RadioAstron reveals a change in the jet collimation profile of 3C 84

TL;DR

RadioAstron observations of 3C 84 reveal a significant morphological evolution of its jet, including a change from cylindrical to parabolic collimation, providing insights into jet dynamics and magnetic field properties.

Contribution

This study presents the first high-resolution RadioAstron image of 3C 84's jet, demonstrating morphological changes and evolution in collimation profile over three years.

Findings

Jet collimation profile evolved from quasi-cylindrical to parabolic.

Confirmed limb-brightened jet and counter-jet structures.

Constrained magnetic field strength in the core and hotspot.

Abstract

Due to its brightness and proximity, the radio galaxy 3C 84 (optical counterpart NGC 1275 in the Perseus cluster) has been the target of extensive studies investigating the central parsec region of its active galactic nucleus. In 2003, its most recent active phase resulted in a plasma ejection visible in the southern jet, which presented a unique opportunity to study jet formation and evolution at high angular resolution with very long baseline interferometry (VLBI). We aim to study the morphology, evolution, and spectral properties of the restarted jet three years after the first ultra-high angular resolution observations with the RadioAstron space-VLBI satellite in September 2013. To study 3C 84, we used space-VLBI observations carried out in September 2016 at 22 GHz with a global VLBI network and the 10 m Spektr-R radio telescope in orbit as well as quasi-simultaneous multifrequency observations at 4.8, 8, 15, and 43 GHz from the Very Long Baseline Array, including the Effelsberg 100 m telescope. We present the 22 GHz RadioAstron image of 3C 84 from 2016, which reveals the source's central region at a 58 microarcsecond effective resolution. During the three years that elapsed between the first and second space-VLBI observations, the source underwent significant morphological changes. We confirm the existence of the limb-brightened jet and counter-jet reported earlier as well as a flip in the position of the hotspot discovered recently via VLBI monitoring at 43 GHz. Based on measuring the collimation profile, we find that it has evolved from being quasi-cylindrical to parabolic. This is most likely the result of the decreased pressure of the mini-cocoon, which was inflated by the jet and contains hot gas that cannot confine the jet efficiently as it propagates further away from the core. Finally, we also constrained the magnetic field strength in the core region and the hotspot.