Updated binary solution for 1FGL J1018.6–5856 and the implications for gamma-ray production

Gamma-ray binaries are high mass binary systems that emit most of the non-thermal energy in gamma-rays above 1 MeV. All gamma-ray binaries consist of a compact object, in the mass range of a neutron star or black hole, in orbit around an O/Oe or B/Be type star. The gamma-ray binary 1FGL J1018.6–5856 shows modulated radio, X-ray and gamma-ray emission, with an orbital period of approximately 16.54 d. While radio and GeV light curves both showed a single maximum, the X-ray light curve shows two maxima. The orbital parameters of the system have been considered by several authors, considering both radial velocity measurements and model dependent solutions. However, the model dependent solutions contradict the most recent solution found from radial velocity measurements. Using new high resolution optical spectroscopic observations undertaken with the Southern African Large Telescope (SALT) and an updated binary period determination using Fermi-LAT data, suggests a more eccentric orbit than previously found as well, and a slightly longer orbital period. We present these results and discuss the implications for producing the non-thermal emission observed from the source.