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◦, and 90◦. Including X-ray heating enabled us to reduce the compact object’s mass by ∼0.5−1M compared to the case with no heating. Based on the K0III spectral type of the optical component (with a probable mass of mv 2.9M), we concluded that mx 2.45−2.55M (for i = 75◦−90◦). 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 mv 0.65−0.75M, the compact object’s mass is close to the standard mass of a neutron star, mx 1.4M (for i = 75◦−90◦). Thus, it is probable that the X-ray source in the 2S 0921–630 binary is an accreting neutron star. PACS numbers: 97.80.J