The Massive Neutron Star or Low-Mass Black Hole in 2S 0921-630

We report on optical spectroscopy of the eclipsing Halo LMXB 2S0921-630, thatreveals the absorption line radial velocity curve of the K0III secondary starwith a semi-amplitude K_2=92.89 +/- 3.84 km/s, a systemic velocity$\gamma$=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 effectsof X-ray irradiation. Using the previously determined rotational broadening ofthe mass donor, and applying conservative limits on the orbital inclination, weconstrain the compact object mass to be 2.0-4.3 Msolar (1-sigma), ruling out acanonical neutron star at the 99% level. Since the nature of the compact objectis unclear, this mass range implies that the compact object is either alow-mass black hole with a mass slightly higher than the maximum neutron starmass (2.9 Msolar) or a massive neutron star. If the compact object is a blackhole, 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 constrainsthe equation of state of nuclear matter.Comment: Accepted by ApJ