# 18-22 cm VLBA Faraday rotation studies of six AGN jets

## TL;DR

This study uses VLBA observations at 18-22 cm to analyze Faraday rotation in six AGN jets, revealing evidence of toroidal magnetic fields likely linked to helical magnetic structures.

## Contribution

It presents the first detection of transverse Faraday rotation gradients across multiple AGN jets at these wavelengths, indicating the presence of toroidal magnetic fields.

## Key findings

- Transverse Faraday rotation gradients detected in four AGN jets.
- Evidence supports the existence of toroidal magnetic fields in AGN jets.
- Potential indication of helical magnetic field structures.

## Abstract

The formation of relativistic jets in active galactic nuclei (AGN) is related to accretion on to their central supermassive black holes, and magnetic fields are believed to play a central role in launching, collimating and accelerating the jet streams from very compact regions out to kiloparsec or megaparsec scales. In the presence of helical or toroidal magnetic fields threading the AGN jets and their immediate vicinity, gradients in the observed Faraday rotation measures are expected due to the systematic change in the line-of-sight component of the magnetic field across the jet. We have analysed total intensity, linear polarization, fractional polarization and Faraday rotation maps based on Very Long Baseline Array data obtained at four wavelengths in the 18-22 cm range for six AGN (OJ 287, 3C 279, PKS 1510-089, 3C 345, BL Lac and 3C 454.3). These observations typically probe projected distances out to tens of parsecs from the observed core, and are well suited for Faraday rotation studies due to the relatively long wavelengths used and the similarity of the structures measured at the different wavelengths. We have identified statistically significant, monotonic, transverse Faraday rotation gradients across the jets of four of these six sources, as well as a tentative transverse Faraday rotation gradient across the jet of OJ 287, providing evidence for the presence of toroidal magnetic fields, which may be one component of helical magnetic fields associated with these AGN jets.