2016A&A...585A..33B

High-resolution Very-Long-Baseline Interferometry (VLBI) observations of relativistic jets are essential for constraining the fundamental parameters of jet formation models. At a distance of 249Mpc, Cygnus A is a unique target for such studies, since it is the only Fanaroff-Riley Class II radio galaxy for which a detailed subparsec scale imaging of the base of both jet and counter-jet can be obtained. Observing at millimeter wavelengths unveils those regions that appear self-absorbed at longer wavelengths and enables an extremely sharp view toward the nucleus to be obtained. We performed 7mm Global VLBI observations, achieving ultra-high resolution imaging on scales down to 90µas. This resolution corresponds to a linear scale of only ∼400 Schwarzschild radii (for M_BH=2.5x10⁹M_☉). We studied the kinematic properties of the main emission features of the two-sided flow and probed its transverse structure through a pixel-based analysis. We suggest that a fast and a slow layer with different acceleration gradients exist in the flow. The extension of the acceleration region is large (∼10⁴R_S), indicating that the jet is magnetically driven. The limb brightening of both jet and counter-jet and their large opening angles (φ_J∼10°) strongly favour a spine-sheath structure. In the acceleration zone, the flow has a parabolic shape (r∝z^0.55±0.07). The acceleration gradients and the collimation profile are consistent with the expectations for a jet in ``equilibrium'', achieved in the presence of a mild gradient of the external pressure (p∝z^–k, k≤2).