The Mass of the Compact Object in the Low-Mass X-ray Binary 2S 0921-630

TL;DR

This study models the mass of the compact object in the low-mass X-ray binary 2S 0921-630, considering X-ray heating effects, and suggests it is likely a neutron star based on spectral analysis and mass estimates.

Contribution

The paper introduces a Roche model incorporating X-ray heating and anisotropy effects to estimate the compact object's mass in 2S 0921-630, providing new insights into its nature.

Findings

X-ray heating reduces the estimated mass of the compact object by 0.5-1 solar masses.

The compact object is likely a neutron star with a mass around 1.4 solar masses.

If the optical star has lost mass, the compact object remains consistent with a neutron star.

Abstract

We interpret the observed radial-velocity curve of the optical star in the low-mass X-ray binary 2S 0921-630 using a Roche model, taking into account the X-ray heating of the optical star and screening of X-rays coming from the relativistic object by the accretion disk. Consequences of possible anisotropy of the X-ray radiation are considered.We obtain relations between the masses of the optical and compact (X-ray) components, mv and mx, for orbital inclinations i=60, 75, 90 degrees. Including X-ray heating enabled us to reduce the compact object's mass by near 0.5-1Msun, compared to the case with no heating. Based on the K0III spectral type of the optical component (with a probable mass of mv=2.9Msun, we concluded that mx=2.45-2.55Msun (for i=75-90 degrees). If the K0III star has lost a substantial part of its mass as a result of mass exchange, as in the V404 Cyg and GRS 1905+105 systems, and its mass is $m_v=0.65-0.75Msun, the compact object's mass is close to the standard mass of a neutron star, mx=1.4Msun (for i=75-90 degrees). Thus, it is probable that the X-ray source in the 2S 0921-630 binary is an accreting neutron star.