Correlation between radio spectral index and X-ray surface brightness $({\alpha }_{143\mathrm{MHz}}^{1283\mathrm{MHz}}/I_{\mathrm{X}})$ for Abell 2142 at 60 arcsec resolution. Note the inverted y-axis, to facilitate comparison with previous similar studies. The strength of the correlation is negligible, with Spearman (Pearson) coefficients r_S = 0.17 (r_P = 0.26). Blue markers indicate upper limits to the spectral index, which were excluded from the fitting and are shown here for illustrative purposes only.