Linear Polarization Properties of Parsec-Scale AGN Jets

Alexander Pushkarev,Tuomas Savolainen,Yuri Kovalev,Hugh Aller,Matthew Lister,Mary Hodge,Margo Aller

doi:10.3390/galaxies5040093

Alexander Pushkarev, Tuomas Savolainen + Show 5 more

Open Access

https://doi.org/10.3390/galaxies5040093

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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.

Highlights

IntroductionDespite substantial progress in the understanding of the phenomenon of the jets of active galactic nuclei (AGN) achieved over the last several decades with the method of very long baseline interferometry (which probes jets from sub- to several hundred parsec scales) the question of how jets are launched, confined and collimated remains an active area of research
Despite substantial progress in the understanding of the phenomenon of the jets of active galactic nuclei (AGN) achieved over the last several decades with the method of very long baseline interferometry the question of how jets are launched, confined and collimated remains an active area of research
Since a jet expands with distance, magnetic field decreases as B ∝ r −b (b = 1 for a pure toroidal field, b = 2 for a pure axial field) but becomes more regular, as is evident from the observed increase in fractional polarization

Summary

Introduction

Despite substantial progress in the understanding of the phenomenon of the jets of active galactic nuclei (AGN) achieved over the last several decades with the method of very long baseline interferometry (which probes jets from sub- to several hundred parsec scales) the question of how jets are launched, confined and collimated remains an active area of research. The dominant acceleration mechanisms, which could be steady or impulsive in nature, are still unknown. All these questions are expected to be tightly connected with the key agent of jet dynamics, the magnetic (B) field. Construction of the Very Long Baseline Array (VLBA) in 1994 has allowed polarimetric observations of large samples of AGN jets on a regular basis facilitating detailed studies of the polarization characteristics of jet synchrotron radio emission. Galaxies 2017, 5, 93 publication, we analyze the polarization properties of a large sample of AGN jets mainly observed within the MOJAVE program [1] with the VLBA at 15 GHz

Observational Data and the Sample

Results

Discussion