Gamma-gamma absorption above a molecular cloud torus in blazars

Gamma rays have been observed from two blazars at TeV energies. One of these, Markarian 421, has been observed also at GeV energies and has roughly equal luminosity per decade at GeV and TeV energies. Photon-photon pair production on the infrared background radiation is expected to prevent observation above $\sim 1$ TeV. However, the infrared background is not well known and it may be possible to observe the nearest blazars up to energies somewhat below $\sim 100$ TeV where absorption on the cosmic microwave background will give a sharp cut-off. Blazars are commonly believed to correspond to low power radio galaxies, seen down along a relativistic jet; as such they are all expected to have the nuclear activity encircled by a dusty molecular torus, which subtends an angle of 90 degrees or more in width as seen from the central source. Photon-photon pair production can also take place on the infrared radiation produced at the AGN by this molecular torus and surrounding outer disk. We calculate the optical depth for escaping gamma rays produced near the central black hole and at various points along the jet axis for the case of blazars where the radiation is observed in a direction closely aligned with the jet. Assuming typical dimensions of the torus, we find that the TeV emission site must be above the top of the torus, and that if blazars are observed at higher energies in the future this will constrain the emission region to be well above the torus. This has important implications for models of gamma ray emission in active galactic nuclei.