Gamma Ray and Infrared Emission from the M87 Jet and Torus

The existence of intrinsic obscuration of Fanaroff-Riley I objects is acontroversial topic. M87, the nearest such object, is puzzling in that althoughit has very massive central black hole it has a relatively low luminosity,suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) itis not known with certainty whether or not M87 has a dusty torus. Infrared observations indicate that if a torus exists in M87 it must have arather low infrared luminosity. Using arguments from unification theory ofactive galactic nuclei, we have earlier suggested that the inner parsec-scaleregion of M87 could still harbour a small torus sufficiently cold such that itsinfrared emission is dwarfed by the jet emission. The infrared emission fromeven a small cold torus could affect through photon-photon pair productioninteractions the escape of 100 GeV to TeV energy gamma rays from the centralparsec of M87. The TeV gamma-ray flux from the inner jet of M87 has recently been predictedin the context of the Synchrotron Proton Blazar (SPB) model to extend up to atleast 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection ofgamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported.We discuss the interactions of gamma-rays produced in the inner jet of M87 withthe weak infrared radiation expected from a possible dusty small-scale torus,and show that the HEGRA detection shows that the temperature of any torussurrounding the gamma-ray emission region must be cooler than about 250 K. Wesuggest that if no gamma-rays are in future detected during extreme flaringactivity in M87 at other wavelength, this may be expected because of torusheating.Comment: 7 pages, submitted to Prog. Theor. Phys. Suppl., ps fil