The orbit of the gamma-ray binary 1FGL J1018.6-5856

Abstract

Gamma-ray binaries are a small subclass of the high mass X-ray binary population which exhibit emission across the whole electromagnetic spectrum. We present radial velocities of 1FGL J1018.6$-$5856 based on observations obtained with the Southern African Large Telescope (SALT). We combine our measurements with those published in the literature to get a broad phase coverage. The mass function obtained supports a neutron star compact object, although a black hole mass is possible for very low inclination angles. The improved phase coverage allows constraints to be placed on the orbital eccentricity (e = 0.31 $\pm$ 0.16), which agrees with estimates from high-energy data.