ECHELLE SPECTROSCOPY OF γ-RAY BINARY 1FGL J1018.6-5856

We observed the Fermi-discovered gamma-ray binary 1FGL J1018.6-5856 at 20 epochs over 50 days using the CHIRON spectrograph, obtaining spectra at R similar to 25,000 covering lambda lambda 4090-8908 angstrom. The average spectrum confirms an O6 V((f)) spectral type and extinction E (B - V) = 1.35 +/- 0.04. 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 <= 40 km s(-1) for the He II lines and substantially lower for H I. We argue that this indicates a most likely compact object mass <2.2 M-circle dot. While black hole solutions are not excluded, a neutron star source of the.-ray emission seems preferred.