Neutrinos and Ultra-High-Energy Cosmic-Ray Nuclei from Blazars

TL;DR

This paper models neutrino and ultra-high-energy cosmic-ray production in blazars, identifying two regimes based on luminosity and composition, and explores how different blazar types contribute to observed neutrino and cosmic-ray fluxes.

Contribution

It introduces a detailed modeling framework for nuclear cascades in blazar jets, distinguishing regimes of nuclear survival and cascade, and analyzes their implications for neutrino and cosmic-ray emissions.

Findings

High-frequency BL Lacs mainly produce cosmic rays.

HL-FSRQs are the most efficient neutrino sources.

Neutrino spectra depend on the injection composition, especially in nuclear cascade regimes.

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 demonstrate that neutrinos and CRs come from very different object classes. For example, high-frequency peaked BL Lacs (HBLs) tend to produce CRs, and HL-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.