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Paper: Evidence for Helical B Fields in AGN Jets from Faraday-Rotation Gradients
Volume: 345, From Clark Lake to the Long Wavelength Array: Bill Erickson's Radio Science
Page: 264
Authors: Gabuzda, D.C.
Abstract: It has long been known that some radio-loud Active Galactic Nuclei (AGN) show a tendency for the magnetic (B) fields in their parsec-scale jets to be orthogonal to the local jet direction. Although this was initially interpreted as reflecting the presence of transverse shocks in the jets, it now appears more likely that many of these transverse B fields represent the nt toroidal component of the intrinsic B fields of the jets themselves. The most recent evidence supporting this hypothesis is the observation of systematic gradients in the Faraday rotation across the parsec-scale jets of a number of AGN, which we believe are due to the systematic change in the line-of-sight component of a toroidal or helical jet B field across the jet. Such fields would come about in a natural way as a result of the “winding up” of an initial “seed” field by the rotation of the central accreting object. The detection of toroidal or helical jet B fields is a result of cardinal importance for our understanding of the jets of AGN that was made possible by measurements of a fundamentally low-frequency phenomenon — Faraday rotation. These measurements required polarization observations at relatively low frequencies with high resolution that could be accurately calibrated. This underscores the importance of polarization sensitivity for future low-frequency instruments currently being planned.
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