Wang, Xiang-Yu 2011 用伽马射线限制甚高能宇宙射线

主要内容:
由于甚高能宇宙线会被微波背景或者红外背景散射而看不到远处的，但是这些宇宙线和它们作用之后的二级粒子，特别是伽马射线还是能被看到的，通过这个可以反推有多少甚高能宇宙线。

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Title:		Constraining the Emissivity of Ultrahigh Energy Cosmic Rays in the Distant Universe with the Diffuse Gamma-Ray Emission
Authors:		Wang, Xiang-Yu; Liu, Ruo-Yu; Aharonian, Felix
Affiliation:		AA(Department of Astronomy, Nanjing University, Nanjing 210093, China ; Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany ; Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210093, China), AB(Department of Astronomy, Nanjing University, Nanjing 210093, China ; Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210093, China), AC(Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany ; Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland)
Publication:		The Astrophysical Journal, Volume 736, Issue 2, article id. 112 (2011). (ApJ Homepage)
Publication Date:		08/2011
Origin:		IOP
Astronomy Keywords:		cosmic rays, gamma rays: diffuse background, neutrinos
DOI:		10.1088/0004-637X/736/2/112
Bibliographic Code:		2011ApJ...736..112W

Abstract

Ultrahigh cosmic rays (UHECRs) with energies >~ 10¹⁹ eV emitted at cosmological distances will be attenuated by cosmic microwave and infrared background radiation through photohadronic processes. Lower energy extragalactic cosmic rays (~10¹⁸-10¹⁹ eV) can only travel a linear distance smaller than ~Gpc in a Hubble time due to the diffusion if the extragalactic magnetic fields are as strong as nano-Gauss. These prevent us from directly observing most of the UHECRs in the universe, and thus the observed UHECR intensity reflects only the emissivity in the nearby universe within hundreds of Mpc. However, UHECRs in the distant universe, through interactions with the cosmic background photons, produce UHE electrons and gamma rays that in turn initiate electromagnetic cascades on cosmic background photons. This secondary cascade radiation forms part of the extragalactic diffuse GeV-TeV gamma-ray radiation and, unlike the original UHECRs, is observable. Motivated by new measurements of extragalactic diffuse gamma-ray background radiation by Fermi/Large Area Telescope, we obtained upper limit placed on the UHECR emissivity in the distant universe by requiring that the cascade radiation they produce not exceed the observed levels. By comparison with the gamma-ray emissivity of candidate UHECR sources (such as gamma-ray bursts (GRBs) and active galactic nuclei) at high redshifts, we find that the obtained upper limit for a flat proton spectrum is ~= 10^1.5 times larger than the gamma-ray emissivity in GRBs and ~= 10 times smaller than the gamma-ray emissivity in BL Lac objects. In the case of iron nuclei composition, the derived upper limit of UHECR emissivity is a factor of 3-5 times higher. Robust upper limit on the cosmogenic neutrino flux is further obtained, which is marginally reachable by the Icecube detector and the next-generation detector JEM-EUSO.

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