2006ApJ...653.1089L

In this paper, we study the photon-photon pair production optical depth for gamma rays with energies from 10 to 200 GeV emitted by powerful blazars due to the diffuse radiation field of the broad-line region (BLR). There are four key parameters in the BLR model employed to determine the γ-γ attenuation optical depth of these gamma rays. They are the gamma-ray emitting radius R_γ, the BLR luminosity L_BLR, the BLR half thickness h, and the ratio τ_BLR/f_covof the Thomson optical depth to the covering factor of BLR. For FSRQs, on average, it is impossible for gamma rays with energies from 10 to 200 GeV to escape from the diffuse radiation field of the BLR. If GLAST could detect these gamma rays for most of FSRQs, the gamma-ray emitting region is likely to be outside the cavity formed by the BLR. Otherwise, the emitting region is likely to be inside the BLR cavity. As examples, we estimate the photon-photon absorption optical depth of gamma rays with energies from 10 to 200 GeV for two powerful blazars, HFSRQ PKS 0405-123 and FSRQ 3C 279. Comparing our results with GLAST observations in the future could test whether the model employed and the relevant assumptions in this paper are reliable and reasonable, and then limit constraints on the position of the gamma-ray emitting region relative to the BLR and the properties of the BLR.