Extragalactic transient phenomena and preparations for the future Cerenkov Telescope Array Observatory

Active galactic nuclei (AGN) are among the most energetic extragalactic objects in the universe. Blazars are AGN with a relativistic jet oriented towards the observer. This jet is composed of accelerated particles that can emit across the entire electromagnetic spectrum. The γ rays produced can be studied through Cerenkov astronomy. The Cerenkov Telescope Array Observatory (CTAO) will be the next generation of telescopes operating at very high energies, with an increased sensitivity by a factor of five to ten depending on the energy compared to the current generation of imaging atmospheric Cerenkov telescopes. The NectarCAM will be the camera that equips the nine medium-sized telescopes of the northern CTAO site. In this manuscript, we address, through the description of nectarchain, the development of the calibration code for the NectarCAM. We particularly study gain calibration, with two distinct methods which we compare. We also study the optimization of the signal extraction method in the pixels. Blazars exhibit variability on multiple timescales. For example, we can focus on long-term behavior, ranging from months to decades. The increased sensitivity of CTAO will allow for a more refined reconstruction of the power spectral density of these objects than is currently possible. In this thesis, we aim at quantifying this hypothesis through CTAO observation simulations for the blazars listed in the Key Science Projects AGN of CTAO. Real data from H.E.S.S. are also used to validate our analysis method. Blazar variability on timescales shorter than a week is probed during eruptive transient phenomena. To quantify the future prospects of CTAO for studying these eruptions, we rely on phenomenological models, adjusted to significant eruptions of the past twenty years. Here again, we simulate CTAO observations of these eruption models. The objective is to quantify the capabilities of CTAO to detect variability, or to discriminate between different emission models, hadronic or leptonic. Finally, we also attempt to study the prospects offered by CTAO for detecting periodicity in the very high energy γ rays emission of a blazar. (author)