Spectral index map of the radio shock between 46 and 144 MHz made at a common resolution of 20″ × 20″. The three regions used to compute the integrated spectral index of the shock are shown in grey, where the contribution from the bright narrow-angle-tailed sources is subtracted, shown in purple. The contours indicate the LOFAR 144 MHz total intensity at [3, 6, 12, 24, 48] times the background noise.