Jets, Superluminal Motion, and Gamma-Ray Bursts Radio synthesis maps have shown jets in hundreds of AGN, on scales from subparsec to megaparsecs. The continuity of jets in direction indicates that the central generator has a memory over millions of years, and disk structures provide a natural way to control the direction of the jets. There is a vast literature on the collimation and production of jets; I will mention only a few points here.How fast are they? Structures of jets can indicate their Mach numbers (with respect to the external medium), but not immediately their absolute velocities. Some sources look as if the jets are rather slow and flexible, while others look like highly relativistic blowtorches. We do not even know for sure whether we are seeing a phase or group velocity when motions can be measured. Strong evidence for relativistic bulk motions comes from superluminal sources, in which the projected speed of motion (always outward from the core) of distinct blobs is 1-10c. A natural explanation is (backwards) time dilation in material approaching us at ~0.9
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