On the Mass of the Neutron Star in V395 Carinae/2S 0921-630

We report high-resolution optical spectroscopy of the low-mass X-ray binary V395 Car/2S 0921-630 obtained with the MIKE echelle spectrograph on the Magellan-Clay telescope. Our spectra are obtained near the inferior conjunction of the mass donor star, and we exploit the absorption lines originating from the back side of the K-type object to accurately derive its rotational velocity. Using K0-K1 III templates, we find vsini=32.9+/-0.8 km s^-1. We show that the choice of template star and the assumed limb-darkening coefficient has little impact on the derived rotational velocity. This value is a significant revision downward compared to previously published values. We derive new system parameter constraints in light of our much lower rotational velocity. We find M₁=1.44+/-0.10 M_solar, M₂=0.35+/-0.03 M_solar, and q=0.24+/-0.02, where the errors have been estimated through a Monte Carlo simulation. A possible remaining systematic effect is the fact that we may be overestimating the orbital velocity of the mass donor due to irradiation effects. However, any correction for this effect will only reduce the compact object mass further, down to a minimum mass of M₁=1.05+/-0.08 M_solar. There is thus strong evidence that the compact object in this binary is a neutron star of rather typical mass and that the previously reported mass values of 2-4 M_solar were too high due to an overestimate of the rotational broadening.