Absorption of Very High Energy Gamma-Rays by Intergalactic Infared Radiation: A New Determination

We present a new calculation of the intergalactic γ-ray pair-production absorption coefficient as a function of both energy and redshift. In reexamining this problem, we make use of a new empirially based calculation (as opposed to previous model calculations) of the intergalactic infrared radiation field (IIRF). We find smaller opacities than those given previously (Stecker & De Jager 1997). We apply our results to the new observations of the flaring γ-ray spectra of Mrk 421 and Mrk 501, both at a redshift of ∼0.03. Our new calculations indicate that there should be no significant curvature in the spectra of these sources for energies below 10 TeV, as indicated by recent observations. However, the intrinsic spectra of these sources should be harder by amounts of ∼ 0.25 to 0.45 in the spectral index (in the 1 10 TeV range), with an intergalactic absorption cutoff above ∼ 20 TeV.