Shock waves and non-thermal phenomena in merging galaxy clusters

In this Thesis, we used state of the art radio and X-ray datasets and techniques to derive constraints on the formation mechanisms of diffuse radio emission in merging galaxy clusters. In particular, turbulence is believed to be responsible for the formation of the central and likely spherical sources called radio halos, while shocks are the origin of the elongated and polarized emissions found in cluster outskirts known as radio relics. Although this scenario seems supported by current observations, the processes that originate these synchrotron sources are still poorly constrained. An important goal achieved during the Thesis is a progress on the relic-shock connection and on the origin of radio relics. This was obtained thanks to the detection in the X-rays of new shocks in merging galaxy clusters. In combination with the analysis of radio observations, this allowed us to derive efficient constraints on the mechanisms of particle (re)acceleration and on the magnetic fields in relics. Notably, we demonstrated for the first time, in an homogeneous way, that merger shocks can not reproduce the luminosity of radio relics if particles are accelerated from the thermal pool. This strongly support that other mechanisms, such as shock re-acceleration, are involved in the formation of this kind of sources. LOFAR is a new generation interferometer that is providing a revolutionary view of clusters at low frequencies. For this reason, the exploitation of LOFAR observations represented a central task of the Thesis. We used LOFAR observations in combination with X-ray and radio data coming from other facilities to study non-thermal phenomena in two dynamically complex cluster mergers providing also first hints of a radio bridge of emission connecting two clusters in a pre-merging phase. Our results proved the extraordinary potential of LOFAR in galaxy cluster science.