Photon-photon Absorption of Very High Energy Gamma Rays from Microquasars: Application to Ls 5039

Very high energy (VHE) γ-rays have recently been detected from the Galactic black-hole candidate and microquasar LS 5039. A plausible site for the production of these VHE γ-rays is the mildly relativistic outflow close to the base of the jet. However, at distances comparable to the binary separation, the intense photon field of the stellar companion will lead to substantial γγ absorption of VHE γ-rays. If the system is viewed at a substantial inclination (i 6= 0), this absorption feature will be modulated on the orbital period of the binary as a result of both a phase-dependent intensity of the stellar radiation field and the pair-production threshold. We apply our results to the case of LS 5039 and find that (1) γγ absorption effects will be substantial if the photon production site is located at a distance from the central compact object of the order of the binary separation (≈ 2.5× 10 cm) or less; (2) the γγ absorption depth will be largest at a few hundred GeV, leading to a characteristic absorption trough; (3) the γγ absorption feature will be strongly modulated on the orbital period of the binary, characterized by a spectral hardening accompanying periodic dips of the VHE γ-ray flux; and (4) gamma rays can escape virtually unabsorbed, even from within ≈ 10 cm, when the star is located behind the production site as seen by the observer. Subject headings: gamma-rays: theory — radiation mechanisms: non-thermal — X-rays: binaries — stars: winds, outflows Astrophysical Institute, Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA E. O. Hulburt Center for Space Research, Code 7653 Naval Research Laboratory, Washington, D.C. 20375-5352