Implications of Cosmological Gamma-Ray Absorption II. Modification of gamma-ray spectra

Abstract

Bearing on the model for the time-dependent metagalactic radiation field developed in the first paper of this series, we compute the gamma-ray attenuation due to pair production in photon-photon scattering. Emphasis is on the effects of varying the star formation rate and the fraction of UV radiation assumed to escape from the star forming regions, the latter being important mainly for high-redshift sources. Conversely, we investigate how the metagalactic radiation field can be measured from the gamma-ray pair creation cutoff as a function of redshift, the Fazio-Stecker relation. For three observed TeV-blazars (Mkn501, Mkn421, H1426+428) we study the effects of gamma-ray attenuation on their spectra in detail.