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4.1. Background

The aim of this section is to discuss recent evidence bearing on the question "why are strong-flavor jets predominantly one-sided?". Three main classes of explanation have been proposed:

Doppler boosting: Jets are intrinsically symmetrical and have relativistic bulk velocities. The nearer jet appears to be brighter as a result of Doppler boosting. Most attention has been focused on this idea, since it makes several simple predictions. With the exception of M 87 (Biretta, these proceedings), there is no direct evidence for apparent motions in excess of c on kiloparsec scales. The indirect arguments are:

Continuity from parsec scales There is good evidence for relativistic flow in VLBI jets (e.g. Marscher, these proceedings) and the continuity of sidedness from parsec to kiloparsec scales implies that the mechanism for generating the asymmetry should be independent of scale.

Depolarization asymmetry Laing (1988) and Garrington et al. (1988, 1991) showed that, in FRII radio sources with one-sided jets (predominantly quasars), the jetted lobe depolarizes less rapidly with increasing wavelength than the unjetted lobe. The obvious explanation (Laing 1988; Garrington & Conway 1991) is that the nearer lobe is seen through less magnetoionic material (hot gas in the galaxy or cluster halo, primarily) and therefore shows less depolarization. This effect is discussed in more detail in the following subsection.

Emission-line velocities van Breugel (1989) noted a tendency for the stronger (or only) jet to be on the side of the source with blueshifted forbidden-line emission and the counter-jet side to have redshifted lines. It is extremely important to confirm this result, as it is the only direct piece of evidence for motion towards the observer associated with the stronger jet (the velocities are, of course, much less than those inferred for the jet and are presumed to be due to interaction with outflowing material).

Asymmetric dissipation: One of the jets radiates a significantly higher proportion of its energy as synchrotron radiation. Models for such a process are not yet well-developed (but see Fraix-Burnet 1992 for a possible approach).

Alternating ejection: Only one jet is active at any time. This idea encounters severe difficulties when applied to objects where the synchrotron lifetimes of electrons in components on both sides of the nucleus are short compared with the light-travel times between components and nucleus. This was originally pointed out by Hargrave & Ryle (1974) for Cygnus A and applies to many other powerful radio sources. The recent detection of optical synchrotron radiation from the counterjet side of M 87 (Stiavelli et al. 1992; Sparks et al. 1992) allows a similar argument to be made for an FRI source.

For the moment, we adopt the hypothesis that strong-flavor jets are symmetrical and relativistic and attempt to test its predictions.

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