Cosmogenic neutrinos as a probe of the transition from Galactic to extragalactic cosmic rays

There are two promising scenarios that explain the ankle, which is a dip in the spectrum of cosmic rays at ∼ 10¹⁹ eV. A scenario interprets the ankle as the transition from Galactic to extragalactic cosmic rays (ankle-transition scenario), while the other is that the dip is caused by pair production on the cosmic microwave background radiation (proton-dip scenario). In this paper, we considered whether cosmogenic neutrinos can be a clue to judge which scenario is favored. We calculated the fluxes of cosmogenic neutrinos following these scenarios with plausible physical parameter sets, and found several important features as follows. First of all, the neutrino flux at ∼ 10²⁰ eV becomes much higher in the ankle-transition scenario as long as the maximum energy of the cosmic rays at sources is sufficiently high. On the other hand, the neutrino spectrum has a characteristic peak at ∼ 10¹⁶ eV in the proton-dip scenario on the condition that extragalactic protons significantly contribute to the observed cosmic rays down to 10¹⁷ eV. Thus, we concluded cosmogenic neutrinos should give us a clue to judge which scenario is favored, unless these features are masked by the neutrino background coming from possible, powerful neutrino sources such as active galactic nuclei and γ-ray bursts. We also found an interesting feature that the neutrino flux at ∼ 10¹⁸ eV depends only on the cosmological evolution of the cosmic ray sources. That means cosmogenic neutrinos with the energy bring us information on the cosmological evolution of the sources of ultra-high energy cosmic rays. Finally, we compared the fluxes of cosmogenic neutrinos with the expected sensitivity curves of several neutrino detectors, and conclude the detection of cosmogenic neutrinos in the near future is promising.