Neutrino Background Flux from Sources of Ultrahigh-Energy Cosmic-Ray Nuclei

arXiv:1003.4959v1 [astro-ph.HE] 25 Mar 2010 Neutrino Background Flux from Sources of Ultrahigh-Energy Cosmic-Ray Nuclei Kohta Murase1,2 and John F. Beacom2,3,4 1Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto, 606-8502, Japan 2 CCAPP, The Ohio State University, Columbus, OH 43210, USA 3 Department of Physics, The Ohio State University, Columbus, OH 43210, USA 4 Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA (Dated: March 25, 2010) Motivated by Pierre Auger Observatory results favoring a heavy nuclear composition for ultrahigh- energy (UHE) cosmic rays, we investigate implications for the cumulative neutrino background. The requirement that nuclei not be photodisintegrated constrains their interactions in sources, therefore limiting neutrino production via photomeson interactions. Assuming a dNCR/dECR ∝E−2 CR injection spectrum and photodisintegration via the giant dipole resonance, the background flux of neutrinos is lower than E2 νΦν ∼10−9 GeV cm−2 s−1 sr−1 if UHE nuclei ubiquitously survive in their sources. This is smaller than the analogous Waxman-Bahcall flux for UHE protons by about one order of magnitude, and is below the projected IceCube sensitivity. If IceCube detects a neutrino background, it could be due to other sources, e.g., hadronuclear interactions of lower-energy cosmic rays; if it does not, this supports our strong restrictions on the properties of sources of UHE nuclei. PACS numbers: 95.85.Ry, 98.70.Sa I. INTRODUCTION The much-anticipated era of high-energy neutrino as- tronomy seems near [1, 2]. The IceCube detector at the South Pole is nearing completion [3], and the compa- rable KM3Net detector in the Mediterranean is being planned [4]. These and higher-energy neutrino detectors, e.g., ANITA [5], are expected to reveal unseen aspects of the extreme universe. One of the main goals is to identify the sources of the cosmic rays, a long-standing mystery. While cosmic rays below the knee at ∼1015.5 eV are likely produced by Galactic supernovae, those at higher energies have less certain origins. There is special interest in ultrahigh- energy cosmic rays (UHECRs) [6], which have energies above the ankle at ∼1018.5 eV and which are almost cer- tainly from extragalactic sources. Plausible accelerators include active galactic nuclei (AGN) [7, 8], gamma-ray bursts (GRBs) [9, 10], newly born magnetars [11] and clusters of galaxies [8, 12]. Neutrinos and gamma rays will be important diagnostics of UHECRs, either directly, by pointing to nearby sources, or indirectly, by the levels of the cumulative background fluxes from all sources. For the usually-assumed possibility that the UHECRs are protons, there is a large literature on neutrino pro- duction through photomeson interactions inside (e.g., for AGN [13, 14], GRBs [13, 15], newly born magnetars [16], and clusters [17]) or outside (e.g., via the Greisen, Zat- sepin, and Kuzmin process [18] or a lower-energy vari- ant [19]) sources. Due to large model uncertainties, more general arguments are useful. The Waxman and Bahcall (WB) [20] and the Mannheim, Protheroe and Rachen (MPR) [21] upper bounds on the neutrino background follow from an assumption that the UHECR sources are at least semi-transparent to photomeson interactions, i.e., that each accelerated proton loses at most ∼1/2 of its energy via this process before escape; the details are discussed below. These fluxes define reasonable land- marks to assess the sensitivity of neutrino telescopes (we use “landmark” instead of “bound” to emphasize that this is a nominal scale instead of an observational bound). Observations of UHECRs have recently been greatly improved by the High-Resolution Fly’s Eye (HiRes) and the Pierre Auger Observatory (PAO). Both report a spec- trum cutoffat ∼60×1018 eV [22–24]. For UHE protons, this is consistent with attenuation due to photomeson in- teractions with the cosmic microwave background [25]. For UHE heavy nuclei such as iron, as in models in Refs. [24, 26, 27], it is consistent with attenuation due to photodisintegration interactions with the cosmic in- frared background [24, 26]. Surprising new results suggest that UHECRs may be nuclei instead of protons. The UHECR composition is probed by the average depth of shower maximum, Xmax, and the r.m.s. fluctuations around it, δXmax; while both are subject to uncertainties in the hadronic models, these are much less for δXmax. HiRes data on Xmax favor a proton composition [28]. However, with the larger PAO data set, and results on both Xmax and δXmax, a heavier nuclear composition is favored [29]. We derive new results for the neutrino background due to UHECR sources, taking into account that the PAO results would require that UHE nuclei survive photodis- integration interactions in their sources. Our landmarks for the neutrino background due to UHE nuclei are sig- nificantly lower than the analogous WB and MPR land- marks for UHE protons. For all these landmarks, neu- trino

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