Spectral index and radiative age trend along the radio arcs obtained with the sampling method shown in the bottom-right panel. On the x-axis is shown the sampling numeration (indicated in the bottom right panel), on the left y-axis the spectral index scale and on the right y-axis the radiative age scale. In each panel the top blue plot displays the radiative age estimated by assuming B = B_ml = 2.1μG, and the bottom red plot displays the radiative age estimated by assuming the equipartition magnetic field B = B_eq = 12μG. Top-left: northern arc trends; top-right: western arc trends; bottom-left: southern arc trends; bottom-right: sampling method. The black contours are the 1.4 GHz reference levels reported in Fig. 2, middle panel.