Assessment of the non-thermal nature of the filamentary structures found by pGMCA. The northeast region of Cas A from our merged observations is shown in the top left panel. The top right panel shows an image of the synchrotron retrieved by our method on the 2.5–4 keV band. The blue and green contours in these latter two panels show the regions of extraction, and the white rectangle shows the fourth reverse shock box from Fig. 1. The extracted spectra from these two regions are shown in the panels below, both fitted with a simple phabs*powerlaw model in Xspec in red.