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We present new LOFAR observations of the massive merging galaxy cluster MACSJ0717.5+3745. The cluster hosts the most powerful radio halo known to date.These new observations, in combination with published uGMRT (300$-$850 MHz) andVLA (1$-$6.5 GHz) data, reveal that the halo is more extended than previouslythought, with a largest linear size of $\sim2.2 \rm Mpc$. The halo shows asteep spectrum ($\alpha_{144\,\text{MHz}}^{1.5\,\text{GHz}}\sim-1.4$) and asteepening ($\alpha_{1.5 \text{GHz}}^{5.5 \text{GHz}}\sim-1.9$) above 1.5 GHz.We find a strong scattering in spectral index maps on scales of 50$-$100 kpc.We suggest that such a strong scattering may be a consequence of the regimewhere inverse Compton dominate the energy losses of electrons. The spectralindex becomes steeper and shows an increased curvature in the outermost regionsof the halo. We combined the radio data with \textit{Chandra} observations toinvestigate the connection between the thermal and non-thermal components ofthe intracluster medium (ICM). Despite a significant substructure in the haloemission, the radio brightness correlates strongly with the X-ray brightness atall observed frequencies. The radio-versus-X-ray brightness correlation slopesteepens at a higher radio frequency (from $b_{144 \text{MHz}}=0.67\pm0.05$ to$b_{3.0 \text{GHz}}=0.98\pm0.09$) and the spectral index shows a significantanti correlation with the X-ray brightness. Both pieces of evidence furthersupport a spectral steepening in the external regions. The compelling evidencefor a steep spectral index, the existence of a spectral break above 1.5 GHz,and the dependence of radio and X-ray surface brightness correlation onfrequency are interpreted in the context of turbulent reacceleration models.Under this scenario, our results allowed us to constrain that the turbulentkinetic pressure of the ICM is up to 10%.

Physical insights from the spectrum of the radio halo in MACS J0717.5+3745