Neutrino signatures of the origins of cosmic rays

The intensity of extraterrestrial neutrinos discovered by IceCube [M. G. Aartsen et al. (IceCube Collaboration), Science 342 (2013) 1242856] is in reasonable agreement with predictions of neutrinos from the jets of active galactic nuclei due to pion production by accelerated protons [K. Mannheim, Astropart. Phys. 3 (1995) 295]. The observed deficit of Glashow-resonance events at 6.3 PeV could result from the suppression of events with energies larger than PeV due to the presence of a strong “big blue bump” radiation field in flat-spectrum radio quasars. The total neutrino spectrum could exhibit a two-component structure in which the sub-PeV component is dominated by the jets from AGN with high accretion rates and another component peaking at EeV energies due to those with low accretion rates. Each component of the neutrino spectrum should carry the energy flux that corresponds to its relative contribution to the extragalactic gamma ray background. The arrival directions should correlate with known sources, and a simple test shows that the PeV events can indeed be explained by known blazars with prominent radio jets. If a Galactic component of cosmic rays with energies per nucleon above knee energies exists, as air shower array data seem to indicate, the neutrinos due to pion production from these sources are also detectable, pinpointing them an energies where gamma-ray observations are not yet possible.