Modeling TeV gamma-rays from LS 5039: An active OB star at the extreme

摘要

Perhaps the most extreme examples of "Active OB stars" are the subset ofhigh-mass X-ray binaries -- consisting of an OB star plus compact companion --that have recently been observed by Fermi and ground-based Cerenkov telescopeslike 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 ofa massive O6.5V star in a 3.9-day-period, mildly elliptical (e = 0.24) orbitwith its companion, assumed here to be a black-hole or unmagnetized neutronstar. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star windonto the companion, we find that the orbital phase variation of the accretionfollows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the localradius and wind speed. Moreover, a simple model, wherein intrinsic emission ofgamma-rays is assumed to track this accretion rate, reproduces quite well Fermiobservations of the phase variation of gamma-rays in the energy range 0.1-10GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkovtelescope, it is important to account also for photon-photon interactionsbetween the gamma-rays and the stellar optical/UV radiation, which effectivelyattenuates much of the strong emission near periastron. When this is included,we find that this simple BHL accretion model also quite naturally fits the HESSlight curve, thus making it a strong alternative to the pulsar-wind-shockmodels commonly invoked to explain such VHE gamma-ray emission in massive-starbinaries.