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H.D. Aller, M.F. Aller, P.A. Hughes (U. Mich.)
Early Very Long Baseline Interferometry Polarization measurements by Gabuzda and others indicated that many BL Lacertae type objects preferentially exhibited polarized jet components in which the magnetic field is aligned perpendicular to the jet axis, while Quasi Stellar Objects often exhibited polarized structures where the magnetic field is predominantly parallel to the jet axis. Recent VLBI observations by many investigators reveal that extragalactic jets often contain curved or bent structures, and these complex structures are often accompanied by large deviations of the EVPAs of the polarized emission within the jets from alignment either along or perpendicular to the jet axis. One possible explanation of the difference between jet components with perpendicular or parallel fields has been the interplay between transverse shocks which align the magnetic fields in the perpendicular direction and the effects of shear in the jet flow which are believed to align the magnetic fields in the direction parallel to the jet axis. However, neither mechanism would produce EVPAs in other directions (neither parallel nor perpendicular to the jet axis), and the bent structures suggest the presence of oblique shocks in the jets. Following earlier work by Cawthorne & Cobb (ApJ, 350, 536, 1990), we present evidence that these deviations in EVPA can be produced by oblique shocks in the jet flows that are comparable in strength to those used in transverse shock models (e.g. Hughes, Aller, & Aller, ApJ, 341, 54, 1989). We also find that with the inclusion of relativistic aberration effects, even relatively weak oblique shocks can produce EVPAs which appear to be perpendicular to the jet axis. We suggest that oblique shocks may be the major mechanism for magnetic field alignment in jets, and that flow shear is not required to explain the polarization behavior of extragalactic jets.
This work was supported in part by NSF grant AST9900723.