2015ApJ...805...18W

We observed the Fermi-discovered γ-ray binary 1FGL J1018.6-5856 at 20 epochs over 50 days using the CHIRON spectrograph, obtaining spectra at covering Å. The average spectrum confirms an O6 V((f)) spectral type and extinction. Variable absorption line equivalent widths suggest substantial contamination by wind line features. This variability and limited signal-to-noise ratio hindered accurate continuum definition and prevented measurement of a high quality radial velocity curve. Nevertheless, the best data indicate a radial velocity amplitude for the He ii lines and substantially lower for H i. We argue that this indicates a most likely compact object mass. While black hole solutions are not excluded, a neutron star source of the γ-ray emission seems preferred.