3.3. Limits on Bulk Flow Velocities from Brightness and Separation Ratios
An upper limit to the average advance speed of the hot-spots in FRII sources, h, can be derived from the distribution of arm-length ratios, which includes a contribution from light-travel effects. Typical values are h < 0.3c (e.g. Longair & Riley 1979). The distribution is almost certainly dominated by intrinsic factors, rather than by light-travel, and the limit is therefore rather crude.
The overall symmetry in lobe flux can also be used to argue that the majority of the low surface brightness emission comes from material whose mean flow velocity is not highly relativistic. This applies to lobes in FRI and FRII sources and to weak-flavor jets. Typical limits are 0.2c.