Spectral index evolution of the limb-brightened jet in 3C 84

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
² Finnish Centre for Astronomy with ESO, University of Turku, 20014 Turku, Finland
³ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, Massachusetts 02138, USA
⁵ Dept. of Physics & Astronomy, Sejong University, Guangjin-gu, Seoul 05006, Republic of Korea
⁶ Department of Astrophysics, Institute for Mathematics, Astrophysics and Particle Physics (IMAPP), Radboud University, P.O. Box 9010, 6500 GL Nijmegen, The Netherlands
⁷ Department of Physics, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 44919, Republic of Korea
⁸ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059 Valparaíso, Chile
⁹ Department of Physics and Astronomy, University of Mississippi, University, Mississippi 38677, USA
¹⁰ Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile
¹¹ Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 12 December 2025
Accepted: 13 April 2026

Abstract

Relativistic jets launched by active galactic nuclei are fundamental for understanding the physics of accreting supermassive black holes and their immediate environment, but the mechanisms that drive the jet launching remain uncertain. We investigate the sub-parsec jet of 3C 84 using multi-epoch multi-frequency very long baseline interferometry (VLBI) observations with the European VLBI Network and the Very Long Baseline Array at 22 and 43 GHz. We analysed the evolution of the spectral index gradient in the core region to relate the observed structure to physical interpretations and to distinguish between competing jet-launching models. Furthermore, we examined the effect of the ambient medium and magnetic field configuration on the jet morphology and dynamics over time and explored their connection to a coinciding γ-ray flare. Our spectral analysis reveals significant changes across three epochs, indicating dynamic activity between filamentary structures on sub-parsec scales, evolving magnetic fields, and a complex interaction with the surrounding medium, all of which shape the innermost jet and might affect its high-energy emission.