Left: 141 MHz–1.4 GHz spectral index map in the halo region. The discrete sources were removed from the data before the spectral index map was made. The red annuli and black dashed sectors are where the spectral indices are estimated, and they are shown in the right panel. The outermost regions of the N and S sectors are where X-ray SB discontinuities are found in Zhang et al. (2020). The thin dashed lines are the regions where discrete sources are subtracted or show locally flat spectra, which are excluded from the profiles. The LOFAR grey contours are ±[1, 2, 4, 8, 16] × 3σ, where σ = 345 μJy beam⁻¹ (beam_FWHM = 46″). Right: spectral index profiles from the halo centre towards the outskirts and towards the N and S sectors.