Discovery of peculiar double radio relics in the merging cluster ZwCl J1447.2+2619

Cluster mergers serve as powerful laboratories for cosmology and astrophysics. Radio relics, diffuse radio emissions found in the outskirts of merging clusters, are believed to originate from the merger shocks and thus provide a unique laboratory for plasma acceleration physics. Here we report the discovery of remarkable double radio relics in the merging cluster ZwCl J1447.2+2619 at z=0.376. Using multi-wavelength observations and numerical simulations, we find that the cluster is comprised of two main substructures and demonstrate that the position and morphology of the double radio relics can be reproduced by a near head-on collision ~0.7 Gyr ago. Interestingly, the observed surface brightness ratio between the two radio relics is significantly offset from the relation seen in other double radio relic systems. We propose that this offset may arise from the difference in upstream environments when cosmic-ray electrons are injected only into the smaller radio relic. Our study shows that double radio relic systems are a powerful laboratory for understanding plasma acceleration when the analysis is aided with high-quality multi-wavelength observations and careful hydro-simulations.