Injection spectral index calculated in 15″-box (∼48 kpc separation) along the relics’ edges. Left: zoom-in over the RN (top) and RS (bottom). Right: injection spectral index profiles extracted from 45 to 145 MHz 15″-resolution images. The right panels display the density-normalized histograms together with a kernel density estimation (KDE) curve for a smooth and continuous representation of the data distribution. For each relic, the data are colour-coded by the different groups identified by the GMM. The GMM means are plotted as horizontal dashed lines.