X-ray surface brightness versus spectral index of the radio halo emission. The Bayesian linear regression is performed for data points with at least a 3σ radio noise level. The red and green dashed lines correspond to the best-fit obtained separately for the H1 and H2 region of the halo. The anticorrelation between these two quantities is evident, indicating that the spectral index is flatter at high X-ray brightness and tends to get steeper in low X-ray brightness regions.