Keck Measurement of the Xte J2123–058 Radial Velocity Curve

We measured the radial velocity curve of the companion of the neutron star X-ray transient XTE J2123–058. Its semi-amplitude (K2) of 298.5±6.9 km s−1 is the highest value that has been measured for any neutron star LMXB. 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± 0.047M⊙, along with our constraints on the companion’s spectral type (K5V-K9V) and previous constraints on the inclination, gives a likely range of neutron star masses from 1.2 to 1.8M⊙. 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 corotates with the disk of the Galaxy. Subject headings: accretion, accretion disks — X-ray transients: general — stars: individual (XTE J2123–058) — stars: neutron stars — X-rays: stars