The radial velocity of the companion star in the low-mass X-ray binary 2S 0921-630: limits on the mass of the compact object

In this Paper we report on optical spectroscopic observations of the low-mass X-ray binary 2S 0921-630 obtained with the Very Large Telescope. We found sinusoidal radial velocity variations of the companion star with a semi-amplitude of 99.1+-3.1 km s^-1 modulated on a period of 9.006+-0.007 days, consistent with the orbital period found before for this source, and a systemic velocity of 44.4+-2.4 km s^-1. Due to X-ray irradiation the centre-of-light measured by the absorption lines from the companion star is likely shifted with respect to the centre-of-mass. We try to correct for this using the so-called K-correction. Conservatively applying the maximum correction possible and using the previously measured rotational velocity of the companion star, we find a lower limit to the mass of the compact object in 2S 0921-630 of M_X sin^3 i > 1.90+-0.25 Msun (1 sigma errors). The inclination in this system is well constrained since partial eclipses have been observed in X-rays and optical bands. For inclinations between 60< i<90 degrees we find 1.90+-0.25<M_X<2.9+-0.4 Msun. However, using this maximum K-correction we find that the ratio between the mass of the companion star and that of the compact object, q, is 1.32+-0.37 implying super-Eddington mass transfer rates; however, evidence for that has not been found in 2S 0921-630. We conclude that the compact object in 2S 0921-630 is either a (massive) neutron star or a low-mass black hole.