Super-GZK photons from photon–axion mixing

We show that photons with energies above the GZK cutoff can reach us from very distant sources if they mix with light axions in extragalactic magnetic fields. The effect which enables this is the conversion of photons into axions, which are sufficiently weakly coupled to travel large distances unimpeded. These axions then convert back into high energy photons close to the Earth. We show that photon–axion mixing facilitates the survival of super-GZK photons most efficiently with a photon–axion coupling scale M ≳ 10 11 GeV, which is in the same range as the scale for the photon–axion mixing explanation for the dimming of supernovae without cosmic acceleration. We discuss possible observational consequences of this effect.