Photons may convert into axion-like particles and back in the magnetic field of various astrophysical objects, including active galaxies, clusters of galaxies, intergalactic space and the Milky Way. This is a potential explanation for the candidate neutral ultra-high energy (E>10¹⁸ eV) particles from distant BL Lac type objects which probably have been observed by the High Resolution Fly's Eye experiment. Axions of the same mass and coupling may explain also TeV photons detected from distant blazars.
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Fairbairn et al. (2011) studied this question.
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