Study of Gamma-Ray Counterparts of Galactic and Extragalactic Multi-Messenger Sources

This PhD work focuses on the study of gamma-ray counterparts of candidate multi-messenger sources, with a particular attention on those which are believed to be production and acceleration sites of cosmic rays. Initially reliant on optical observations, astronomical science underwent a revolution during the second half of the previous century, due to the opening of new windows of the electromagnetic spectrum. The new observations in the radio, infrared, ultraviolet, and high-energy wavebands paved the path for the birth of new disciplines, first Multiwavelength and later Multi-Messenger Astronomy. These provide a huge contribution in the study of cosmic rays (CRs) which, in Galactic environments, may originate from sources such as supernova remnants (SNRs). The SNR paradigm, strengthened by Fermi’s particle acceleration model, positions SNRs as primary CR sources. However, the deflection of CRs by interstellar magnetic fields does not allow a direct tracking back to the source. In this context, gamma rays play a crucial role, being able to trace the non-thermal emission of the accelerated CRs. The thesis is structured as follows: Chapter 1 presents the neutral and charged messengers whose combined information can help to shed light on the violent phenomena they are emitted from and to identify the sources of CRs, a problem still unsolved. Chapter 2 delves into SNRs and CR acceleration mechanisms. Chapter 3 reviews past, current and future gamma-ray observational technologies, from space-born instruments for the elusive MeV gap to ground-based observatories for the TeV regime. Moreover, the first results of an R&D activity for the development of a small Compton telescope equipped with a novel silicon tracker for low-energy gamma rays are reported. Finally, chapter 4 presents the study of the SNR W44, based on the analysis of data collected by the gamma-ray space telescope Fermi-LAT and by the ground-based Cherenkov telescopes belonging to the MAGIC experiment. The results of these analyses were interpreted in terms of the CR escape process, in a novel scenario where also a certain fraction of low-energy particles are able to escape from the forward shock of the SNR.