Investigating Extended Gamma-Ray Sources with ASTRI and CTAO: Observation, Analysis and Modelling

This thesis investigates the gamma-ray emission from Galactic sources with next-generation IACTs (Imaging Atmospheric Cherenkov Telescopes) like the telescopes of the ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Mini-Array and CTAO (Cherenkov Telescope Array Observatory), with the dual aim of addressing the role of young massive star clusters (YMSCs) in the acceleration of Galactic cosmic rays and improving their characterization capabilities of Galactic sources in the multi-TeV energy regime. The first part focuses on the characterization of ASTRI, using Monte Carlo simulations to test the analysis chain and to benchmark sensitivity. The point-spread function (PSF) was derived from both simulations and real data from the first ASTRI telescopes, i.e. ASTRI–Horn and ASTRI–1, leading to an energy-dependent description suitable for morphological analyses. In addition, the residual background across the field of view was studied, and a two-dimensional model was implemented in the instrument response functions, improving the reliability of extended source reconstructions. The second part addresses the scientific potential of the first stereoscopic phase of the ASTRI Mini-Array. A survey strategy was proposed to target ultra-high-energy sources discovered by LHAASO (Large High Altitude Air Shower Observatory). Dedicated simulations of the Cygnus region, the Boomerang Nebula, and SS 433 demonstrated that the Mini-Array can resolve complex morphologies, constrain emission mechanisms, and identify candidate PeVatrons in the multi-TeV energy range. The final part examines YMSCs as possible gamma-ray emitters. Source models were constructed and compared to TeV halo templates. Simulations with ASTRI and CTAO response functions were used to test a morphological classification scheme based on excess count profiles. The results show that the mechanical efficiency of stellar winds influences the gamma-ray morphology, and that some of the extended sources observed by LHAASO may be consistent with a YMSC origin. In summary, the thesis has: validated the ASTRI analysis pipeline with simulations; introduced refined PSF and background modelling methods; developed survey and analysis strategies for the ASTRI Mini-Array; and it also proposed a morphological framework to identify YMSCs as potential Galactic cosmic-ray accelerators. These contributions strengthen the technical and scientific preparation of ASTRI and provide concrete improvements and methodologies for the study of gamma-ray sources.