Cutout of the filament with a flat spectral index visible in the HBA to L band (145 MHz to 1.5 GHz) spectral index map (‘Filament 1’). On the left is the region as seen in the Chandra Low-Energy Transmission Grating with the High-Resolution Camera Spectroscopy detector (Weisskopf et al. 2012), in the middle as seen in the LOFAR HBA at 145 MHz, and in the right, the HBA to L band (145 MHz to 1.5 GHz) spectral index map (same as Fig. 2, right). The contours correspond to spectral index values of α = −0.15 and 0.05.