Gamma-Ray and Neutrino Backgrounds as Probes of the High-Energy Universe: Hints of Cascades, General Constraints, and Implications for TeV Searches

Abstract

Recent observations of isotropic diffuse backgrounds by Fermi and IceCube allow us to get more insight into distant very-high-energy (VHE) and ultra-high-energy (UHE) gamma-ray/neutrino emitters, including cosmic-ray accelerators/sources. First, we investigate the contribution of intergalactic cascades induced by gamma-rays and/or cosmic rays (CRs) to the diffuse gamma-ray background (DGB) in view of the latest Fermi data. We identify a possible VHE Excess from the fact that the Fermi data are well above expectations for an attenuated power law, and show that cascades induced by VHE gamma rays (above ~10 TeV) and/or VHECRs (below ~10^19 eV) may significantly contribute to the DGB above ~100 GeV. The relevance of the cascades is also motivated by the intergalactic cascade interpretations of extreme TeV blazars such as 1ES 0229+200, which suggest very hard intrinsic spectra. This strengthens the importance of future detailed VHE DGB measurements. Then, more conservatively, we derive general constraints on the cosmic energy budget of high-energy gamma rays and neutrinos based on recent Fermi and IceCube observations of extragalactic background radiation. We demonstrate that these multi-messenger constraints are useful and the neutrino limit is very powerful for VHE/UHE hadronic sources. Furthermore, we show the importance of constraints from individual source surveys by future imaging atmospheric Cherenkov telescopes such as Cherenkov Telescope Array, and demonstrate that the cascade hypothesis for the VHE DGB can be tested by searching for distant emitters of cascaded gamma rays.

35 pages, 22 figures, replaced to match the published version (JCAP, 08, 030). Interested readers may wish to consult the recent paper by Inoue and Ioka (arXiv:1206.2923)

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• Background model systematics for the Fermi GeV excess
• Constraining High-Energy Cosmic Neutrino Sources: Implications and Prospects
• Star-forming galaxies as the origin of diffuse high-energy backgrounds: Gamma-ray and neutrino connections, and implications for starburst history
• Constraining Very Heavy Dark Matter Using Diffuse Backgrounds of Neutrinos and Cascaded Gamma Rays
• Hidden Hearts of Neutrino Active Galaxies
• An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?
• Extragalactic star-forming galaxies with hypernovae and supernovae as high-energy neutrino and gamma-ray sources: the case of the 10 TeV neutrino data
• Galaxy Clusters as Reservoirs of Heavy Dark Matter and High-Energy Cosmic Rays: Constraints from Neutrino Observations
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• EeV Astrophysical neutrinos from FSRQs?
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• Cumulative neutrino background from quasar-driven outflows
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• Search for High-Energy Neutrinos from Ultra-Luminous Infrared Galaxies with IceCube
• Near-future discovery of point sources of ultra-high-energy neutrinos
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• Gamma-ray counterparts of 2WHSP high-synchrotron-peaked BL Lac objects as possible signatures of ultra-high-energy cosmic-ray emission
• Deciphering the Sources of Cosmic Neutrinos
• Revisiting the propagation of highly-energetic gamma rays in the Galaxy
• The IceCube Pie Chart: Relative Source Contributions to the Cosmic Neutrino Flux
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