Active Galaxies as Particle Accelerators
  We present the theoretical arguments and describe the accumulating experimental evidence that jets, powered by supermassive black holes, are true cosmic accelerators. They produce photons of TeV energy, possible higher, and may be the enigmatic source of the highest energy cosmic rays. The features of the multi-wavelength emission spectrum are dictated by the interactions of electrons and protons, accelerated in the vicinity of the black hole, with the ambient light in the galaxy. Photoproduction of neutral pions by protons on UV light is the source of the highest energy photons, in which most of the bolometric luminosity of the galaxy may be emitted. They initiate an electromagnetic cascade which via pair production on the magnetic field and photon-photon interactions determines the emerging gamma-ray spectrum at lower energies. The lower energy photons, observed by conventional astronomical techniques, are, as a result of the cascade process, several generations removed from the primary high energy beams. The case that proton beams power active galaxies is far from conclusive. A three-prong technological assault on the problem will, in the near future, hopefully provide new insights into the structure of active galactic nuclei with the construction of second-generation atmospheric Cherenkov telescopes, large-scale EeV cosmic ray detectors and high energy neutrino telescopes. 
F. Halzen