  Perhaps the most extreme examples of "Active OB stars" are the subset of
high-mass X-ray binaries -- consisting of an OB star plus compact companion --
that have recently been observed by Fermi and ground-based Cerenkov telescopes
like HESS to be sources of very high energy (VHE; up to 30 TeV) gamma-rays.
This paper focuses on the prominent gamma-ray source, LS5039, which consists of
a massive O6.5V star in a 3.9-day-period, mildly elliptical (e = 0.24) orbit
with its companion, assumed here to be a black-hole or unmagnetized neutron
star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind
onto the companion, we find that the orbital phase variation of the accretion
follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local
radius and wind speed. Moreover, a simple model, wherein intrinsic emission of
gamma-rays is assumed to track this accretion rate, reproduces quite well Fermi
observations of the phase variation of gamma-rays in the energy range 0.1-10
GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov
telescope, it is important to account also for photon-photon interactions
between the gamma-rays and the stellar optical/UV radiation, which effectively
attenuates much of the strong emission near periastron. When this is included,
we find that this simple BHL accretion model also quite naturally fits the HESS
light curve, thus making it a strong alternative to the pulsar-wind-shock
models commonly invoked to explain such VHE gamma-ray emission in massive-star
binaries.
