Blazars as sources of neutrinos and ultra-high-energy cosmic rays

The origin of ultra-high-energy cosmic rays (UHECRs) is still unclear, but there is evidence that their origin lies in extragalactic sources. At the same time, neutrino telescopes like Ice- Cube have observed a flux of high-energy astrophysical neutrinos, expected to originate in cosmic ray (CR) interactions. However, the arrival directions of the observed neutrinos do not seem to significantly correlate with the coordinates of known high-energy astrophysical sources. In this thesis we contribute to the understanding of this problem by exploring blazars, a class of active galactic nuclei (AGNs), as potential sites for the acceleration and interaction of UHECRs. Motivated by evidence that a fraction of the observed UHECRs are heavier than protons, we model numerically the interactions of a population of accelerated nuclei with the environment photon fields present in blazars. We then estimate the emitted neutrinos and UHECR spectrum and composition. We conclude that in low-luminosity blazars, accelerated CRs do not interact efficiently due to the low density of the photon fields, but instead escape the source unscathed, while in high-luminosity blazars (such as flat-spectrum radio quasars, FSQRs), photo-hadronic and photo-nuclear interactions are efficient, leading to abundant neutrino production and the development of a nuclear cascade of secondary nuclei that are lighter than the accelerated isotope. We then use our model to quantify the neutrino emission from the entire cosmological distribution of blazars. We conclude that a population of low-luminosity blazars, currently unobserved but expected theoretically, can explain the entire IceCube flux at the highest energies. However, if that is the case, then high-luminosity blazars must have a comparatively low hadronic content in order to explain the lack of correlations between neutrinos and bright gamma-ray sources. We also model neutrino and photon emission from one particular blazar, object TXS 0506+056, from whose direction a neutrino was recently detected during a state of enhanced electromagnetic activity. We test the hypothesis that a signal of 13±5 muon neutrinos observed by IceCube from the same direction in 2014-15 may have originated in the same source. Given the constraints from multi-wavelength observations, we show that such photo-hadronic models can explain at most 5 events observed by IceCube, which seems to disfavor the hypothesis that this blazar was the source of the signal. Finally, we turn our attention from blazars to the remnants of neutron star mergers, and study their potential as CR emitters. The only neutron star merger ever observed was detected recently in gravitational waves, and its remnant has since been monitored by telescopes in different wavelengths. We model the non-thermal interactions in the source and show that radio and X-ray observations, as well as the non-observation of the remnant in gamma rays, can provide crucial constraints on the magnetic field strength. Given these constraints, we estimate that this source class is capable of accelerating and emitting very-high-energy CRs. This result emphasizes the importance of future gravitational wave observations to better constrain the population of these sources. While the results of this thesis provide steps towards an understanding of CR and neutrino production, further work is still necessary, including a joint source-propagation model capable of better constraining the sources based on UHECR data.