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A fraction of AGN producing VHE gamma-rays are located in galaxy clusters.The magnetic field present in the intra-cluster medium would lead toconversions of VHE photons into axion-like particles (ALPs), which are ageneric prediction of several extensions of the Standard Model. ALPs producedin this way would traverse cosmological distances unaffected by theextragalactic background light at variance with VHE photons which undergo asubstantial absorption. Eventually, a nontrivial fraction of ALPs wouldre-convert into VHE photons in the magnetic field of the Milky Way. Thismechanism produces a significant hardening of the VHE spectrum of AGN in galaxyclusters. As a specific example we consider the energy spectra of two observedVHE gamma-ray sources located in galaxy clusters, namely 1ES 0414+009 atredshift z=0.287 and Mkn 501 at z=0.034. We find that the hardening in theobserved spectra becomes relevant at E > 1 TeV. The detection of this signaturewould allow to indirectly probe the existence of ultra-light ALPs with mass m_a< 10^{-8} eV and photon-ALP coupling g_{a gamma} < 10^{-10} GeV^{-1} with thepresently operating Imaging Atmospheric Cherenkov Telescopes like H.E.S.S.,MAGIC, VERITAS and CANGAROO-III and even more likely with the planned detectorslike CTA, HAWC and HiSCORE. An independent laboratory check of ultra-light ALPsinvoked in this mechanism can be performed with the planned upgrade of thephoton regeneration experiment ALPS at DESY and with the next generation solaraxion detector IAXO.

Hardening of TeV gamma spectrum of AGNs in galaxy clusters by conversions of photons into axion-like particles