Spectral index distribution. Left panel: LBA radio image at 60 MHz and resolution of 80″. The 2σ_rms contours and the grid to extract the spectral index between 60 and 144 MHz are overlaid. Different areas of the bridge are covered by differently coloured cells. The coloured cells cover the LBA emission above 2σ_rms, and the red cells cover the 1σ_rms LBA emission and 2σ_rms HBA emission. Top right: spectral index distribution across the bridge. The black points show values extracted from each cell, separated by colour, and the red arrows indicate 1σ_rms limits extracted from the red cells. The dashed horizontal green line shows the mean spectral index, and the filled horizontal green region represents the standard deviation. Bottom right: distribution of residuals (Δα) of α with respect to the mean spectral index.