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

We update the list of GeV-TeV extragalactic γ-ray sources using the 2-year catalog from the Fermi-LAT and recent results 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, and 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 Bayeasian model to describe this dataset and use it to constrain two simple models for the EBL-induced breaks.

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