Transverse Faraday-Rotation Gradients Across the Jets of 15 Active Galactic Nuclei

TL;DR

This study confirms the presence of transverse Faraday rotation measure gradients across jets in 15 active galactic nuclei, supporting the existence of helical magnetic fields, with rigorous error analysis validated by recent Monte Carlo simulations.

Contribution

The paper provides new measurements of transverse RM gradients in 15 AGN jets, including some first-time observations, with a thorough error analysis using Monte Carlo simulations.

Findings

All 15 AGN jets show monotonic transverse RM gradients with at least 3 sigma significance.

Previous uncertainties in RM gradient measurements have been addressed and validated through recent simulations.

The results support the presence of helical magnetic fields in AGN jets.

Abstract

The presence of a helical magnetic field threading the jet of an Active Galactic Nucleus (AGN) should give rise to a gradient in the observed Faraday rotation measure (RM) across the jet, due to the associated systematic change in the line-of-sight magnetic field. Reports of observations of transverse RM gradients across AGN jets have appeared in the literature starting from 2002, but concerns were raised about the resolution required for these gradients to be reliable, and there was a lack of a full understanding of the best approach to accurate estimation of the uncertainties of local RM values. These questions have now been resolved by recent Monte Carlo simulations carried out by various groups, enabling both a verification of previously published results and reliable analyses of new data. We consider here RM gradients across the jet structures of 15 AGN, some previously published in the refereed literature but without a correct and complete error analysis, and some published for the first time here, all of which have monotonic transverse RM gradients with significances of at least 3 sigma.