  A fraction of AGN producing VHE gamma-rays are located in galaxy clusters.
The magnetic field present in the intra-cluster medium would lead to
conversions of VHE photons into axion-like particles (ALPs), which are a
generic prediction of several extensions of the Standard Model. ALPs produced
in this way would traverse cosmological distances unaffected by the
extragalactic background light at variance with VHE photons which undergo a
substantial absorption. Eventually, a nontrivial fraction of ALPs would
re-convert into VHE photons in the magnetic field of the Milky Way. This
mechanism produces a significant hardening of the VHE spectrum of AGN in galaxy
clusters. As a specific example we consider the energy spectra of two observed
VHE gamma-ray sources located in galaxy clusters, namely 1ES 0414+009 at
redshift z=0.287 and Mkn 501 at z=0.034. We find that the hardening in the
observed spectra becomes relevant at E > 1 TeV. The detection of this signature
would 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 the
presently operating Imaging Atmospheric Cherenkov Telescopes like H.E.S.S.,
MAGIC, VERITAS and CANGAROO-III and even more likely with the planned detectors
like CTA, HAWC and HiSCORE. An independent laboratory check of ultra-light ALPs
invoked in this mechanism can be performed with the planned upgrade of the
photon regeneration experiment ALPS at DESY and with the next generation solar
axion detector IAXO.
