Constraints on Ultrahigh-Energy Cosmic-Ray Sources from a Search for Neutrinos above 10 PeV with IceCube

Abstract

We report constraints on the sources of ultra-high-energy cosmic rays (UHECRs) above 10 9 GeV, based on an analysis of seven years of IceCube data. This analysis efficiently selects very high energy neutrino-induced events which have deposited energies from 5 × 10 5 GeV to above 10 11 GeV. Two neutrino-induced events with an estimated deposited energy of (2.6 ± 0.3) × 10 6 GeV, the highest neutrino energy observed so far, and (7.7 ± 2.0) × 10 5 GeV were detected. The atmospheric background-only hypothesis of detecting these events is rejected at 3.6σ. The hypothesis that the observed events are of cosmogenic origin is also rejected at >99% CL because of the limited deposited energy and the non-observation of events at higher energy, while their observation is consistent with an astrophysical origin. Our limits on cosmogenic neutrino fluxes disfavor the UHECR sources 3 having cosmological evolution stronger than the star formation rate, e.g., active galactic nuclei and γ-ray bursts, assuming proton-dominated UHECRs. Constraints on UHECR sources including mixed and heavy UHECR compositions are obtained for models of neutrino production within UHECR sources. Our limit disfavors a significant part of parameter space for active galactic nuclei and new-born pulsar models. These limits on the ultra-high-energy neutrino flux models are the most stringent to date. Introduction -The sources of ultra-high-energy cosmic rays (UHECRs; cosmic-ray energy E CR 10 18 eV) remain unidentified [1]. The majority of the candidate objects are extra-Galactic, such as Active Galactic Nuclei (AGN) [2-6] , γ-ray bursts [7][8][9][10][11][12][13], and starburst galaxies [14][15][16][17][18][19]. UHECR interactions with ambient photons and matter at sources generate astrophysical neutrinos with 5% of the parent UHECR energy, on average [20][21][22]. Thus, a substantial fraction of these Extremely High-Energy (EHE) astrophysical neutrinos is expected to have an energy above 10 7 GeV. Moreover, neutrinos with energies above ∼107 GeV are expected to be produced in the interactions between the highest-energy cosmic rays and background photons in the universe [23]. In the following we refer to the neutrinos produced in these interactions as cosmogenic neutrinos [24]. These astrophysical and cosmogenic EHE neutrinos can constitute key messengers identifying currently unknown cosmic accelerators, possibly in the distant universe, because their propagation is not influenced by background photon or magnetic fields.

Combined fit of spectrum and composition data as measured by the Pierre Auger Observatory

“…We briefly discuss some possible extensions in section 6. A description of the spectrum in terms of elementary fluxes injected from astrophysical sources, all the way from log 10 (E/eV) ≈ 18 up to the highest energies, with a single component, is only possible if UHECRs are protons, since protons naturally exhibit the ankle feature as the electron-positron production dip; however this option is at strong variance with Auger composition measurements [10,43] and, to a lesser extent, with the measured HE neutrino fluxes [44][45][46]. On the other hand, the ankle can also be interpreted as the transition between two (or more) different populations of sources.…”

Comprehensive approach to tau-lepton production by high-energy tau neutrinos propagating through the Earth

“…The propagation of UHE cosmic rays through the cosmic photon backgrounds is expected to produce a flux of UHE neutrinos [4][5][6][7]. The detection of or tighter limits on this cosmogenic UHE neutrino flux has the potential to significantly constrain models of UHE cosmic-ray sources [8,11,12]. UHE neutrinos have been searched for in the past decades with several experiments using the Moon [13][14][15], the Earth's crust with the Pierre Auger Observatory [11], and the Antarctic ice cap with IceCube [12] and ANITA [16], each producing the most constraining limits to date in different parts of the UHE band.…”

IceCube real-time alert system

“…The top panel shows the all-sky effective area, while the down-going (southern sky) and up-going (northern sky) effective areas are shown in the middle and bottom panels, respectively. The EHE effective areas are increased relative to the published offline selection [46], while the HESE effective areas shown here are just for track-like events and show reduced effective area relative to the published analysis [12]. The cumulative distribution for the opening angle showing the radius containing 50% and 90% of simulated events for HESE and EHE selected neutrino events.…”