Left: regions from which the spectral index between 54 and 144 MHz was extracted, overlaid on the high-resolution HBA map. Each region is as large as the smallest common circular beam (9.7″ × 9.7″). The AGN, the tip of the AGN tail and source A are labelled. Right: results of the fit of the spectral index. Top panel: flux density at 54 (blue) and 144 (orange) MHz for each region, starting from the AGN position. Bottom panel: spectral index, fitted with a JP ageing model. The curve becomes dashed from the point where the model was extrapolated, indicated by the grey vertical line.