The mass of the compact object in the low-mass X-ray binary 2S 0921-630

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.