Extragalactic Absorption of High Energy Gamma-Rays

The pair-production absorption of high-energy -rays by intergalactic low-energy photons is expected to produce a high-energy cuto in the spectra of sources which is a sensitive function of redshift. We rst discuss the expected absorption coe cient as a function of energy and redshift derived by Stecker and De Jager by making use of a new empirically based calculation of the spectral energy distribution of the intergalactic infrared radiation eld as given by Malkan and Stecker. We then discuss the fact that new data on the high energy -ray source Mrk 501 appear to show the amount of intergalactic absorption predicted. The implications of this new HEGRA data, should they be con rmed, are signi cant for the astrophysics of this source, implying that (1) there is no signi cant intrinsic absorption inside the source, and (2) the physics of the emission spectrum produces a power-law to energies above 20 TeV. As a further test for intergalactic absorption, we give a predicted spectrum, with absorption included, for PKS 2155-304. This XBL lies at a redshift of 0.12, the highest redshift source yet observed at an energy above 0.3 TeV. We also discuss the determination of the -ray opacity at higher redshifts (out to z = 3), following the treatment of Salamon and Stecker.