Pair-production opacity at high and very-high gamma-ray energies

The propagation of high energy (HE, $E_\gamma>100$ MeV) and very high-energy gamma-rays (VHE, $E_\gamma>100$ GeV) in the extra-galactic photon field leads to pair-production and consequently energy- and distance-dependent attenuation of the primary intensity. The spectroscopy of an increasing number of extra-galactic objects at HE and VHE energies has demonstrated indeed the presence of such an attenuation which in turn has been used to constrain the photon density in the medium. At large optical depth ($\tau\gtrsim 2$) potential modifications of pair-production due to competing but rare processes (as, e.g., the presence of sub-neV axion-like particle) may be found. Indications for a pair-production anomaly have previously been found with VHE-spectra. Here, we present further indications (at the level of $3.68 \sigma$) for a reduced optical depth at high energies from an analysis of Fermi-\textit{LAT} data.