Neutrinos and Ultra-high-energy Cosmic-ray Nuclei from Blazars

Abstract

We discuss the production of ultra-high-energy cosmic-ray (UHECR) nuclei and neutrinos from blazars. We compute the nuclear cascade in the jet for both BL Lac objects and flat-spectrum radio quasars (FSRQs), and in the ambient radiation zones for FSRQs as well. By modeling representative spectral energy distributions along the blazar sequence, two distinct regimes are identified, which we call “nuclear survival” (typically found in low-luminosity BL Lacs) and “nuclear cascade” (typically found in high-luminosity FSRQs). We quantify how the neutrino and cosmic-ray (CR) emission efficiencies evolve over the blazar sequence, and we demonstrate that neutrinos and CRs come from very different object classes. For example, high-frequency-peaked BL Lacs (HBLs) tend to produce CRs, and high-luminosity FSRQs are the more efficient neutrino emitters. This conclusion does not depend on the CR escape mechanism, for which we discuss two alternatives (diffusive and advective escape). Finally, the neutrino spectrum from blazars is shown to significantly depend on the injection composition into the jet, especially in the nuclear cascade case: Injection compositions heavier than protons lead to reduced neutrino production at the peak, which moves at the same time to lower energies. Thus, these sources will exhibit better compatibility with the observed IceCube and UHECR data.