A lot of high luminosity extragalactic radio sources have jets on one side only, while they are symmetric in many respects. This phenomenon can be explained by different models. One possibility is the fact that the one-sidedness is only apparent due to Doppler beaming. If the source is not seen in the plane of the sky, but is pointed towards the observer, and if the material in the jets have relativistic speeds, then the nearby lobe is brighter than the distant one. Especially if the angle of view is small this effect can be quite dramatic. The other possibility is of course that the jets are intrinsically asymmetric. The dissipation on one side could be stronger or the outflow of material from the core can flip alternately between the lobes.
In 1988 Laing and Garrington reported that almost invariably the jet side depolarizes less rapidly with increasing wavelength than the opposite side. Differences in the polarization characteristics of the components of double sources had been noted before, but the connection with jet-sidedness only becomes apparent when maps with a high enough resolution are compared with the polarization data.
The sources that these authors used were all high-luminosity FR-II type objects
with small angular sizes ( arcsec). The redshifts were typically between
1 and 2. They included 23 quasars and 2 radiogalaxies. At least 23, and
possibly 24 out of the 25 sources showed less depolarization on the jet side,
and there were no clear counter-examples. The depolarization did not correlate
with the size or brightness of the components. It is known that depolarization
may become stronger near the core. The thermal plasma that is responsible for
the depolarization can be within the radio components (internal depolarization)
or in an irregular foreground screen (external depolarization). If the effect
of relativistic beaming is important then you expect that the two lobes are
intrinsically similar. Asymmetric depolarization then occurs in a foreground
screen.
The visible jet always points towards the observer. As a result of this the counterjet is always seen through more of this screen and hence has a higher Faraday dispersion. The screen can be made under several physical conditions. They include an emission-line disk perpendicular to the radio axis, a cocoon of thermal plasma surrounding the emitting regions or a halo of X-ray emitting gas around the central object.
If, on the other hand, jets are intrinsically one-sided, then different internal Faraday depths lead, of course, to differences in the two lobes. With this hypothesis you don't need a screen to cause the asymmetric depolarization.
