Neutrinos and ultra-high-energy nuclei from blazars: From a single-source model to a population study

Five years after the discovery of the diffuse astrophysical neutrino flux, its origin is still unclear. At the same time, composition measurements of ultra-high-energy cosmic rays (UHECRs) seem to indicate a component heavier than protons. In this talk we present an interaction model of protons and heavier nuclei in blazar jets. In bright blazars, photo-nuclear interactions lead to extensive nuclear cascades of lighter elements, and we calculate numerically for the first time their effect on blazar neutrino emission. In the case of flat-spectrum radio quasars (FSRQs), our model considers the effect of CR propagation in the broadline region, which boosts neutrino production in these sources. We then apply this model to the entire cosmological distribution of blazars using the updated blazar sequence, based on a large sample from the Fermi 3LAC catalog. We conclude that the IceCube flux of sub-PeV and PeV neutrinos can be explained by a population of low-luminosity blazars, not yet observed in gamma rays but predicted by cosmological evolution models. On the other hand, the IceCube stacking analysis limits the contribution from bright FSRQs, which places constraints on their hadronic content. Based on this model, we can estimate the number of future coincident neutrino detections like that of TXS 0506+056.