Macroscopic and microscopic aspects of particle acceleration by cosmic shocks

Banfi, Serena (2022) Macroscopic and microscopic aspects of particle acceleration by cosmic shocks, [Dissertation thesis], Alma Mater Studiorum Università di Bologna. Dottorato di ricerca in Astrofisica, 34 Ciclo. DOI 10.48676/unibo/amsdottorato/10060.
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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.

Tipologia del documento
Tesi di dottorato
Banfi, Serena
Dottorato di ricerca
Settore disciplinare
Settore concorsuale
Parole chiave
shocks, cosmic web, numerical simulations, magnetohydrodynamics, particle-in-cell, particle acceleration, galaxy clusters
Data di discussione
21 Marzo 2022

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