Linear Polarization Properties of Parsec-Scale AGN~Jets

TL;DR

This study analyzes 20 years of VLBA polarization data of 484 AGN jets, revealing polarization increases along jets, differences between quasars and BL Lacs, and the role of shocks in magnetic field alignment.

Contribution

It provides a comprehensive analysis of parsec-scale AGN jet polarization properties using multi-epoch VLBA data, highlighting differences between AGN types and the impact of shocks.

Findings

Polarization increases with distance from the core.

BL Lacs have higher polarization and more stable EVPAs than quasars.

Shocks likely enhance magnetic field order in jets.

Abstract

We used 15 GHz multi-epoch Very Long Baseline Array (VLBA) polarization sensitive observations of 484 sources within a time interval 1996--2016 from the MOJAVE program, and also from the NRAO data archive. We have analyzed the linear polarization characteristics of the compact core features and regions downstream, and their changes along and across the parsec-scale active galactic nuclei (AGN) jets. We detected a significant increase of fractional polarization with distance from the radio core along the jet as well as towards the jet edges. Compared to quasars, BL Lacs have a higher degree of polarization and exhibit more stable electric vector position angles (EVPAs) in their core features and a better alignment of the EVPAs with the local jet direction. The latter is accompanied by a higher degree of linear polarization, suggesting that compact bright jet features might be strong transverse shocks, which enhance magnetic field regularity by compression.