Parsec-scale Faraday rotation and polarization of 20 active galactic nuclei jets

TL;DR

This study uses VLBA polarimetry across multiple frequencies to analyze Faraday rotation and polarization in 20 AGN jets, revealing complex magnetic field structures and high RM values near cores, advancing understanding of jet magnetism.

Contribution

It provides detailed parsec-scale polarization and Faraday rotation measurements of AGN jets, highlighting the presence of helical magnetic fields and complex Faraday structures, which were not previously well characterized.

Findings

Core RM up to 16,900 rad/m^2 indicates dense, magnetized media.

Transverse RM gradients observed in seven sources.

Jets contain large-scale ordered magnetic fields, often aligned with the jet flow.

Abstract

We perform polarimetry analysis of 20 active galactic nuclei (AGN) jets using the Very Long Baseline Array (VLBA) at 1.4, 1.6, 2.2, 2.4, 4.6, 5.0, 8.1, 8.4, and 15.4 GHz. The study allowed us to investigate linearly polarized properties of the jets at parsec-scales: distribution of the Faraday rotation measure (RM) and fractional polarization along the jets, Faraday effects and structure of Faraday-corrected polarization images. Wavelength-dependence of the fractional polarization and polarization angle is consistent with external Faraday rotation, while some sources show internal rotation. The RM changes along the jets, systematically increasing its value towards synchrotron self-absorbed cores at shorter wavelengths. The highest core RM reaches 16,900 rad/m^2 in the source rest frame for the quasar 0952+179, suggesting the presence of highly magnetized, dense media in these regions. The typical RM of transparent jet regions has values of an order of a hundred rad/m^2. Significant transverse rotation measure gradients are observed in seven sources. The magnetic field in the Faraday screen has no preferred orientation, and is observed to be random or regular from source to source. Half of the sources show evidence for the helical magnetic fields in their rotating magnetoionic media. At the same time jets themselves contain large-scale, ordered magnetic fields and tend to align its direction with the jet flow. The observed variety of polarized signatures can be explained by a model of spine-sheath jet structure.