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

G. F. Paraschos, L. C. Debbrecht, J. A. Kramer, E. Traianou, I. Liodakis, T. P. Krichbaum, J. -Y. Kim, M. Janssen, D. G. Nair, T. Savolainen, E. Ros, U. Bach, J. A. Hodgson, M. Lisakov, N. R. MacDonald, J. A. Zensus

Published 2024-04-30Version 1

The spatial scales of relativistic radio jets, probed by relativistic magneto-hydrodynamic jet launching simulations (RMHDs) 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. 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 global VLBI measurements of 3C 84 we aim to study its sub-parsec region in both total intensity and linear polarisation, to explore the properties of this jet, with a linear resolution of $\sim0.1$ parsec. We test different simulation setups by altering the bulk Lorentz factor $\Gamma$ 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.