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tokens: line), along with the combined contribution (black dashed line) as given in van Soelen et al. ( 2012 ). A 33,000 K blackbody distribution is plotted for comparison (blue dashed line). 3. Gamma–Gamma Absorption Gamma-ray photons emitted in binary systems are subject to γγ absorption as they pass through the star’s photon field (see, e.g., Dubus 2006 ). The interaction of a γ -ray photon with a low-energy photon can result in electron–positron pair production (Gould Schréder 1967 ), if the energy exceeds the threshold condition for pair production, which is given bywhere and denote the photon energies of the target low-energy photon and the γ -ray photon, respectively, normalized to the electron rest-mass energy, i.e., , and θ is the interaction angle between the two photons. The minimum threshold occurs for a head-on collision (), and for a 1 TeV gamma-ray photon this will require photons with a threshold frequency ofwhich is within the mid-IR regime (where the disk is the primary photon source; Figure 2 ). The γγ optical depth is given by (Gould Schréder 1967 )where l is the distance over which the γ -ray photon travels, , , and is the number density of the low-energy target photons. Here, is the γγ cross section (Jauch Rohrlich 1976 ),whereand is the Thomson cross section. The maximum γγ cross section occurs when the energy of the interacting photons is twice that of the energy threshold (Equation ( 8 )). This implies that for TeV γ -ray photons the γγ interaction is a maximum for IR photons at a frequency We treat the two components of the target radiation field (stellar radiation and circumstellar disk radiation) separately and numerically calculate the γγ optical depth of both components for a γ -ray photon traveling in the direction of the observer. The total optical depth is given by the sum of the optical depths of the two components. Figure 3 shows the optical depth for a γ -ray photon with an energy of 1 TeV as a function of time from periastron. As expected, the γγ optical depth due to the photons from the disk is lower than that due to the star but still results in a decrease of the flux at TeV energies. The total decrease of the flux at 1 TeV due to γγ absorption may reach ∼60%. For the geometrical configuration used in our calculations (see Section 2.1 ) the maximum absorption due to the disk is at ≈4 days prior to periastron and that due to the star is at ≈2 days before periastron. Figure 3. The γγ optical depth of the Be star’s radiation field as a function of time around periastron for a 1 TeV γ -ray photon. The blue dashed line denotes the stellar component, the red dot-dashed line denotes the circumstellar disk component, and the combined effect is shown by the solid black line. 4. TeV Emission We assume that the TeV emission from PSR B1259-63/LS 2883 is produced by relativistic particles accelerated at the termination shock that occurs between the pulsar and stellar winds. The location of the termination shock depends on the