Spectral index maps. Top panel: spectral index map of the emission in the radio bridge in A399–A401 between 60 and 144 MHz, with a resolution of 80″. The distribution of the spectral index values (histogram at the top) only refers to the bridge emission inside the 2σ_rms level, which is not covered by the grey mask over the radio haloes. Bottom panel: associated spectral index error map. The LOFAR LBA contours at 2σ_rms and 3σ_rms are overlaid in black.