Macroscopic and microscopic aspects of particle acceleration by cosmic shocks

Cosmic collisionless shocks are highly energetic phenomena in which different non-thermal processes take place. Above all, particle acceleration is arguably the most important, and observations of its signatures can be a powerful tool to constrain the local plasma and magnetic properties at the acceleration site. Within large-scale structures, the presence of cosmic rays can be revealed by means of different detection approaches: relativistic electrons can emit in radio via synchrotron radiation (radio relics), while energetic protons can interact with thermal protons of the intracluster medium and emit in the gamma-ray band. Both electrons and protons should in principle be accelerated by the first order Fermi acceleration mechanism known as diffusive shock acceleration: however, only evidence of cosmic-ray electrons has been detected so far in galaxy clusters, while no signatures of cosmic-ray proton have been reported. With a comprehensive analysis from 'macroscopic' to 'microscopic' scales, I address the missing gamma-ray issue by means of advanced numerical simulations, in which extragalactic magnetic fields are evolved together with the dynamics of large-scale structures. The primary factor at play is obliquity, the angle between the shock propagation direction and the magnetic field. I first determine the distribution of obliquity in cosmological simulations as a function of the medium in which the shock takes place. Then, I detect a pattern in the arrangement of the magnetic field around filaments and perform a quantitative study of the topological properties of the magnetic field surrounding the cosmic web. Finally, shocks in filaments are more closely investigated with new particle-in-cell simulations on much smaller scales, where the actual acceleration efficiency by realistic shocks can be measured. The new insights obtained from these numerical simulations provide a tool for the correct interpretation of observations and to estimate the magnetic properties that can be inferred, e.g. from Faraday rotation measurements.