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tokens: of 18.5arcsec. This procedure was also applied to the E relic, but since the emission of the NAT is extended, it is impossible to properly separate the contribution of the tail from the relic. Hence, we decided not to subtract the radio galaxy. This choice allows us to study the spectral index behaviour from the core of the NAT to the lobes, and to investigate its connection with the diffuse source. We computed the average flux density and spectral index of the E relic (and of the NAT radio galaxy) and of the W relic and the radio halo emission (sources subtracted) between 1.5 and 3GHz. The values are reported in Table 3 . The uncertainties on the flux density measurements are computed as where is the calibration error, is the rms noise listed in Table 2 and n beam is the number of beams in the sampled region. These uncertainties were then propagated to the spectral index. Table 3. Flux density, spectral index between 1.5 and 3GHz and radio power of extended sources measured from the images used for the spectral index study. We separated the region of the W relic and the halo on the basis of the spectral index profile as described in Section 4.2.2 . The stands for point-like sources subtracted. We extrapolated the radio power at 1.4GHz from the 1.5GHz flux density measurement, considering a luminosity distance D L 1253.3 Mpc (Wright 2006 ) and using the spectral indices for the k -correction. The uncertainties on flux densities were propagated to the radio power. The sum of the radio power of the two relics (sources subtracted) is consistent with the relation found by de Gasperin etal. ( 2014 ) between the radio power of double radio relics and the cluster mass. Finally, we computed the spectral index for each pixel with value 3 in both frequency bands. We show the spectral index map of the E relic in Fig. 5 . We propagated the uncertainties on the flux densities pixel by pixel on the spectral index. The error map of the spectral index for the E relic is shown in the bottom panel of Fig. 5 , while the spectral index image of the western region and its error map are shown in Fig. 6 . Figure 5. Top panel: Spectral index image of the E relic region between 1.5 and 3GHz. The 1.5GHz total intensity contours (BCD configuration) are overlaid in black: the levels start at 3, with 0.04 mJy beam 1 , and are separated by a factor of 2. Bottom panel: Error map of the spectral index image with the same contours overlaid. The restoring beam of the two images is shown in the left-hand corner and its size is 18.5arcsec 18.5arcsec. Figure 6. Top panel: Spectral index image of the W relic and halo region computed between 1.5 and 3GHz. Point-like sources were subtracted. The two dashed red polygons mark the flux density extraction regions for the halo and the West relic. Bottom panel: Error map of the spectral index image. Contours and restoring beam are as in Fig. 5 . Spectral index images obtained from interferometric data should be treated with