The stratified two-sided jet of CygnusA Acceleration and collimation

Aims. 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 249 Mpc, CygnusA 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. Methods. 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 similar to 400 Schwarzschild radii (for M-BH = 2. 5 x 10(9) M-circle dot). 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. Results. 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 (similar to 10(4) RS), indicating that the jet is magnetically driven. The limb brightening of both jet and counter-jet and their large opening angles (phi(J) similar to 10 degrees) strongly favour a spine-sheath structure. In the acceleration zone, the flow has a parabolic shape (r infinity 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 infinity z(-k) ,k <= 2).