High resolution mm-VLBI imaging of Cygnus A

TL;DR

This study uses 7 mm VLBI imaging to resolve the sub-parsec structure of Cygnus A's jets, revealing limb-brightening, acceleration zones, and a transverse velocity gradient, providing insights into jet physics near the supermassive black hole.

Contribution

First ultra-high resolution 7 mm VLBI imaging of Cygnus A's jet base, revealing detailed transverse structure and jet kinematics at unprecedented scales.

Findings

Jets are limb-brightened with large opening angles (~10°).

Jet flow accelerates within the inner 1 pc, then stabilizes.

Counter-jet shows a stationary component.

Abstract

At a distance of 249 Mpc ($z$=0.056), Cygnus A is the only powerful FR II radio galaxy for which a detailed sub-parsec scale imaging of the base of both jet and counter-jet can be obtained. Observing with VLBI at millimeter wavelengths is fundamental for this object, as it uncovers those regions which appear self-absorbed or free-free absorbed by a circumnuclear torus at longer wavelengths. We performed 7 mm Global VLBI observations, achieving ultra-high resolution imaging on scales down to 90 $\mu$as. This resolution corresponds to a linear scale of only $\sim$400 Schwarzschild radii. We studied the transverse structure of the jets through a pixel-based analysis, and kinematic properties of the main emission features by modeling the interferometric visibilities with two-dimensional Gaussian components. Both jets appear limb-brightened, and their opening angles are relatively large ($\phi_\mathrm {j}\sim 10^{\circ}$). The flow is observed to accelerate within the inner-jet up to scales of $\sim$1 pc, while lower speeds and uniform motions are measured further downstream. A single component seen in the counter-jet appears to be stationary. These observational properties are explained assuming the existence of transverse gradient of the bulk Lorentz factor across the jet, consisting of a fast central spine surrounded by a slower boundary layer.