Study of cosmic-ray interactions in candidate sources. Applications to the interpretation of the energy spectrum and mass composition data of the Pierre Auger Observatory.

In the last decades significant progresses in the understanding of Ultra-HighEnergy Cosmic rays (UHECRs) have been made, thanks to the measurements provided by very large ground-based experiments such as the Pierre Auger Observatory. However, it is still a mystery what the sources of UHECRs are and by which processes they are accelerated. This thesis has been developed inside the Auger Collaboration and was devoted to study astrophysical models capable to describe the UHECRs measurements, in particular the energy spectrum and mass composition, in order to infer information about the properties of UHECRs sources. As a first approach, an astrophysical scenario with a population of identical extra-galactic point-like sources homogeneously and isotropically distributed has been tested, thanks to the use and the implementation of a software for the fit of the energy spectrum and mass composition, with the aim to describe the Auger data above the so-called ankle feature (E = 1018.7 eV). At this stage only the extra-galactic propagation has been taken into account (“propagation model”). In order to extend the analysis below the ankle, two populations of sources capable to accelerate cosmic-ray nuclei with different spectral parameters have been considered. To this purpose, the software previously used within the Auger Collaboration has been reorganized and improved. The results suggest that, within this scenario, a reasonable description of the measurements over a large energy range can be found, in terms of spectral parameters and mass composition at the escape from the sources. In the second part, the hypothesis of point-like sources has been abandoned and a study of the UHECRs interactions in the environment surrounding the sources has been performed (“source-propagation model”), applied in particular to Starburst Galaxies. This could be helpful to explain the origin of some features observed in the UHECR measurements, therefore motivating the results found with the “propagation model”. As an additional novelty, we have been included the hadronic interactions in the source environment. By exploring several configurations of source characteristics, we demonstrate that it is possible to accommodate the Auger measurements with the scenario of cosmic rays accelerated and processed in Starburst Galaxies. Secondary messengers directly produced in these sources and along the UHECR travel to the Earth have been computed, in order to complete the multi-messenger picture and improve the constraining capability of our model.