Discovery of spectral curvature in the shock downstream region: CIZA J2242.8+5301

Context. Giant cluster radio relics are thought to form at shock fronts in the course of collisions between galaxy clusters. Via processes that are still poorly understood, these shocks accelerate or re-accelerate cosmic-ray electrons and might amplify magnetic fields. The best object to study this phenomenon is the galaxy cluster CIZA J2242.8+5301 as it shows the most undisturbed relic. By means of Giant Metrewave Radio Telescope (GMRT) and Westerbork Synthesis Radio Telescope (WSRT) data at seven frequencies spanning from 153 MHz to 2272 MHz, we study the synchrotron emission in this cluster. Aims. We aim at distinguishing between theoretical injection and acceleration models proposed for the formation of radio relics. We also study the head-tail radio sources to reveal the interplay between the merger and the cluster galaxies. Methods. We produced spectral index, curvature maps, and radio colour–colour plots and compared our data with predictions from models. Results. We present one of the deepest 153 MHz maps of a cluster ever produced, reaching a noise level of 1.5 mJy beam-1. We derive integrated spectra for four relics in the cluster, discovering extremely steep spectrum diffuse emission concentrated in multiple patches. We find a possible radio phoenix embedded in the relic to the south of the cluster. The spectral index of the northern relic retains signs of steepening from the front towards the back of the shock also at the radio frequencies below 600 MHz. The spectral curvature in the same relic also increases in the downstream area. The data is consistent with the Komissarov-Gubanov injection models, meaning that the emission we observe is produced by a single burst of spectrally-aged accelerated radio electrons.