On the mass of the neutron star in Cyg X-2

TL;DR

This study refines the orbital parameters and mass estimate of the neutron star in Cyg X-2 using high-resolution spectroscopy, suggesting it may be more massive than the typical neutron star mass.

Contribution

It provides new measurements of orbital and rotational parameters that lead to a revised neutron star mass estimate, clarifying previous discrepancies.

Findings

Neutron star mass estimated at 1.71 solar masses.

No evidence of irradiation effects influencing radial velocity.

Results align with previous studies, contrasting with some earlier findings.

Abstract

We present new high resolution spectroscopy of the low mass X-ray binary Cyg X-2 which enables us to refine the orbital solution and rotational broadening of the donor star. In contrast with Elebert et al (2009) we find a good agreement with results reported in Casares et al. (1998). We measure $P=9.84450\pm0.00019$ day, $K_2=86.5\pm1.2$ km s$^{-1}$ and $V \sin i=33.7\pm0.9$ km s$^{-1}$. These values imply $q=M_{2}/M_{1}=0.34 \pm 0.02$ and $M_{1}=1.71\pm 0.21$ M$_{\odot}$ (for $i=62.5 \pm 4^{\circ}$). Therefore, the neutron star in Cyg X-2 can be more massive than canonical. We also find no evidence for irradiation effects in our radial velocity curve which could explain the discrepancy between Elebert et al's and our $K_2$ values.