Multi-epoch VLBI observations of the blazar 3C 66A: Spatial twisting and temporal oscillation of the parsec-scale jet

¹ Korea Astronomy and Space Science Institute Daejeon 34055, Republic of Korea
² LUX, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université 92190 Meudon, France
³ Department of Astronomy & Center for Galaxy Evolution Research, Yonsei University Seoul 03722, Republic of Korea
⁴ Max-Planck Institut für Radioastronomie Auf dem Hügel 69 Bonn 53121, Germany
⁵ Korea National University of Science and Technology Daejeon 34113, Republic of Korea

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Received: 15 September 2025
Accepted: 14 December 2025

Abstract

Context. High-resolution very long baseline interferometry (VLBI) observations have revealed a growing number of active galactic nuclei (AGNs) that exhibit variations in their inner jet position angle (PA). Investigations of such jets can shed light on the understanding of precession mechanisms and instabilities occurring in the jet and the coupled accretion disk, since changes in the spatial orientation arise in the innermost region.

Aims. Previous VLBI kinematic studies of the blazar 3C 66A have unveiled complex jet kinematic behaviors (e.g., inward/outward, sub- to super-luminal and nonradial motions). Using follow-up high-resolution VLBI observations and archival data, we investigate the morphology and the variations in orientation and core flux density of the 3C 66A jet to gain a deeper insights into its kinematic behavior and physical origins.

Methods. We performed KVN and VERA array (KaVA) observations at 22/43 GHz over three epochs in 2014 and collected 109 sets of Very Long Baseline Array (VLBA) archival data at 43 GHz between 1996 – 2025. We imaged the parsec-scale jet and parameterized it using circular Gaussian fittings to the UV visibilities. Finally, we derived the inner jet PA and the core flux densities for the VLBA data.

Results. The jet presents a twisted morphology in the KaVA maps. The PA of the fitted Gaussian components is in the range between 170° and 195°. Our kinematic analysis using the VLBA data indicates that the PA oscillates with an amplitude of 7.77 ± 0.79° and a period of 10.94 ± 0.22 years, presented for the first time in this work. This oscillation is topped by a continuous clockwise shift of the PA by −0.83 ± 0.07°/year. We also identified a strong core flux variability with possible periodicity and a 2σ correlation between the core flux density and the inner jet PA change. We discuss possible physical models that could explain the observed features for this object; in particular, a supermassive black hole binary (SMBHB) system, Lense-Thirring (LT) effect, and jet or disk instabilities.

Conclusions. The oscillation and continuous shift of the PA and the possible radio flux periodicity, together with the optical flux periodicity of ∼2 years that had previously been confirmed in several independent studies, favor a jet precession scenario driven by orbital motion and disk-orbit misalignment in a SMBHB system. For the estimated central mass of M = (1.42 ± 0.19)×10⁸ M_⊙ from variability timescales, the separation between the putative black holes is r = (1.65 ± 0.08)×10⁻² pc.