A fraction of active galactic nuclei producing very high-energy (VHE) $\gamma$ rays are located in galaxy clusters. The magnetic field present in the intracluster medium would lead to conversions of VHE photons into axionlike 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 reconvert into VHE photons in the magnetic field of the Milky Way. This mechanism produces a significant hardening of the VHE spectrum of active galactic nuclei 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 $E\gtrsim 1\mathrm{TeV}$. The detection of this signature would allow one to indirectly probe the existence of ultralight ALPs with mass $m_a\lesssim {10}^{-8}\mathrm{eV}$ and photon-ALP coupling $g_{a\gamma }\lesssim {10}^{-10}{\mathrm{GeV}}^{-1}$ 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 Cherenkov Telescope Array, High Altitude Water Cherenkov Experiment and Hundred*i Square-km Cosmic ORigin Explorer. An independent laboratory check of ultralight ALPs invoked in this mechanism can be performed with the planned upgrade of the photon regeneration experiment Any Light Particle Search at Deutsches Elektronen-Synchrotron and with the next generation solar axion detector International Axion Observatory.

• Received 16 July 2012

DOI:https://doi.org/10.1103/PhysRevD.86.075024