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

摘要

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, m(v) and m(x), for orbital inclinations i = 60 degrees, 75 degrees, and 90 degrees. Including X-ray heating enabled us to reduce the compact object's mass by similar to 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 M-v similar or equal to 2.9 M-.), we concluded that m(x) similar or equal to 2.45-2.55 M-. (for i = 75 degrees-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) similar or equal to 0.65-0.75 M-., the compact object's mass is close to the standard mass of a neutron star, m(x) similar or equal to 1.4 M-. (for i = 75 degrees-90 degrees). Thus, it is probable that the X-ray source in the 2S 0921-630 binary is an accreting neutron star.