Derivation of a Relation for the Steepening of TeV Selected Blazar
  Gamma-ray Spectra with Energy and Redshift
  We derive a relation for the steepening of blazar gamma-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 gamma-rays with low energy photons of the intergalactic
background light (IBL). Given this relation, with good enough data on the mean
gamma-ray SED of TeV selected BL Lacs, 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.
