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arXiv · astro-ph/0409752

The massive neutron star or low-mass black hole in 2S0921-630

T. Shahbaz·J. Casares·C. Watson·P. A. Charles·R. I. Hynes·S. C. Shih·D. Steeghs
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Abstract

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.

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T. Shahbaz, J. Casares, C. Watson, P. A. Charles, R. I. Hynes, S. C. Shih, D. Steeghs. 2004-09-30. The massive neutron star or low-mass black hole in 2S0921-630. https://doi.org/10.1086/426504

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