Using Faraday Rotation Gradients to probe Magnetic Tower Models

TL;DR

This study uses multi-wavelength VLBA polarization observations to detect Faraday Rotation gradients in AGN jets, revealing phenomena consistent with magnetic tower models and advancing understanding of magnetic field structures in these jets.

Contribution

It provides the first observational evidence linking transverse FR gradient reversals to magnetic tower models in AGN jets.

Findings

Detected transverse FR gradient reversals in multiple sources.

Proposed link between gradient behavior and magnetic tower structures.

Suggested that core and jet gradients correspond to different helical fields.

Abstract

Parsec-scale multi-wavelength VLBA polarization observations can be used to study the magnetic-field structures of Active Galactic Nuclei (AGN) based on Faraday Rotation (FR) gradients. A number of transverse FR gradients have been found, and interpreted as corresponding to helical magnetic fields wrapped around the jets; the gradients reflect the systematic change in the line-of-sight component of a toroidal or helical magnetic field across the jet (e.g Gabuzda, Murray & Cronin 2004). Our observations of a sample of BL Lac objects at six wavelengths near 2, 4 and 6 cm have also revealed a previously undetected phenomena: these transverse gradients sometimes change their direction with distance from the core. We have observed this behaviour in at least five sources, which display gradients in their VLBI core region opposite to those in the jet. We suggest that this may be linked to magnetic tower models. In magnetic tower models, the field lines go outward with the jet and return and close in the accretion disk (or vice versa); differential rotation of the accretion disk winds up the inner and outer field lines into two helices (the inner helix "nested" in the outer helix). The total observed FR gradient is a sum of the effect of these two helical fields. It may be that gradients detected relatively far from the core correspond to the outer helix, while gradients detected in the core region correspond to dominance of the inner helix. This provides tentative evidence for the unification of helical magnetic fields and magnetic tower models, which could provide crucial new information for understanding AGN jets. Further VLBI studies with resolution sufficient to reliably detect these gradients in the cm-wavelength core and inner jet will be important for further investigations of this phenomena.