Active Galactic Nuclei (AGN) are among the brightest, persistent, extra-galactic sources in the sky, characterised by boosted emission spanning the entire electromagnetic spectrum, from radio to very high-energy gamma-rays. They are powered by a central supermassive black hole (SMBH) that accretes mass from its surroundings. In some AGN, the gravitational energy released by accreting gas is efficiently converted into kinetic energy, launching twosided, collimated, and highly relativistic jets. Blazars are a special class of AGN in which the jet is pointed toward the observer’s line of sight, resulting in strongly Doppler-boosted emission. In the standard picture of a blazar’s inner structure, the SMBH is surrounded by a geometrically thin, optically thick accretion disk (AD). Above it, a hot corona scatters photons from the disk up to X-ray energies. On larger scales, a dusty torus of molecular gas absorbs and re-emits radiation in the infrared (IR). The ultraviolet (UV) light from the AD photo-ionises gas clouds in the broad line region (BLR), which move rapidly in the gravitational potential of the BH closer to it, while the narrow line region (NLR) lies further out at lower velocities. According to the AGN “unification model”, blazars are traditionally divided into BL Lacertae (BL Lacs) objects and flat spectrum radio quasars (FSRQs) based on the strength and presence of optical emission lines. However, this purely empirical classification has been increasingly challenged in favour of more physically motivated distinctions based on the underlying accretion properties and jet power. Theoretical models predict that relativistic particles in blazar jets can be accelerated to high energies. Their radiative cooling results in the observed multi-wavelength (MWL), predominantly non-thermal, jet emission seen in blazar spectral energy distributions (SED). These accelerated particles can also interact with photons from the accretion disk or external radiation fields (such as the photo-ionised radiation from BLR and NLR), potentially leading to the production of high-energy neutrinos. These elusive particles are unique messengers to study our Universe: nearly massless and barely interacting with matter, they travel long cosmological distances essentially undisturbed, carrying information about their production site and direction. The IceCube Neutrino Observatory, operating since 2008 at the South Pole, is currently the most sensitive detector for investigating the nature of astrophysical neutrinos. These particles are thought to arise from interactions between ultra-high-energy cosmic rays and ambient matter or radiation, making them key tracers of cosmic-ray sources. Yet, the precise origin of astrophysical neutrinos and the mechanisms that produce them remain open questions in astrophysics. Among the most promising candidate sources for high-energy neutrino emission are AGN, and blazars in particular. Several studies have pointed to spatial correlations between IceCube neutrino events and blazars. This Thesis focuses, for the first time, on a sample of 58 blazars found to coincide with regions of anisotropies (so-called “hotspots”) in IceCube’s neutrino sky maps. Through a multi-wavelength approach, this work investigates the physical properties of these neutrino candidate sources, aiming to uncover the conditions that may favour efficient neutrino and cosmic-ray production. The analysis combines archival data, literature sources, and new observations across the radio, optical and gamma-ray bands, with primary attention on optical spectroscopy, to characterise both the accretion regime and jet properties of these sources. The results are then placed in the broader context of the general blazar population to explore how the sample behaves and to gain insight into the potential link with neutrinos. The findings suggest that radiatively efficient accretion may be a promising marker for neutrino emission; however, this remains far from a complete picture. Expanding this work within a more comprehensive multiwavelength and multimessenger framework, including observational constraints from other bands, from neutrino analyses and theoretical SED modelling, will be key to advancing our understanding of this complex and still unsolved puzzle.
Alessandra Azzollini (Thu,) studied this question.