The evidence of Chap. 21 shows that the huge fluxes of relativistic material needed to power extended extragalactic radio sources originate close to the active galactic nuclei of the host galaxies. Direct evidence for extreme events in active galactic nuclei is provided by the superluminal motions observed in compact radio sources, by the properties of variable extragalactic γ-ray sources and by the γ-ray bursts. The extreme properties of these sources require them to be moving at highly relativistic velocities.

Compact radio sources

Direct evidence for the presence of ultra-relativistic electrons in the nuclei of active galaxies is provided by very long baseline interferometric (VLBI) studies of radio quasars and BL-Lac objects at centimetre wavelengths. Combining the angular sizes of these ultra-compact radio sources with their flux densities Sv, the brightness temperature Tb = (λ2/2k)(Sv/Ω) of the source region can be determined, where Ω is the solid angle subtended by the radio source. Observations of large samples of strong compact radio sources with structures on the scale of 1 milliarcsecond have shown that the maximum brightness temperatures are of the order of 1011–1012 K, none of them exceeding the limit of 1012 K at which catastrophic synchrotron self-Compton radiation would take place, as described in Sect. 9.6 (Kellermann et al., 1998).