Evidence for a cosmological effect in γ-ray spectra of BL Lacertae

We update the list of GeV-TeV extragalactic γ-ray sources using the two-year catalog from the Fermi Large-Area Telescope (LAT) and recent results from ground-based γ-ray telescopes. Breaks in the spectra between the high-energy (100 MeV < E < 300 GeV) and the very high-energy (E > 200 GeV) ranges, as well as their dependence on distance, are discussed in the context of absorption on the extragalactic background light (EBL). We calculate the size of the expected break using a model for the EBL and compare it to the data, taking into account systematic uncertainties in the measurements. We develop a novel Bayesian model to describe this dataset and use it to constrain two simple models for the EBL-induced breaks.