Gamma-ray absorption in massive X-ray binaries

Context. Gamma-ray emission in the TeV (10(12) eV) 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 gamma-rays. This results in a modulation of the gamma- ray flux as the relative positions of the gamma-ray source and companion star change with orbital phase for the observer. Aims. The extent to which this variable absorption can provide useful diagnostics for the location and nature of gamma- ray emission is examined. Methods. The absorption spectrum and transmitted flux are calculated by integrating the gamma gamma 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 degrees. 303, a system similar to LS 5039 but still undetected at TeV energies. Results. In LS 5039, emission close to a black hole or a neutron star primary is considered. In both cases, the transmitted flux > 250 GeV drops by an order-of-magnitude near periastron (phi = 0). A black hole yields a clear spectral signature in the average spectrum at approximate to 400 GeV. A neutron star yields more variability, with the spectral feature moving from 200 GeV (phi = 0.1) to 3 TeV (phi = 0.7). Only 20% of the flux is absorbed at f = 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 degrees. 303, significant absorption (up to 90% of the 100 GeV flux) is predicted only slightly before periastron, accompanied by a spectral hardening above 1 TeV. In PSR B1259-63, although 40% of the flux is absorbed before periastron, the large variability seen by HESS is due to the gamma-ray emission process. Conclusions. The predictions made here are essential to distinguish variability in the emission of gamma-rays from that due to absorption. A modulation would provide a novel way to constrain the gamma-ray source. Its absence would imply that gamma- ray emission occurs on large scales.