Axion-like particle imprint in cosmological very-high-energy sources
  Discoveries of very high energy (VHE) photons from distant blazars suggest
that, after correction by extragalactic background light (EBL) absorption,
there is a flatness or even a turn-up in their spectra at the highest energies
that cannot be easily explained by the standard framework. Here, it is shown
that a possible solution to this problem is achieved by assuming the existence
of axion-like particles (ALPs) with masses ~1 neV. The ALP scenario is tested
making use of observations of the highest redshift blazars known in the VHE
energy regime, namely 3C 279, 3C 66A, PKS 1222+216 and PG 1553+113. In all
cases, better fits to the observed spectra are found when including ALPs rather
than considering EBL only. Interestingly, quite similar critical energies for
photon/ALP conversions are also derived, independently of the source
considered.
