Probing circular polarization and magnetic field structure in active galactic nuclei

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
² Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
³ National Radio Astronomy Observatory, P.O. Box O Socorro, NM 87801, USA
⁴ Instituto de Astrofísica de Andalucía, Gta. de la Astronomia, s/n, Genil, 18008 Granada, Spain

^⋆ Corresponding author: jah@lanl.gov, jkramer@mpifr.de

Received: 11 August 2023
Accepted: 9 March 2025

Abstract

Context. The composition and magnetic field morphology of relativistic jets can be studied using circular polarization (CP) measurement. Recent three-dimensional relativistic magnetohydrodynamic (3D RMHD) simulations coupled with radiative transfer (RT) calculations make strong predictions about the level (and morphology) of the jet’s CP emission. These simulations show that the sign of CP and the electric vector position angle (EVPA) are both sensitive to the jet’s magnetic field morphology within the radio core.

Aims. We probe this theory by exploring whether the jet’s radio core EVPA orientation is consistent with the observed sign of the core CP in deep full-track polarimetric observations. Based on a selection of sources from earlier MOJAVE observations, we aim to probe the nature of linear polarization (LP) and CP in the innermost regions of jets from a small sample of nine blazars. This sample includes sources that have exhibited: (i) positive CP; (ii) negative CP; or (iii) positive and negative CP simultaneously in the radio core region. By coupling deep polarimetric observations of a carefully selected sample of blazars with state-of-the-art RMHD and RT calculations, we hope to gain a deeper understanding of the physics of blazar jets.

Methods. Nine blazar sources were observed using the VLBA at both 15 GHz and 23 GHz. Standard AIPS calibration was applied. Our self-calibration relies on a physically based model applied in DoG-HiT resulting in more accurate gains. To improve the imaging quality, we used specialized algorithms, such as DoG-HiT, which excel in their handling of compact emission.

Results. We observe robust, relatively high degrees of fractional circular polarization |m̄_c| ≃ (0.32 ± 0.2)% at 15 GHz and |m̄_c| ≃ (0.59 ± 0.56)% at 23 GHz. We observe consistent polarized structure and EVPA orientation over time when comparing our analysis with archival MOJAVE data. Theoretical predictions indicate a clearly favored toroidal magnetic field orientation within the extended jet emission of the reconstructed signal of the blazar 0149+218. At 23 GHz, the jet structures of 1127–145 and 0528+134 (even at super-resolution) exhibit characteristics that are aligned with a helical or poloidal magnetic nature. Changes in the CP sign as the frequency transitions from 15 GHz to 23 GHz suggest the influence of optical depth effects.