Spectral index maps for NGC 1052. Panel a: 2D-distribution of the spectral index for the OP jet model, panel b: VLBA observations of NGC 1052 and panel c: PM jet model. The variation of the spectral index along the jet axis (black dashed line in panels a–c) can be seen in panel d. In panels a–c, the convolving beam is plotted in the lower left corner of each panel. The contours correspond to the 22 GHz flux density distribution and the lowest flux density contour is drawn at 1 mJy and increases by factors of two. The dash-dotted line in panel d corresponds to the optically thin spectral index of the models (α = 1.4).