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

TL;DR

This study provides a detailed high-resolution spectral analysis of the double radio relic system in MACS J1752.0+4440, revealing complex shock structures and challenging simple acceleration models.

Contribution

It presents new wideband radio observations and spectral analysis that uncover substructures and spectral curvature, suggesting multiple shocks and re-acceleration processes.

Findings

Flat integrated spectral indices for both relics, around -0.9 and -0.83.

Identification of a 'bright bar' substructure in the NE relic.

Spectral curvature analysis shows concave spectra, indicating complex shock dynamics.

Abstract

Radio relics are diffuse, extended synchrotron sources located at the outskirts of merging galaxy clusters. Their origin has been linked with shock waves injected into the intracluster medium, but the acceleration mechanism at the shock front is still under debate. Some clusters, like MACS J1752.0+4440, host a double relic system, with two relics found on opposite sides with respect to the cluster center. To investigate the acceleration mechanism that generates radio relics, we study the morphological and spectral properties of the double relic system in MACS J1752. We present new wideband radio continuum observations made with uGMRT and JVLA, and LOFAR data. We perform a detailed, high-resolution spectral analysis of the double relic system in MACS J1752, observing and characterizing substructures, particularly for the brighter relic. We find a double-peaked surface brightness and spectral index profile for the NE relic and identify a "bright bar" substructure. Moreover, we observed surprisingly flat integrated spectral indices for both relics, at $\alpha_{\mathrm{int}}^{\mathrm{NE}} = -0.91 \pm 0.06$ and $\alpha_{\mathrm{int}}^{\mathrm{SW}} = -0.83 \pm 0.05$. We study the spatial variation of the spectral index, observing a coherent trend with the observed substructures. We estimate an injection Mach number of $\mathcal{M}_{\mathrm{NE}} = 3.1^{+0.1}_{-0.1}$ and $\mathcal{M}_{\mathrm{SW}} = 3.2^{+0.1}_{-0.1}$. By performing a spectral curvature analysis for both relics, generating color-color plots and a spectral curvature maps, we observe two "concave" spectra represented by positive spectral curvature, in contrast with particle population ageing models. The observed properties of the NE relic are not consistent with a simple scenario with a single shock front. Multiple shock surfaces, re-acceleration, and projection effects likely play a role in shaping the morphology of the relic.