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

TL;DR

This study uses multi-frequency VLBI observations to analyze the spectral index evolution of the 3C 84 jet, revealing dynamic activity, magnetic field changes, and complex interactions influencing jet structure and high-energy emission.

Contribution

It provides new insights into the physical processes of jet launching and evolution in 3C 84 through detailed spectral and morphological analysis over multiple epochs.

Findings

Significant spectral index changes over three epochs.

Evidence of evolving magnetic fields and filamentary structures.

Interaction between jet and ambient medium linked to gamma-ray activity.

Abstract

Relativistic jets launched by active galactic nuclei are fundamental for understanding the physics of accreting supermassive black holes and their immediate environment, yet the mechanisms driving jet launching remain uncertain. In this study, 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 analyse the evolution of the spectral index gradient in the core region to relate the observed structure to physical interpretations and to discriminate between competing jet launching models. Furthermore, we examine the impact of the ambient medium and magnetic field configuration on jet morphology and dynamics over time, and explore their connection to a coinciding $\gamma$-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 may influence its high-energy emission.