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M.F. Aller (U. Michigan)
We review results from recent studies of radioband variability, in all 4 Stokes parameters, which focus on investigating the physical conditions in Blazars and on identifying class-dependent differences between QSOs and radio-bright BL Lacs. Within the framework of a scenario based on relativistic jet flow: 1) The observed range in flux and linear polarization can be accounted for by weak, propagating shocks oriented obliquely to the flow direction, without invoking shear to explain apparent magnetic field alignment parallel to the flow direction, found in some QSOs; 2) Longterm preferred orientations of the electric vector of the polarized emission relative to the flow direction are identified from multi-decade data, suggesting that a dominant magnetic field alignment persists over distinct events; statistically this offset differs for QSOs and BL Lacs, supporting class-dependent alignments found from VLBI studies of individual components; 3) Increased source opacity, identified by multifrequency flux and linear polarization observations, is temporally associated with enhanced levels of circularly-polarized emission (CP), supporting the view that CP results from linear-to-circular mode conversion in a partially opaque region of the source. Changes in polarity in the Stokes parameter V are apparent in dual frequency CP observations, suggesting that the magnetic field does not retain its handedness over time, whereas no polarity change might be expected for plasma permeated by magnetic fields generated near the central engine; 4) Temporally associated activity in the X-ray and radioband in 3C 120 are identified using simultaneous RXTE monitoring; this suggests a causal relationship, plausibly yielding the first connection between activity in the radio jet and the accretion disk/central engine.
This research has been supported in part by grants from the NSF and from NASA. NRAO is operated by AUI, under cooperative agreement with the NSF. UMRAO is supported by funds from the University of Michigan Astronomy Department.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.