Scatter plots of spectral index and radio SB (I_R) as a function of X-ray SB (I_X). The regions for extracting data are shown in the overlaid image (left panel). The data points are plotted in the same colour as the regions in the overlaid image (i.e. black circles for the central regions, magenta squares for the regions in the SW direction, and red diamonds for the regions in the NE direction). The best-fit parameters are obtained for the central regions.