Rotational Broadening Measurement for the Neutron Star X‐Ray Transient XTE J2123−058

We used optical spectroscopy of the neutron star X-ray transient XTEJ2123-058 to measure the rotational broadening of the binary companion'sstellar absorption lines and determined that the companion's projectedrotational velocity is vsini = 121 (+21)(-29) km/s. This value is considerablylarger than the measurements of vsini obtained previously for three otherneutron star systems where the values are between 34 and 65 km/s. The largervalue is likely due to the combination of high binary inclination and short (6hr) orbital period for XTE J2123-058. Along with previously measuredparameters, the rotational broadening measurement allowed us to determine thebinary parameters, including the ratio of the neutron star mass to thecompanion mass, q = M_1/M_2 = 2.7 (+1.3)(-0.9), the neutron star mass, M_1 =1.46 (+0.30)(-0.39) solar masses, and the companion mass, M_2 = 0.53(+0.28)(-0.39) solar masses, assuming a Roche lobe filling companionsynchronously rotating with the binary orbit. These values are consistent witha previous measurement where the H alpha emission line was used to determinethe semiamplitude of the neutron star's radial velocity curve (K_1). It is asignificant result that the two methods give consistent values. We also reportthe first measurement of the XTE J2123-058 companion's radius. Assumingsynchronous rotation, we obtain R_2 = 0.62 (+0.11)(-0.15) solar radii, which isin-line with that expected for a late K-type star on or close to the mainsequence. Finally, we report the first precise spectroscopic determination ofthe binary orbital period (P_orb = 21442.3 +/- 1.8 seconds).Comment: 6 pages, accepted by Ap