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

Abstract

There are two promising scenarios that explain the ankle, which is a dip in the spectrum of cosmic rays at ∼ 10 19 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 caused by pair production on the cosmic microwave background radiation (proton-dip scenario). In this paper, we consider 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 ∼ 10 20 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 ∼ 10 16 eV in the protondip scenario on the condition that extragalactic protons significantly contribute to the observed cosmic rays down to 10 17 eV. Thus, we conclude 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 AGNs and GRBs. We also found an interesting feature that the neutrino flux at ∼ 10 18 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 compare 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.2

Constraints on the origin of ultra-high-energy cosmic rays from cosmogenic neutrinos and photons

“…6. The experimental spectrum can only be matched above 10 19 eV for this model, and high-energy neutrinos expectations are then not higher than for the dip model (see Takami et al 2009;Kotera et al 2010) despite the harder spectral, owing to the lower luminosity needed to match only the highest energy point of the spectrum (see discussion below). The constraints brought by the Fermi diffuse flux are then much looser than for the dip model and comparable to those of the mixed composition model.…”

“…Because these cosmogenic neutrinos can travel from their production site without undergoing interactions or deflection, they were, soon after this pioneering work, considered as potentially interesting probes of the highest energy phenomena in the universe. Their expected flux on earth was intensively calculated in the past decades for different astrophysical scenarios on the cosmological evolution of the sources, the composition or the maximum energy at the sources (see for instance Stecker 1979;Hill & Schramm 1985;Engel et al 2001;Kalashev et al 2002;Secker & Stanev 2005;Hooper et al 2005;Ave et al 2005;Stanev et al 2006;Allard et al 2006;Anchordoqui et al 2007;Takami et al 2009;Berezinsky 2009;Ahlers et al 2009;Kotera et al 2010).…”

Cosmic neutrino cascades from secret neutrino interactions

“…The cascade bump makes the spectrum harder below the absorption dip. The γ = 1.4 case roughly mimics an astrophysical spectrum with a pγ origin (e.g., [97,98]), as opposed to a flatter power-law spectrum with a pp origin (e.g., [99]). For this case, the spectrum with νSI can have twin bumps, separated by an absorption dip.…”

Extragalactic propagation of ultrahigh energy cosmic-rays

“…Since these cosmogenic neutrinos can travel from their production site without undergoing interactions or deflections, they were, soon after this pioneering work, considered as potentially interesting probes of the highest energy phenomena in the universe. Their expected flux on earth was intensively calculated in the past decades for different astrophysical scenarios on the cosmological evolution of the sources, the composition or the maximum energy at the sources (see for instance [153,53,154,155,156,157,158,159,66,160,161,162,163,164]). …”