Letter to the Editor

Evidence of a toroidal magnetic field in the core of 3C 84

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: gfparaschos@mpifr-bonn.mpg.de
² Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
³ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía, 18008 Granada, Spain
⁴ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁵ Institute of Astrophysics, Foundation for Research and Technology, Hellas, Heraklion 7110, Greece
⁶ Department of Astronomy and Atmospheric Sciences, Kyungpook National University, Daegu 702-701, Republic of Korea
⁷ Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁸ Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁹ Aalto University Department of Electronics and Nanoengineering, PL 15500, 00076 Aalto, Finland
¹⁰ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
¹¹ Dept. of Physics & Astronomy, Sejong University, Guangjin-gu, Seoul 05006, Republic of Korea
¹² Lebedev Physical Institute of the Russian Academy of Sciences, Leninsky prospekt 53, 119991 Moscow, Russia
¹³ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
¹⁴ Department of Physics and Astronomy, The University of Mississippi, University, Mississippi 38677, USA

Received: 2 April 2024
Accepted: 26 April 2024

Abstract

The spatial scales of relativistic radio jets, probed by relativistic magneto-hydrodynamic (RMHD) jet launching simulations and by most very long baseline interferometry (VLBI) observations differ by an order of magnitude. Bridging the gap between these RMHD simulations and VLBI observations requires selecting nearby active galactic nuclei (AGN), the parsec-scale region of which can be resolved. The radio source 3C 84 is a nearby bright AGN fulfilling the necessary requirements: it is launching a powerful, relativistic jet powered by a central supermassive black hole, while also being very bright. Using 22 GHz globe-spanning VLBI measurements of 3C 84 we studied its sub-parsec region in both total intensity and linear polarisation to explore the properties of this jet, with a linear resolution of ∼0.1 parsec. We tested different simulation set-ups by altering the bulk Lorentz factor Γ of the jet, as well as the magnetic field configuration (toroidal, poloidal, helical). We confirm the persistence of a limb brightened structure, which reaches deep into the sub-parsec region. The corresponding electric vector position angles (EVPAs) follow the bulk jet flow inside but tend to be orthogonal to it near the edges. Our state-of-the-art RMHD simulations show that this geometry is consistent with a spine-sheath model, associated with a mildly relativistic flow and a toroidal magnetic field configuration.