR BINARIES

We present an improved method for determining the mass of neutron stars in eclipsing X-ray pulsar binaries and apply the method to six systems, namely, Vela X-1, 4U 1538-52, SMC X-1, LMC X-4, Cen X-3, and Her X-1. In previous studies to determine neutron star mass, the X-ray eclipse duration has been approximated analytically by assuming that the companion star is spherical with an effective Roche lobe radius. We use a numerical code based on Roche geometry with various optimizers to analyze the published data for these systems, which we supplement with new spectroscopic and photometric data for 4U 1538-52. This allows us to model the eclipse duration more accurately and thus calculate an improved value for the neutron star mass. The derived neutron star mass also depends on the assumed Roche lobe filling factor beta of the companion star, where beta = 1 indicates a completely filled Roche lobe. In previous work a range of beta between 0.9 and 1.0 was usually adopted. We use optical ellipsoidal light-curve data to constrain beta. We find neutron star masses of 1.77 +/- 0.08 M-circle dot for Vela X-1, 0.87 +/- 0.07 M-circle dot for 4U 1538-52 (eccentric orbit), 1.00 +/- 0.10 M-circle dot for 4U 1538-52 (circular orbit), 1.04 +/- 0.09M(circle dot) for SMC X-1, 1.29 +/- 0.05 M-circle dot for LMC X-4, 1.49 +/- 0.08 M-circle dot for Cen X-3, and 1.07 +/- 0.36 M-circle dot for Her X-1. We discuss the limits of the approximations that were used to derive the earlier mass determinations, and we comment on the implications our new masses have for observationally refining the upper and lower bounds of the neutron star mass distribution.