2002ApJ...581..570T

We used optical spectroscopy of the neutron star X-ray transient XTE J2123-058 to measure the rotational broadening of the binary companion's stellar absorption lines and determined that the companion's projected rotational velocity is vsini=121⁺²¹_–29km.s^–1. This value is considerably larger than the measurements of vsini obtained previously for three other neutron star systems, where the values were between 34 and 65 km.s^–1. The larger value is likely due to the combination of high binary inclination and short (6 hr) orbital period for XTE J2123-058. Along with previously measured parameters, the rotational broadening measurement allowed us to determine the binary parameters, including the ratio of the neutron star mass to the companion mass, q=M₁/M₂=2.7^+1.3_–0.9, the neutron star mass M₁=1.46^+0.30_–0.39M_☉, and the companion mass M₂=0.53^+0.28_–0.39M_☉, assuming a Roche lobe-filling companion synchronously rotating with the binary orbit. These values are consistent with a previous measurement in which the Hα emission line was used to determine the semiamplitude of the neutron star's radial velocity curve (K₁). It is a significant result that the two methods give consistent values. We also report the first measurement of the XTE J2123-058 companion's radius. Assuming synchronous rotation, we obtain R₂=0.62^+0.11_–0.15R_☉, which is in line with that expected for a late K-type star on or close to the main sequence. Finally, we report the first precise spectroscopic determination of the binary orbital period (P_orb=21,442.3±1.8 s).