Vol. 555
In section 3. Cosmology

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

This map shows the spectral index of the radio emission of a galaxy cluster, obtained through the measurement of seven frequencies, between 153 MHz and 2272 MHz. Four radio relics are found in the field, of which the north one is the most spectacular, with a size 1.8 Mpc × 150 kpc. These radio relics correspond to large-scale shocks formed during mergers of galaxy clusters. Synchrotron emission is emitted by cosmic-ray electrons re-accelerated in the shocks. Measuring the spectral index of the radio emission and its gradient helps to better understand the electron acceleration mechanism. In the northern relic, a clear steepening of the index from the front towards the back of the shock supports some injection models in which the emission is produced by a single burst of spectrally-aged accelerated radio electrons.