Variation of the spectral index along the radio lobes at low frequency (top panel) and at high frequency (bottom panel). In both cases the spectral index shows a systematic and quick steepening from the inner region of the lobes towards the edges. Values have been extracted from the black rectangular regions shown in the middle panel of Fig. 5. The grey line represents a spectral index equal to α = 1 as a reference. The 0 value on the x-axis corresponds to the innermost point in each lobe, where the flattest spectral index is measured. We note that one beam corresponds to five pixels. An average error bar is shown in the bottom left corner of both plots.