Derivation of a relation for the Steepening of Tev-selected Blazar γ-ray spectra with energy and redshift

Abstract

We derive a relation for the steepening of blazar γ-ray spectra between the multi-GeV Fermi energy range and the TeV energy range observed by atmospheric Cerenkov telescopes. The change in spectral index is produced by two effects: (1) an intrinsic steepening, independent of redshift, owing to the properties of emission and absorption in the source and (2) a redshift-dependent steepening produced by intergalactic pair production interactions of blazar γ-rays with low-energy photons of the "intergalactic background light" (IBL). Given this relation, with good enough data on the mean γ-ray spectral energy distribution of TeV-selected BL Lac objects, the redshift evolution of the IBL can, in principle, be determined independently of stellar evolution models. We apply our relation to the results of new Fermi observations of TeV-selected blazars. © 2010. The American Astronomical Society. All rights reserved.