Present and Future Constraints on Axion-Like Particles from Blazars: From current gamma-ray telescopes to the Cherenkov Telescope Array Observatory

The field of very high energy (VHE) gamma-ray astrophysics has been revolutionized in the last few decades by the advent of stereoscopic imaging atmospheric Cherenkov telescope arrays, designed and operated by large international scientific collaborations. Astrophysical targets of various kinds have been routinely observed with increasing precision and sensitivity, in coordination with instruments working in other wavebands, with the common aim of providing a unified multi-wavelength view of the physical phenomena governing the behavior of these sources at all energy scales. Besides conventional astrophysics, VHE observations are also used to investigate the nature of the dark matter in the Universe and to study exotic physics effects related to the cosmological propagation of photons. Outside our galaxy, the jetted active galactic nuclei denoted as blazars clearly dominate the extragalactic VHE sky. Their relativistic jets, closely aligned to the line of sight from Earth, are a rich and complex environment, characterised by the presence of relatively strong magnetic fields over parsec-scale lengths. Owing to their cosmological distance from Earth, these sources serve as ideal targets to investigate effects of non-standard gamma-ray propagation. Among these, the existence of axion-like particles (ALPs) may be probed by exploiting their expected coupling to photons. The mechanism of ALP-photon conversions in external magnetic fields might lead to distinct signatures in the blazars' VHE spectra, which could be detected by the present or future generation of Cherenkov telescopes. In this thesis, the present status of ALP searches and experimental constraints from the observation of blazars with Cherenkov telescopes is discussed. Focus is placed on the analysis of data from MAGIC and LST-1, the first Large-Sized Telescope prototype of the Cherenkov Telescope Array Observatory (CTAO). Once the full initial configuration of CTAO is operational, it will provide greatly improved flux sensitivity and energy resolution compared to current instruments. In order to estimate how these improvements will affect the attainable limits on ALPs from blazars in the near future, sensitivity projections based on simulated observations with CTAO are also presented and discussed.