Are There Rotation Measure Gradients Across AGN Jets?

TL;DR

This study uses multi-frequency VLBA polarimetry to investigate Faraday RM across AGN jets, finding no evidence of RM gradients in several sources, challenging previous claims of widespread helical magnetic fields.

Contribution

It introduces objective criteria for identifying RM gradients and compares observational results with simulations to understand RM variations.

Findings

No RM gradients found in the observed AGN jets.

Contrasts with previous literature claiming widespread RM gradients.

Proposes criteria for defining genuine RM gradients.

Abstract

We report on multi-frequency polarimetry Very Long Baseline Interferometry observations of active galactic nuclei using the VLBA. These observations are used to construct images of the Faraday Rotation Measure (RM) in J1613+342, Mrk 501, 3C 371, and BL Lac. Despite having resolved the jets in total intensity and polarization for three of these sources no RM gradients are found. This is in contrast to the large fraction of sources with RM gradients now claimed in the literature, and invoked as evidence in support of helical magnetic fields. We propose objective criteria for establishing what constitutes an RM gradient. Furthermore, although we note the absence of simple, monotonic gradients, comparison with simulations could reveal systematic changes in the RM which may be masked by a varying jet orientation.