Subparsec Acceleration and Collimation of NGC 4261’s Twin Jets

Abstract We report the first robust evidence for a cospatial subparsec acceleration and collimation zone (ACZ) in the twin jets of the nearby low-luminosity active galactic nucleus 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) × 10 3 R s (Schwarzschild radii) for the jet and (0.97 ± 0.29) pc or (6.4 ± 1.9) × 10 3 R s for the counterjet. We also derive the jet velocity field at distances of ∼(10 3 –2 × 10 4 ) R s . While local kinematic variations are present, the jet shows an overall acceleration to relativistic speeds from ∼10 3 to ∼8 × 10 3 R s , with a maximum Lorentz factor of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">Γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>2.6</mml:mn> </mml:math> . Beyond this region, the jet gradually decelerates to subrelativistic speeds. These findings support the existence of a subparsec-scale (≲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 M87 suggests that the ACZ in NGC 4261 may represent a scaled-down analog of that in M87. These results point toward 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.