2001ApJ...558L..43M

We present the first spectroscopic orbit for the massive X-ray binary LS 5039, which we find to be a short-period (P=4.117±0.011 days) and highly eccentric (e=0.41±0.05) system. The low-mass function for the orbit appears to be most consistent with a neutron star companion, although a black hole remains a possibility if the system has a low inclination. The spectrum of the O7 V optical star appears to be normal for its type (suggesting that there is little flux in the red from an accretion disk) except that the C IV λλ5801, 5812 lines are very weak, perhaps indicating the presence of CNO-processed gas in the O star. There is no evidence of Hα emission, so the system is probably not currently undergoing Roche lobe overflow. The projected rotational velocity, Vsini=131±6 km.s^–1, suggests that the optical star is rotating faster than synchronously with the orbit. The peculiar component of the systemic radial velocity is -17±3 km.s^–1, so the system is not a runaway star (at least not in this dimension).