Astrophysics - High Energy Astrophysical Phenomena, High Energy Physics - Phenomenology
Beschreibung:
The propagation of high energy (HE, Eγ>100 MeV) and very high-energy gamma-rays (VHE, Eγ>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 (τ≳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 3.68σ) for a reduced optical depth at high energies from an analysis of Fermi-LAT data.