Astronomy & Astrophysics
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.
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