Astronomy and Astrophysics – Astrophysics
Scientific paper
2004-09-30
Astrophys.J. 616 (2004) L123-126
Astronomy and Astrophysics
Astrophysics
Accepted by ApJL
Scientific paper
10.1086/426504
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 $\gamma$=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 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.
Casares Jorge
Charles Phil A.
Hynes Robert I.
Shahbaz Tariq
Shih Sheng-Chi
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