Sub-Parsec Acceleration and Collimation of NGC 4261's Twin Jets

TL;DR

This study provides the first clear evidence of a sub-parsec acceleration and collimation zone in NGC 4261's twin jets, revealing jet dynamics and structural transition details through multifrequency VLBA imaging.

Contribution

It identifies the location and properties of the acceleration and collimation zone in NGC 4261's jets, demonstrating a parabolic-to-conical transition and jet acceleration behavior.

Findings

Jet accelerates to relativistic speeds within ~8,000 R_s

Structural transition from parabolic to conical occurs near 1 pc

Jet shows a maximum Lorentz factor of approximately 2.6

Abstract

We report the first robust evidence for a co-spatial sub-parsec acceleration and collimation zone (ACZ) in the twin jets of the nearby low-luminosity active galactic nucleus (LLAGN) NGC 4261. This result is derived from multifrequency Very Long Baseline Array imaging, combined with the frequency-dependent properties of the radio core (core shift and core size) and jet kinematics determined from the jet-to-counterjet brightness ratio. By applying multiple analysis methods and incorporating results from the literature, we identify a parabolic-to-conical structural transition in both the jet and counterjet, with the transition occurring at $(1.23\pm0.24)$ pc or $(8.1\pm1.6)\times10^3 R_{\rm s}$ (Schwarzschild radii) for the jet and $(0.97\pm0.29)$ pc or $(6.4\pm1.9)\times10^3 R_{\rm s}$ for the counterjet. We also derive the jet velocity field at distances of $\sim (10^3-2\times10^4) R_{\rm s}$. While local kinematic variations are present, the jet shows an overall acceleration to relativistic speeds from $\sim 10^3$ to $\sim8\times10^3 R_{\rm s}$, with a maximum Lorentz factor of $\Gamma_{\rm max} \approx 2.6$. Beyond this region, the jet gradually decelerates to sub-relativistic speeds. These findings support the existence of a sub-parsec-scale ($\lesssim 1.5$ pc) ACZ in NGC 4261, where the jet is accelerated via magnetic-to-kinetic energy conversion while being confined by external pressure. A brief comparison with M 87 suggests that the ACZ in NGC 4261 may represent a scaled-down analogue of that in M 87. These results point towards a potential diversity in jet ACZ properties, emphasizing the importance of extending such studies to a broader AGN population to elucidate the physical mechanisms at play.