Intergalactic Extinction of High Energy Gamma-Rays

We discuss the determination of the intergalactic pair-production absorption coefficient as 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 field as given by Malkan and Stecker. We show that the results of the Malkan and Stecker calculation agree well with recent data on the infrared background. We then show that previous spectral data from observations of Mrk 421 and Mrk 501 are consistent with the amount of intergalactic absorption predicted by Stecker and De Jager and that the new HEGRA observations of the flaring spectrum of Mrk 501 presented at this conference actually appear to show the amount of intergalactic absorption which we predict. 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. This source should have its spectrum steepened by ∼ 1 in its spectral index between ∼ 0.3 and ∼ 3 TeV and should show an absorption cutoff above ∼ 6 TeV. We also discuss the determination of the γ-ray opacity at higher redshifts (out to z = 3), following the treatment of Salamon and Stecker.