Spectral index analysis between the frequencies 22 GHz and 43 GHz. Left panel: spectral index map, the contours outline the 43 GHz continuum emission. Right panel: average spectral index along one-pixel slices as a function of distance from the core for two different epochs. The two profiles are in agreement within the errors. The black vertical lines demarcate the jet shape transition distance with its uncertainty. Within the collimation region, the spectral index rapidly decreases down to values of α ∼ −1.5, while it rises to typical values of α ∼ −0.75 beyond the shape transition distance.