4.3. Are All Strong-Flavor Jets Relativistic?
An extremely interesting recent discovery is that FRI radio sources also show depolarization asymmetry, this time in the sense that the side of the source with the stronger jet base depolarizes less rapidly with increasing wavelength. This result was first tentatively suggested by De Ruiter et al. 1989 and has subsequently been confirmed by Parma et al. 1993 and by a search of the published literature. The asymmetry of FRI jet bases, their association with depolarization asymmetry and indirect arguments from "Unified Models" (e.g. Urry, Padovani & Stickel 1991) all suggest that they may have relativistic bulk speeds. This section summarizes the results of Laing (in preparation), who shows that a simple model of a decelerating, relativistic jet can explain a number of the systematics outlined in Table 1.
The velocity profile and field structure assumed in the model (Figure 7) have been suggested previously to explain the observed polarization structure of FRI radio jets. Two antiparallel, but otherwise identical jets propagate away from a galactic nucleus along a direction which makes an angle to the line of sight. The jet cores have velocity cc and contain a magnetic field which has no longitudinal component but is otherwise random (Laing 1980). The surrounding shear layer has velocity sc (s < c) and its field is entirely longitudinal. Both c and s decrease as the jets propagate away from the nucleus. Two effects of relativistic aberration cause the appearance of the jets to change as they slow down: firstly, the relative flux from the two components alters, since their Doppler factors are different and secondly, the degree of polarization of radiation from the core varies, since it is viewed at a changing angle to the line of sight in its rest frame.
Figure 7. A sketch of the magnetic-field and velocity structure assumed for the jet model discussed in the text.
If FRI jets have field and velocity structures of this type and decelerate away from the nucleus, the observable consequences are as follows:
Figure 8. The dependence of total intensity (integrated across the jet) on core speed c for the jet model discussed in the text. The upper and lower curves are for the approaching and receding jets, respectively and the angle to the line of sight is = 60°.
Figure 9. The dependence of the degree of polarization on c for the model discussed in the text. The degree of polarization is positive for B and negative for B||.