Relativistic Phenomena in Active Galactic Nuclei
  The idea that the radio jets in AGN contain material in relativistic motion is supported by many lines of observational evidence, including morphology, brightness temperature estimated with interferometers and with intrinsic variations, interstellar scintillations, X-rays, and superluminal motion. These are largely independent, and taken together make an irrefutable case for relativistic motion. 
M. H. Cohen (Caltech), M. L. Lister (Purdue) and R. C. Vermeulen   (ASTRON)