| Description: | Several studies have shown systematic Faraday Rotation gradients across the parsec-scale jets of several BL Lac objects, interpreted as evidence for helical magnetic B-fields - the gradients were taken to be due to the systematic variation of the line-of-sight B-field across the jet. I present here results for the parsec-scale Faraday Rotation distributions for several BL Lac objects based on polarization data obtained with the Very Long Baseline Array (VLBA) at six wavelengths (between 2 - 6cm) observed in 2003 - 2005. The Rotation Measure maps for several of these sources indicate gradients across their jets, as expected if these jets have helical B- fields. We also observe an interesting new feature in at least seven sources: a reversal in the direction of these gradients that happens either with distance from the core or over time in the jet. This provides new evidence to support ''magnetic-tower'' type models in which field lines emerging from the central region of the accretion disk and closing in the outer region of the accretion disk are both ''wound up'' by the differential rotation of the disk. These results thus provide new insights about the geometry of the jet B-fields and confirmation that these gradients are dynamic. Additional multi-epoch, multi-frequency data for some of these sources confirms the presence of the gradients on both parsec and decaparsec scales, providing additional evidence that these are associated with the overall ''global'' structure of the jet magnetic field. Furthermore, we also find evidence for a helical jet in 0954+658 based on the jet ridge line (in Total Intensity) of this source. Helical Jets can also be explained by helical magnetic fields, which is infact the simplest explanation for all these observations.
(above figure is published in MNRAS by Gabuzda et al. 2008)
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