M. Böttcher and C. D. Dermer,
Astronomy & Astrophysics, 302, 37 (1995)
We calculate the time- and energy-dependent opacity of high-energy gamma-rays attenuated by pair-production interactions with accretion-flare photons that are scattered by gas and dust surrounding the nuclei of active galaxies. We show that the temporal behavior of the high-energy opacity cutoff can be used in conjunction with the time history of the accretion flare to determine the location of the gamma-ray emission site, as well as the column density and scale height of the material surrounding the central engine. Reverberation mapping using this technique is now possible for nearby BL Lac objects such as Mkn 421, and will be particularly valuable when the 10 GeV - 100 GeV gamma-ray window is opened, because here the attenuation of distant blazar radiation by the intergalactic infrared background radiation is negligible.