Compact jets and the AGN paradigm

The nature of compact, nonthermal jets in active galactic nuclei is reviewed from both observational and theoretical standpoints. Despite rapid variations in brightness, the smallest emission regions in a jet may be relatively far removed from the central engine. Certain models for the formation of a relativistic particle beam that evolves downstream into the observed radio-infrared jet do, however, predict substantial high energy radiation from regions in the beam close to the accretion disk. Observations of such emission, and the general requirement that the jets be well collimated and accelerated to relativistic bulk velocities by the point where they are observed in the radio, can provide indirect constraints on the massive black hole accretion paradigm. Observations of well collimated, semi-relativistic jets in the accreting stellar systems SS 433 and Cyg X-3, as well as slower jets emanating from star forming regions, encourage one to associate jet formation with accretion of gas onto a compact object.