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Next: Discussion Up: Internal Structure of the Jets in 3C353 Previous: Origin of the Polarization "Rails''


The correlation shown in Figure 3, the slow (zero) spreading rate of the jet, and the constant separation of the "rails" are consistent with a jet whose structure is approximately self-similar everywhere between J1 and J4. We therefore modeled the averaged transverse I and P profiles, rather than individual profiles, to improve our signal-to-noise.

We use tex2html_wrap_inline489 coordinates in a cylindrical jet axisymmetric about the z axis and inclined by angle i to the plane of the sky. The jet is populated by relativistic electrons with a power-law energy distribution, a specified emissivity distribution, and a specified magnetic field geometry, including both organized and random components. The transverse I, Q and U profiles are computed by integrating 1301 equal cells along each of 131 equally-spaced lines of sight through the jet, and are then convolved to the resolution of our data. We matched the index of the power-law electron energy distribution to the flattest spectral index associated with the jets (tex2html_wrap_inline501); our results are insensitive to this choice.

Predicted and observed transverse profiles are compared
Figure 4: Predicted transverse I and P profiles of the model with zero emissivity in the spine, whose boundary is here at r=26 (0.43 that of the jet), compared with the observed profiles. The model profiles have been convolved to the same tex2html_wrap_inline399 FWHM resolution as the observations of the jet (upper panel) and counterjet (lower panel).

Figure 4 shows the average transverse I and P profiles that we infer by assuming that the lobe magnetic field is perpendicular to both jets everywhere. These profiles are fitted well by a model (also shown) in which

  1. the jets are in the plane of the sky (i = 0°),
  2. the radiating particles are restricted to an outer layer ( tex2html_wrap_inline517 of the radius of the jet),
  3. this outer layer has no radial field component tex2html_wrap_inline519,
  4. tex2html_wrap_inline521 (toroidal) and tex2html_wrap_inline523 (axial) in the layer are uniformly distributed zero-mean random variables normalized so that tex2html_wrap_inline525.

The value of  Inner radius of boundary layer in this model is a compromise between the optimal fits for the jet and the counterjet. For the jet, the ranges of parameters that fit the observed I and P profiles within their 2tex2html_wrap_inline475 errors everywhere, holding the other parameters fixed, are

and tex2html_wrap_inline539.

The best-fitting values of  Inner radius of boundary layer in the counterjet are tex2html_wrap_inline479% smaller.

Such models with no emission from the jet "spine'' at tex2html_wrap_inline545 predict the observed I profiles to within their errors, but in detail the model profiles are too edge-brightened. Better fits are obtained if the spine also radiates, but with an emissivity less than half that of the outer layer, and if the spine field has tex2html_wrap_inline549.

The constraints imposed by fitting the I and P profiles simultaneously are strong, so it is unlikely that any fundamentally dissimilar models of magnetic field organization and relative emissivity will fit our data so well. A smaller ratio of tex2html_wrap_inline521 to tex2html_wrap_inline523 can however be traded against orienting the jet away from the plane of the sky to some extent: models with tex2html_wrap_inline523 as the only field component in the radiating layer can reproduce the observed flat-topped I profiles, but they over-predict the degree of linear polarization at the rail centers, typically by a factor of two.

We also computed the emission from several well-ordered field structures previously proposed for large-scale jets, including helical fields and transverse self-similar "flux ropes'' (Chan & Henriksen 1980). We found none that simultaneously produced flat-topped I profiles and symmetric P profiles with ~ 20 to 30% polarization (Swain 1996).

next up previous
Next: Discussion Up: Internal Structure of the Jets in 3C353 Previous: Origin of the Polarization "Rails''
Tue Sep 1 10:56:44 EDT 1998