I_R − I_X relation of the halo in MACS J0717.5+3745, extracted in square boxes with of 8″ (about 50 kpc). Linmix best-fit relations are indicated by red dashed lines. The upper limits represent cells with data points below 2σ radio noise level. The horizontal black dash-dotted lines indicate the 1σ in the radio maps. The best-fit parameters are obtained for the halo emission (blue dots). The cyan lines show samples from the posterior distribution. Lower panel: residuals of log I_R and log I_X with respect to the Linmix best fit line. For the halo, the radio brightness correlates well with the X-ray at all three frequencies. The best-fitting slopes at 144 MHz, 1.5 GHz and 3 GHz are 0.67 ± 0.05, 0.81 ± 0.09, and 0.98 ± 0.09, respectively. The strong dependence of the slope on frequency suggests that the halo shows a spectral steepening toward high frequencies. Displayed errors in the radio flux density measurements include the statistical and systematic uncertainties.