Photon-axion conversion in Active Galactic Nuclei?
  Axion-Like Particles (ALPs) are the focus of intense current research. We
analyze photon-ALP conversion in the context of relativistic jet models of
Active Galactic Nuclei (AGN) for more than 100 sources. Contrary to previous
claims, we find that this process cannot occur above 100 GeV regardless of the
actual AGN model and the values of ALP parameters. This result rules out a
proposed strategy to bypass the cosmic opacity above 100 GeV, as apparently
required by observations. We also show that for some AGN an observable effect
can show up in the X and soft gamma-ray bands.
