Multi-wavelength and multi-messenger studies of blazars as candidate neutrino emitters

Even after a century from their discovery, the origin of high-energy cosmic rays is still an open issue. Cosmic rays are charged particles able to reach Earth at energies up to 1020 eV, requiring their sources to be natural cosmic accelerators. However, the identification of such acceleration sites remains puzzling. Moreover, due to their electric charge, cosmic rays undergo deflection by intergalactic magnetic fields before reaching Earth, preventing a direct association with their emitting sources. A promising approach in establishing this association is represented by multi-messenger astrophysics, a relatively recent field exploiting the diverse information carried by different messengers originating from the same cosmic sources. In this context, the detection of high-energy astrophysical neutrinos and photons from the same objects represents an important step towards the identification of genuine cosmic ray accelerators. While photons can originate in both leptonic and hadronic processes, neutrinos are direct tracers of hadronic interactions, since they are thought to be produced by cosmic rays interacting with ambient matter or radiation fields during their acceleration or propagation. From the same interactions, also γ−rays are expected to be produced, making a joint observation of high-energy photons and neutrinos a stronger indication of hadronic processes from the emitting source. Neutrino astronomy has seen significant advancements in the last decade. In 2013 the discovery of an astrophysical neutrino flux was announced by the IceCube Neutrino Telescope, confirming the existence of high-energy neutrinos of cosmic origin, while in more recent years the first potential neutrino sources were detected. The first evidence for a neutrino source came in 2017, when a high-energy neutrino event detected by IceCube was found to be in spatial and temporal coincidence with the flaring blazar TXS 0506+056. Observatories from all around the world operated a prompt follow-up of the neutrino event, pointing in its direction and finding it consistent with the location of the blazar. The source was detected in both high energy (HE, 0.5MeV < E < 100 GeV) and very high energy (VHE, E > 100 GeV) γ−rays and the chance coincidence probability of the photon-neutrino association was rejected at the ∼ 3σ level. Two more recent studies performed by the IceCube Collaboration found evidence of neutrino emission from the Seyfert-II galaxy NGC 1068 (4.2σ) and from the Galactic plane (4.5σ), involving an analysis of the integral neutrino signal recorded with the IceCube detector in 9 and 10 years of data respectively. The former identified galaxy is an active galaxy as TXS 0506+056. However, differently from the blazar, it does not show a relativistic jet of particles. On the contrary, the latter result implies that a contribution to the astrophysical neutrino flux is given also by nearby sources in our galaxy. The variety of identified objects suggests different processes and astrophysical environments contributing to the neutrino background. This thesis comprises different works dealing with two relevant open problems in this field: the investigation of emission processes at work in the sources emission and the search for new neutrino (and hence cosmic-ray) sources. I mainly concentrated on the study of blazars as possible neutrino emitters, but an unbiased follow-up of neutrino events is also included in this thesis.