Broadband radio spectro-polarimetric observations of high Faraday rotation measure AGN

TL;DR

This study uses broadband radio polarimetric observations to analyze high Faraday RM AGN, revealing complex spectral structures and turbulent magnetic fields, and introduces a new qu-fitting technique to probe their magnetized environments.

Contribution

The paper presents a novel broadband polarimetric analysis of high RM AGN and introduces a new qu-fitting method to distinguish depolarization mechanisms and probe jet magnetic fields.

Findings

Most sources show depolarization caused by turbulent magnetic fields.

Complex spectral structures with multiple synchrotron and Faraday components are observed.

The new qu-fitting technique effectively probes the magnetized environment of AGN.

Abstract

We present broadband polarimetric observations of a sample of high Faraday rotation measure (RM) AGN using the VLA telescope from 1 to 2 GHz, and 4 to 12 GHz. The sample (14 sources) consists of compact sources (linear resolution smaller than ~ 5 kpc) that are unpolarized at 1.4 GHz in the NVSS. Total intensity data have been modelled using combination of synchrotron components, revealing complex structure in their radio spectra. Depolarization modelling, through the so called qu-fitting, have been performed on the polarized data using an equation that attempts to simplify the process of fitting many different depolarization models that we can divide into two major categories: External Depolarization and Internal Depolarization models. Understanding which of the two mechanisms are the most representative, would help the qualitative understanding of the AGN jet environment, whether it is embedded in a dense external magnetoionic medium or if it is the jet-wind that causes the high RM and strong depolarization. This could help to probe the jet magnetic field geometry (e.g. helical or otherwise). This new high-sensitivity data, shows a complicated behaviour in the total intensity and polarization radio spectrum of individual sources. We observed the presence of several synchrotron components and Faraday components in their total intensity and polarized spectra. For the majority of our targets, (12 sources) the depolarization seems to be caused by a turbulent magnetic field. Thus, our main selection criteria (lack of polarization at 1.4 GHz in the NVSS), results in a sample of sources with very large RMs and depolarization due to turbulent magnetic fields local to the source. We show how the new qu-fitting technique can be used to probe the magnetised radio source environment and to spectrally resolve the polarized components of unresolved radio sources.