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

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 =6 0 ◦ , 75 ◦ ,a nd90 ◦ . Including X-ray heating enabled us to reduce the compact object's mass by ∼0.5−1 M� , compared to the case with no heating. Based on the K0III spectral type of the optical component (with a probable mass of mv � 2.9 M� ), we concluded that mx � 2.45−2.55 M� (for i =7 5 ◦ −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 � 0.65−0.75 M� , the compact object's mass is close to the standard mass of a neutron star, mx � 1.4 M� (for i =7 5 ◦ −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