2025/06/16 by Yan, Xi, Cui, Lang, Hada, Kazuhiro +6
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2506.13093
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±0.24) pc or (8.1±1.6)×103 R\rm s (Schwarzschild radii) for the jet and (0.97±0.29) pc or (6.4±1.9)×103 R\rm s for the counterjet. We also derive the jet velocity field at distances of ∼ (103-2×104) R\rm s. While local kinematic variations are present, the jet shows an overall acceleration to relativistic speeds from ∼ 103 to ∼8×103 R\rm s, with a maximum Lorentz factor of Γ\rm max ≈ 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.