PHOTON-PHOTON 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-ph oton pair production on the infrared background radiation is expected to prevent observation above similar to 1 TeV. However, the infrared b ackground is not well known and it may be possible to observe the near est blazars up to energies somewhat below similar to 100 TeV where abs orption on the cosmic microwave background will give a sharp cut-off. Blazars are commonly believed to correspond to low power radio galaxie s, seen down along a relativistic jet; as such they are all expected t o have the nuclear activity encircled by a dusty molecular torus, whic h subtends an angle of 90 degrees or more in width as seen from the ce ntral source. Photon-photon pair production can also take place on the infrared radiation produced at the AGN by this molecular torus and su rrounding outer disk. We calculate the optical depth for escaping gamm a-rays produced near the central black hole and at various points alon g the jet axis for the case of blazars where the radiation is observed in a direction closely aligned with the jet. We find that the TeV emi ssion site must be well above the top of the torus. For example, if th e torus has an inner radius of 0.1 pc and an outer radius of 0.2 pc, t hen the emission site in Mrk 421 would have be at least 0.25 pc above the upper surface of the torus, and if Mrk 421 is observed above 50 Te V in the future, the emission site would have to be at least 0.5 pc ab ove the upper surface. This has important implications for models of g amma-ray emission in active galactic nuclei. (C) 1997 Elsevier Science B.V.