Spectral index in 610 MHz–1.4 GHz vs. spectral index in 144 MHz–610 MHz band, color coded for the X-ray surface brightness I_X. The values were measured in the cells of the green grid that is shown in the top left corner, overlapped on the 3, 24, 96σ levels of the 144 MHz image involved in the spectral analysis. The cells in which the relative error on the spectral index is greater than 50% are shown as upper limits.