Kavli Affiliate: Luis C. Ho
| First 5 Authors: Xi Yan, Lang Cui, Kazuhiro Hada, Sandor Frey, Ru-sen Lu
| Summary:
We study the acceleration and collimation of the two-sided jets in the nearby
low-luminosity active galactic nucleus NGC 4261 (3C 270) using archival
multifrequency, multi-epoch Very Long Baseline Array data. By applying multiple
analysis methods and incorporating results from the literature, we robustly
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.
Assuming that the brightness asymmetry between the twin jets is primarily due
to relativistic Doppler (de)boosting, we derive the jet velocity field at
distances of $sim (10^3-2times10^4),R_{rm s}$ based on the
jet-to-counterjet brightness ratio and spectral index. Although 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 results support the existence of
a (sub)parsec-scale ($lesssim 1.5,$pc) acceleration and collimation zone
(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.
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