2008ApJ...688..148L

We study the photon-photon pair production optical depth for 10 GeV-1 TeV gamma rays from 3C 279 due to the diffuse radiation of the broad-line region (BLR). Assuming a power-law spectrum a₁E^–a2_γfor the photon intensity of very high energy gamma rays, a₁≳405 and a₂≳6.4 are inferred from the integrated photon fluxes measured by MAGIC and H.E.S.S. Based on this power-law spectrum, the preabsorbed spectra are calculated by correcting for photon-photon absorption on the diffuse photons of the BLR (internal absorption) and the extragalactic background light (external absorption). The position of the gamma-ray-emitting region, R_γ, determines the relative contributions of these two diffuse radiation components to the total absorption for 10 GeV-1 TeV gamma rays. The internal absorption could make the spectral shape of the gamma rays more complex than if they were corrected only for external absorption and could lead to the formation of arbitrarily softening and hardening gamma-ray spectra. It should be requisite for the internal absorption to be considered in studying 10 GeV-1 TeV gamma rays from powerful blazars. The energy of the annihilated gamma-ray photons due to the internal absorption is likely to be mainly reradiated around GeV energies. Our results indicate that R_γ may lie between the inner and outer radii of the BLR for 3C 279. This implies for powerful blazars that R_γ might be neither inside the BLR cavity nor outside the BLR, but within the BLR shell. Observations by GLAST, MAGIC, H.E.S.S., and VERITAS in the near future could provide more constraints on the position of the gamma-ray-emitting region relative to the BLR.