  We investigate the propagation of a charged particle in a spatially constant,
but time dependent, pseudoscalar background. Physically this pseudoscalar
background could be provided by a relic axion density. The background leads to
an explicit breaking of Lorentz invariance; as a consequence the process p-> p
gamma is possible and the background acts as a shield against extremely
energetic cosmic rays, an effect somewhat similar to the GZK cut-off effect.
The effect is model independent and can be computed exactly. The hypothetical
detection of the photons radiated via this mechanism would provide an indirect
way of verifying the cosmological relevance of axions.
