We present medium-resolution optical spectroscopy with the SOAR telescope of the O star secondary of the high-mass \gamma -ray binary 1FGL J1018.6–5856 to help determine whether the primary is a neutron star or black hole .
We find that the secondary has a low radial velocity semi-amplitude of 11–12 km s ^ { -1 } , with consistent values obtained for H and He absorption lines .
This low value strongly favors a neutron star primary : while a black hole can not be excluded if the system is close to face on , such inclinations are disallowed by the observed rotation of the secondary .
We also find the high-energy ( X-ray and \gamma -ray ) flux maxima occur when the star is behind the compact object along our line of sight , inconsistent with a simple model of anisotropic inverse Compton scattering for the \gamma -ray photons .