On the mass of the neutron star in V395 Car/2S 0921-630

TL;DR

This study uses high-resolution spectroscopy to accurately measure the neutron star's mass in V395 Car, revealing it to be around 1.44 solar masses, much lower than previous estimates, confirming it as a typical neutron star.

Contribution

The paper provides a revised, more accurate measurement of the neutron star's mass using improved spectroscopic techniques and analysis, correcting prior overestimations.

Findings

Neutron star mass is approximately 1.44 Msun.

Rotational velocity of the donor star is 32.9 km/s.

Previous mass estimates of 2-4 Msun were too high.

Abstract

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 superior 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. 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 downwards compared to previously published values. We derive new system parameter constraints in the light of our much lower rotational velocity. We find M_1=1.44 +/- 0.10 Msun, M_2=0.35 +/- 0.03 Msun, 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 over-estimating 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 M_1=1.05 +/- 0.08 Msun. 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 2-4Msun were too high due to an over-estimate of the rotational broadening.