The likely Fermi detection of the Be X-ray binary GRO J1008-57: hint for gamma-ray production in neutron-star X-ray binaries

Abstract

In our search for gamma-ray emission from Be X-ray binaries from analysis of the data obtained with the Large Area Telescope (LAT) on board the Fermi Gamma-Ray Space Telescope, we find likely detection of GRO J1008-57. The binary has an orbital period of 249.48 days, and it is only significantly detected in its orbital phase 0.8--0.9 (>4 sigma). Further light curve analysis indicates that the detection is probably largely due to an emitting event in one orbital cycle around year 2012--2013, following a giant X-ray outburst of the source. This property of having occasional gamma-ray emitting events is similar to that seen in another high-mass X-ray binary 4U 1036-56. However, models considering possible gamma-ray emission from an accreting neutron star have difficulty in explaining the observed 10^{34} erg s^{-1} luminosity of the source, unless the distance was largely over-estimated. Further observational studies are required, in order to more clearly establish the high-energy emission properties of GRO J1008-57 or similar high-mass X-ray binaries and find clues for understanding how gamma-ray emission is produced from them.