Spectral index maps between 22 GHz (dot-dashed blue contour) and 43 GHz (solid black contours) for three selected observations after alignment based on optically thin features (compare Fig. 2). The restoring beam for each epoch is plotted in the lower left corner. A cut along the direction of the jets (64°) results in the bottom panels, showing the spectral index (bottom panels) and the flux density (middle panel) along the jet axis. The blue star indicates the assumed location of the dynamical center.