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 semiamplitude (K-2) 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 K-2 is, in part, due to the high binary inclination of the system but may also indicate a high neutron star mass. The mass function (f(2)) of 0.684 +/- 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.