Magnetic fields in radio relics and in the outskirts of galaxy clusters

The observation of diffuse synchrotron radio emission in galaxy clusters reveals the existence of relativistic electrons and large-scale magnetic fields permeating the intra-cluster volume. Shocks developed during mergers of galaxy clusters are thought to originate the elongated, arc-shaped, and polarized sources found in cluster outskirts known as radio relics. The properties of the magnetic fields in radio relics, the possible magnetic field amplification caused by the shock passage, and its connection to the relics formation mechanism, as well as the value of large-scale magnetic fields in cluster outskirts, are the main topics addressed in this Thesis. Using polarimetric radio observations performed with the Jansky Very Large Array (JVLA) and with the Low Frequency Array (LOFAR), I have investigated the properties of magnetic fields in galaxy clusters outskirts via both statistical Faraday rotation measures and diffuse source polarimetry. The comparison of measured quantities with advanced numerical simulations of, both, radio relic emission and intra-cluster magnetic fields has proved to be a fundamental tool to constrain magnetic field physical parameters. Among the results, the Faraday rotation studies of two double relic galaxy clusters presented in this Thesis show that magnetic-fields are not significantly amplified in the shock region. Despite this, low Mach numbers shocks can stretch and align magnetic field lines, which appear well ordered on the emission scales of radio relics from polarimetric observations. The magnetic field structure inside radio relics is complex and likely filamentary. These results will be corroborated by an ongoing systematic study of clusters with double radio relics. The results achieved in this Thesis proved the extraordinary potential of low-frequency polarization observations for the study of large-scale magnetic fields in the outskirts of galaxy clusters and beyond.