MOJAVE. XX. Persistent Linear Polarization Structure in Parsec-scale AGN Jets

TL;DR

This study analyzes the long-term linear polarization properties of 436 AGN jets using VLBA data, revealing stable magnetic field structures, helical fields, and differences between BL Lacs and quasars over a 23-year period.

Contribution

It provides the first extensive, stacked polarization maps of AGN jets, uncovering persistent magnetic field configurations and transverse polarization profiles with implications for jet physics.

Findings

Polarization increases towards jet edges, indicating helical magnetic fields.

BL Lacs have higher polarization than quasars on small scales.

Polarization and EVPA distributions are stable over long timescales.

Abstract

We analysed the parsec-scale linear polarization properties of 436 active galactic nuclei (AGN) based on 15 GHz polarimetric Very Long Baseline Array (VLBA) observations. We present polarization and total intensity images averaged over at least five epochs since 1996 January 19 through 2019 August 4. Stacking improves the image sensitivity down to $\sim$30 $\mu$Jy/beam and effectively fills out the jet cross-section both in total intensity and linear polarization. It delineates the long-term persistent magnetic field configuration and its regularity by restoring spatial distributions of the electric vector position angle (EVPA) and fractional polarization, respectively. On average, about ten years of stacking period is needed to reveal the stable and most-complete polarization distribution of a source. We find that the degree of polarization significantly increases down and across the jet towards its edges, typically manifesting U or W-shaped transverse profiles, suggesting a presence of a large-scale helical magnetic field associated with the outflow. In some AGN jets, mainly BL Lacs, we detect quasi-constant fractional polarization profiles across the jet, accompanied by EVPAs that closely follow the outflow. BL Lacs show higher fractional polarization values in their cores and jets than those in quasars up to hectoparsec de-projected scales, while on larger scales, they become comparable. High-synchrotron-peaked BL Lac jets are found to be less polarized than intermediate and low-synchrotron-peaked BL Lacs. The spatial distribution of the EVPAs in BL Lacs tend to align with the local jet direction, while quasars show an excess of orthogonal polarization orientation.