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

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.