2006A&A...451....9D

Gamma-ray emission in the TeV (10¹²eV) range has been detected by HESS from two X-ray binaries: PSR B1259-63 and LS 5039. In both, the early-type star provides large numbers of target photons for pair-production with TeV γ-rays. This results in a modulation of the γ-ray flux as the relative positions of the γ-ray source and companion star change with orbital phase for the observer. The extent to which this variable absorption can provide useful diagnostics for the location and nature of γ-ray emission is examined. The absorption spectrum and transmitted flux are calculated by integrating the γγ cross-section along the line-of-sight, taking into account the orbit, the spectrum and the finite size of the companion star in LS 5039, PSR B1259-63 and LSI +61°303, a system similar to LS 5039 but still undetected at TeV energies. In LS 5039, emission close to a black hole or a neutron star primary is considered. In both cases, the transmitted flux >250GeV drops by an order-of-magnitude near periastron (φ=0). A black hole yields a clear spectral signature in the average spectrum at ≃400GeV. A neutron star yields more variability, with the spectral feature moving from 200GeV (φ=0.1) to 3TeV (φ=0.7). Only 20% of the flux is absorbed at φ=0.7, allowing for an almost direct view of the intrinsic spectrum. Low variability will require emission on large scales, more than 0.7 AU away to have <50% absorption in a jet. In LSI +61°303, significant absorption (up to 90% of the 100GeV flux) is predicted only slightly before periastron, accompanied by a spectral hardening above 1TeV. In PSR B1259-63, although 40% of the flux is absorbed before periastron, the large variability seen by HESS is due to the γ-ray emission process. The predictions made here are essential to distinguish variability in the emission of γ-rays from that due to absorption. A modulation would provide a novel way to constrain the γ-ray source. Its absence would imply that γ-ray emission occurs on large scales.