Annu. Rev. Astron. Astrophys. 1997. 35: 607-36 Copyright © 1997 by Annual Reviews. All rights reserved

4.3. Polarization Properties

The initial motivation for the application of shock models to VLBI jets came from the modeling of total flux density variability and polarization behavior in sources with comparatively simple jet structure, e.g. BL Lac and 3C 279 (Hughes et al 1989b). Polarization imaging, especially at high frequencies, i.e. with adequate angular resolution, is a prerequisite to understanding the internal physics of sources with more complex VLBI jets (Roberts et al 1991, Wardle et al 1994, Aller et al 1994). Linear polarization-sensitive VLBI observations of compact sources yield direct information on the structure and order of the underlying magnetic fields, the presence and nature of thermal material, the energy of relativistic electrons, and the geometry of emission on submilliarcsec scales. There is evidence for a difference in the polarization properties of quasars and BL Lac objects, and the latter show significantly lower speeds, which argues for a physical and not merely an orientation difference between the two classes of sources (Roberts et al 1990, Gabuzda et al 1994a, b). Brown et al (1994), Wardle et al (1994) have studied the polarization structure at 5 GHz of 3C 345 and interpreted their results in terms of a comprehensive shock model. Leppänen et al (1995) obtained high-resolution polarization images of 3C 345 at 22 GHz with the VLBA. Assuming negligible depolarization in the core, they find the electric field predominantly oriented along the jet, i.e. the magnetic field orientation is perpendicular to the jet, in contrast to the lower frequency results that suggested a parallel magnetic field (Wardle et al 1994). Gabuzda (1995) found similar evidence for a perpendicular magnetic field from imaging 3C 345 at 10.7 GHz. An analysis applying the model of Wardle et al to the 22-GHz and subsequent 43-GHz observations suggests that the shock model remains applicable for the case of a moderately strong shock (KJ Leppänen & JA Zensus, in preparation). The 22-GHz polarized structure of 3C 345 shows two additional important features: a turn in the electric vector position angle and a steep decline to zero of the observed polarized intensity between C8 and C7. Both properties can, within the shock model, be explained by a simple helical motion of the moving feature (Steffen et al 1995). For the quasar 3C454.3, 7-mm VLBA imaging also reveals an electric field configuration that is aligned with the predominant jet orientation but orthogonal between the major components of the compact structure (Kemball et al 1996). In both cases, it is not yet clear if the observed changes depend on the strength of the shock, or if they result from changes in the local magnetic field configuration, a helical jet geometry, or from local differential Faraday rotation.