Kavli Affiliate: Luis C. Ho
| First 5 Authors: Xi Yan, Xi Yan, , ,
| Summary:
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 $(1.23pm0.24)$ pc or $(8.1pm1.6)times10^3 R_rm
s$ (Schwarzschild radii) for the jet and $(0.97pm0.29)$ pc or
$(6.4pm1.9)times10^3 R_rm s$ for the counterjet. We also derive the jet
velocity field at distances of $sim (10^3-2times10^4) R_rm s$. While local
kinematic variations are present, the jet shows an overall acceleration to
relativistic speeds from $sim 10^3$ to $sim8times10^3 R_rm s$, 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.
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