The average I and P profiles of 3C353's jets are modeled well by the emission from a thick, constant-depth, boundary layer containing a random magnetic field with no component perpendicular to the jet flow. We associate this layer with a region where is suppressed by velocity shear (presumably faster flow at the center of the jet and slower flow at its surface). The roughly constant layer depth and degree of linear polarization along the jet, plus the absence of , imply that the radiating layer does not contain large-scale turbulent eddies associated with entrainment of ambient material (De Young 1996).
The model has a simple relation to the decelerating relativistic-jet model for the first few kpc of FRI (plumed) sources (Laing 1993, Laing 1996, Laing & Bridle, in preparation). The field configuration () in the outer layer is similar to that inferred by Laing & Bridle in the outer layer of the rapidly-spreading jets in the FRI radio galaxy 3C31, but in 3C353 the layer has constant depth and the jets spread slowly. The apparently low emissivity of 3C353's jet spine could also be due to Doppler "hiding'' of a faster (relativistic) flow near the jet axis.
If parts of 3C353's jets move relativistically, we have modeled their appearance in our rest frame. Relativistic aberration modifies the angle of the fields to the line of sight, so our best fit may correspond to to a different ratio of to in the jet's frame, depending on the velocity field in the outer layer. The modification will be within our uncertainties if the velocities in the outer layer are not too high.
For example, the parameter ranges that fit our data to within their errors imply that we could fit the observed profiles with a jet at to the sky, v < 0.5c and everywhere in the outer layer, and v > 0.8c in the spine (to produce the Doppler hiding of this region).
The spine field geometry is poorly constrained by our data, except that it cannot be dominated by . If everywhere in the outer layer, then Doppler favoritism would also produce a 2:1 jet-counterjet intensity asymmetry. As the observed counterjet in 3C353 is not just a faint replica of the main jet, some intrinsic asymmetry must also be present. Parts, or all, of the outer-layer flow may therefore have v < 0.5c.
A relativistic-jet scenario in which the fields in the jet's frame might differ substantially from our model is that of a high-velocity (v >~ 0.8c) outer-layer flow close enough to the plane of the sky that Doppler favoritism is small (to keep a 2:1 intensity ratio between the jets) but in which the aberration is large.