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

TL;DR

The paper analyzes multi-epoch VLBI data of the blazar 3C 66A to characterize a twisted parsec-scale jet and the evolution of the inner-jet position angle (PA) and core flux. It combines KaVA 22/43 GHz observations (2014) with 43 GHz VLBA archival data (1996–2025) to detect a ~11-year PA oscillation atop a slow clockwise PA drift, and a strong, variable core flux with possible ~3.5–10.5 year periodicities. Interpreting these features with a two-timescale jet precession model, the authors argue for a SMBHB-driven scenario in which orbital-motion explains the short-term PA oscillation and disk–orbit precession explains the long-term drift, supported by an inferred SMBHB separation of about $1.65\times10^{-2}$ pc and a total mass of $M\approx1.42\times10^8\,M_\odot$. They further discuss alternative mechanisms like Lense–Thirring precession and jet instabilities (CDI/KHI), but conclude that the SMBHB interpretation offers a coherent explanation for the observed multiwavelength periodicities and jet dynamics, with gravitational waves currently undetectable by PTAs. Future VLBI polarimetric and astrometric monitoring could provide decisive tests of this scenario.

Abstract

Previous VLBI kinematic studies of the blazar 3C 66A have unveiled complex jet kinematic behaviors. 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. 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. The jet presents a twisted morphology in the KaVA maps. The PA of the fitted Gaussian components is in the range between 170 deg and 195 deg. Our kinematic analysis using the VLBA data indicates that the PA oscillates with an amplitude of 7.77 pm 0.79 deg and a period of 10.94 pm 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 pm 0.07 deg/year. We also identified a strong core flux variability with possible periodicity and a 2 sigma 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. The oscillation and continuous shift of the PA and the possible radio flux periodicity, together with the optical flux periodicity of approximately 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.

Figures (16)

• Figure 1: KaVA intensity maps of the 3C 66A radio jet at 22 GHz (top) and 43 GHz (bottom) in 2014. The black contour levels start at three times the rms value, scaling twice. The grey shaded ellipses above each contour show the restoring beam. The detailed imaging parameters are summarized in Table \ref{['tab1']}. The horizontal spacing is proportional to the observation time gap. The exact observing time is in the unit of years. The red circles are the model components obtained from $modelfit$. The continuous curved jet extends downwards to $\sim$3 mas on both panels.
• Figure 2: PA of the Gaussian components of the 3C 66A jet as a function of the core separation obtained with the KaVA observations at 22 (left) and 43 GHz (right) in 2014. Each epoch is marked by green circles, blue triangles, and red squares.
• Figure 3: (a) Inner jet PA versus time at 43 GHz. The dashed curve in the upper panel is the analytic sinusoidal+linear fit. The lower panel shows the residuals. The fitting parameters are summarized in Table \ref{['tab3_PAfit']}. (b) Time evolution of the total (blue) and core (black) flux density.
• Figure 4: (a) DCF analysis between the PA and the core flux density. The grey dotted and dash-dotted lines show the 2$\sigma$ and 3$\sigma$ significance levels, respectively. (b) Core flux density vs. inner jet PA plot. Each panel displays the data between 1996 and 2010 (left) and between 2010 and 2025 (right). We use changing colors to show the time sequence: grey denotes the values from all observing epochs, purple to dark blue to clear blue denotes the data observed between 1996, 2002, and 2010, while green to orange to red between 2010, 2017, and 2020.
• Figure 5: Corner plots of the likelihood analysis for the precession model given three ranges of $\phi_0$. The parameter values shown are the median and 1$\sigma$ credible intervals of the posterior samples.