A high-resolution study of the double radio relic system in MACS J1752.0+4440

Context. Radio relics are diffuse, extended synchrotron sources located in the outskirts of merging galaxy clusters. Their origin has been linked to shock waves injected into the intracluster medium, but the acceleration mechanism at the shock front remains under debate. Some clusters, such as MACS J1752.0+4440, host a double relics system, with two relics located on opposite sides of the cluster center. Aims. To investigate the acceleration mechanism that generates radio relics, we studied the morphological and spectral properties of double relic system in MACS J1752.0+4440 (z = 0.366). Methods. We present new wideband radio continuum observations obtained with uGMRT and JVLA, and previously published LOFAR data. We performed a detailed, high-resolution spectral analysis of the double relic system in MACS J1752.0+4440, and we observed and characterized substructures, particularly those of the brighter relic. Results. We find a double-peaked surface brightness and spectral index profile for the NE relic and identify a “bright bar” substructure. Moreover, we observe surprisingly flat integrated spectral indices for both relics, at αᵢₙₜNE = -0.91 ± 0.06 and αᵢₙₜSW = -0.83 ± 0.05. We studied the spatial variation of the spectral index, observing a coherent trend with the observed substructures. We estimate injection Mach numbers of MNE = 3.1+0.1-0.1 and MSW = 3.2+0.1-0.1. We performed a spectral curvature analysis for both relics, generating color-color plots, and we observe two “concave” spectra characterized by positive spectral curvature, in contrast to particle population aging models. Conclusions. The observed properties of the NE relic are not consistent with a simple scenario involving a single shock front and a single particle-injection event. Multiple shock surfaces, reacceleration, and projection effects likely play a role in shaping the morphology of the relic.