We report on optical spectroscopy of the eclipsing Halo LMXB 2S0921-630, that
reveals the absorption line radial velocity curve of the K0III secondary star
with a semi-amplitude K_2=92.89 +/- 3.84 km/s, a systemic velocity
γ=34.9 +/- 3.3 \kms and an orbital period P_orb of 9.0035 +/- 0.0029 day
(1-sigma). Given the quality of the data, we find no evidence for the effects
of X-ray irradiation. Using the previously determined rotational broadening of
the mass donor, and applying conservative limits on the orbital inclination, we
constrain the compact object mass to be 2.0-4.3 Msolar (1-sigma), ruling out a
canonical neutron star at the 99% level. Since the nature of the compact object
is unclear, this mass range implies that the compact object is either a
low-mass black hole with a mass slightly higher than the maximum neutron star
mass (2.9 Msolar) or a massive neutron star. If the compact object is a black
hole, it confirms the prediction of the existence of low-mass black holes,
while if the object is a massive neutron star its high mass severely constrains
the equation of state of nuclear matter.Comment: Accepted by ApJ