Evidence for a complex acceleration mechanism in the radio relics of MACS J1752+4440

Radio relics are diffuse, synchrotron, radio sources located at the outskirts of merging galaxy clusters, characterized by elongated arc-like shapes, with sizes ranging between 0.5 to 2Mpc, and highly polarized emission. They are thought to trace shock waves created in the ICM during a binary, head-on, merger event. MACS J1752+4440 is a remarkable case of cluster hosting a double relic system, in which two radio relics are found on opposite sides of the cluster center. In this thesis work, we used LOFAR HBA data from the LoTSS survey, paired with new uGMRT and new JVLA L-band data, to perform a spectral study at unprecedented resolution. This allowed us to observe the complex nature of the North-Eastern relic, highlighting a multiple substructure morphology, which could reveal the fine details of a merger shock propagating into the ICM. We investigated the complexity of the double-relic system, observing a double-peaked surface brightness and spectral index profile for the NE relic, which follow the observed complex structure. Moreover, we observed surprisingly flat integrated spectral indices for both relics. Through a spectral index map, we studied the spatial variation of the spectral index, observing a coherent trend with the observed substructures. Assuming DSA, we estimated the injection Mach numbers, in agreement with a weak shock scenario. We propose a scenario for the observed morphology, in which the observed substructures are generated by a second shock surface. By performing a spectral curvature study, generating color-color plots and a spectral curvature map, we observed two “concave” spectra represented by positive spectral curvature, which can be explained in a scenario in which different components are observed in projection. To investigate this, we performed a polarization study on both relics, finding no correlation between the polarization fraction and the spectral curvature. Finally, we measured an equipartition magnetic field.