We report on the optical spectroscopy of the eclipsing halo low-mass X-ray binary 2S 0921 - 630, which reveals the absorption-line radial velocity curve of the K0 III secondary star with a semiamplitude K2 = 92.89 ± 3.84 km s-1, a systemic velocity γ = 34.9 ± 3. 3 km s-1, and an orbital period Porb of 9.0035 ± 0.0029 days (1 σ). 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 M ⊙ (1 σ), 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 possible neutron star mass (2.9 M ⊙) 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.
Publication Source (Journal or Book title)
Shahbaz, T., Casares, J., Watson, C., Charles, P., Hynes, R., Shih, S., & Steeghs, D. (2004). The massive neutron star or low-mass black hole in 2S 0921-630. Astrophysical Journal, 616 (2 II) https://doi.org/10.1086/426504