Search for extreme blazars signal with the KM3NeT/ARCA telescope

The study presented in this thesis explores the connection between extreme blazars and high-energy neutrinos detected by the KM3NeT neutrino telescope, within the framework of multi-messenger astrophysics. The main motivation for this research originates from recent evidence of a possible spatial and temporal coincidence between an extreme blazar and a high-energy neutrino detected by the IceCube observatory. Blazars are among the most promising candidates for the acceleration of ultra-high-energy cosmic rays (UHECRs) and the subsequent production of astrophysical neutrinos. This work aims to model extreme blazars broadband spectral energy distributions (SEDs), estimate their possible expected neutrino fluxes, and search for correlations between their expected neutrino emission and neutrino observation throught KM3NeT detector. Extreme blazars are considered among the best astrophysical laboratories to search for neutrino production in TeV-PeV range due to their lepto-hadronic production associated with their hard synchrotron peaked target photons. A representative sample of extreme blazars was selected from the 3HSP catalogue of high-synchrotron-peaked sources. Their broadband SEDs were reconstructed using multi-wavelength data from Swift, Beppo-Sax, Fermi-LAT, H.E.S.S., and VERITAS, corrected for extragalactic background light (EBL) absorption. The modeling was carried out using the LeHaMoC code. This allowed the estimation of neutrino fluxes produced via photohadronic interactions within the blazar jets. The modeled neutrino spectra were used to build the physical weight of stacking analysis in KM3NeT-ARCA. The resulting sensitivity limits, derived from KM3NeT’s measured background and effective area, were used to constrain their maximal associated flux. The results of this study confirm that, under realistic physical parameters assumption, extreme blazars can produce neutrino fluxes within the sensitive energy range of KM3NeT, though no significant excess has yet been observed. The limits derived using real KM3NeT data already begin to constrain the most optimistic hadronic scenarios. Importantly, this analysis validates the feasibility of correlating KM3NeT events with well-characterized gamma-ray blazar samples, paving the way for future discoveries as the detector reaches its future detector configurations. The analysis represents one of the first stacking analysis using KM3NeT data and the first dedicated to the EHBL population. The results demonstrate the detector’s growing potential to identify the sources of cosmic neutrinos and to test lepto-hadronic emission models in active galactic nuclei.