2001ApJ...559L.123T

We measured the radial velocity curve of the companion of the neutron star X-ray transient XTE J2123-058. Its semiamplitude (K₂) of 298.5±6.9 km.s^–1 is the highest value that has been measured for any neutron star low-mass X-ray binary. The high value for K₂ is, in part, due to the high binary inclination of the system but may also indicate a high neutron star mass. The mass function (f₂) of 0.684±0.047 M_☉, along with our constraints on the companion's spectral type (K5 V-K9 V) and previous constraints on the inclination, gives a likely range of neutron star masses from 1.2 to 1.8 M_☉. We also derive a source distance of 8.5±2.5 kpc, indicating that XTE J2123-058 is unusually far, 5.0±1.5 kpc, from the Galactic plane. Our measurement of the systemic radial velocity is -94.5±5.5 km.s^–1, which is significantly different from what would be observed if this object corotated with the disk of the Galaxy.