Magnetic fields and cosmic rays in the large-scale filaments

Recent low-frequency radio observations have revealed the presence of diffuse radio emission on megaparsec scales, along the densest regions of the Cosmic-Web filaments. The discovery of these bridges has triggered the development of theoretical models to explain the particle acceleration mechanisms and the properties of magnetic fields in these previously unexplored regions of the Universe. The primary aim of this Thesis is to investigate, for the first time, the properties of cosmic-web bridges and filaments, focusing on their spectral characterisation, magnetic field properties, and occurrence. In particular, in this Thesis I will focus on the multifrequency study of the most prominent and first discovered example of a radio bridge, between the galaxy clusters Abell 0399 and Abell 0401. Additionally, I have expanded the sample of radio bridges by studying other promising systems, such as the merger between Abell 2061 and Abell 2067. The only two intercluster bridges confirmed thus far are associated with highly dynamic regions between merging galaxy clusters, but fainter, more extended filaments on scales of tens of megaparsecs remain undetected. To gain a comprehensive understanding of magnetic fields in these poorly explored environments, I employed the Faraday rotation measure of linearly polarised sources along the line of sight to magnetised plasma in three rich superclusters: Corona Borealis, Leo, and Hercules. With this method, I detected the presence of magnetic fields at levels higher that what is predicted solely by the adiabatic compression of a primordial seed in cosmic filaments. This work is a step toward constraining theoretical models of cosmic magneto-genesis and particle-acceleration mechanisms on the largest scales. Overall, the work presented in this Thesis highlights the importance of radio observations as probe of the non-thermal emissions and magnetic fields in cosmic filaments, advancing our understanding of their physical nature and role in large-scale structure formation.