Cosmic ray particles with energies in excess of 10**(20) eV have been detected. The sources as well as the physical mechanism(s) responsible for endowing cosmic ray particles with such enormous energies are unknown. This report gives a review of the physics and astrophysics associated with the questions of origin and propagation of these Extremely High Energy (EHE) cosmic rays in the Universe. After a brief review of the observed cosmic rays in general and their possible sources and acceleration mechanisms, a detailed discussion is given of possible "top-down" (non-acceleration) scenarios of origin of EHE cosmic rays through decay of sufficiently massive particles originating from processes in the early Universe. The massive particles can come from collapse and/or annihilation of cosmic topological defects (such as monopoles, cosmic strings, etc.) associated with Grand Unified Theories or they could be some long-lived metastable supermassive relic particles that were created in the early Universe and are decaying in the current epoch. The highest energy end of the cosmic ray spectrum can thus be used as a probe of new fundamental physics beyond Standard Model. We discuss the role of existing and proposed cosmic ray, gamma-ray and neutrino experiments in this context. We also discuss how observations with next generation experiments of images and spectra of EHE cosmic ray sources can be used to obtain new information on Galactic and extragalactic magnetic fields and possibly their origin.

Origin and Propagation of Extremely High Energy Cosmic Rays

Origin and propagation of extremely high-energy cosmic rays

The observation of air showers from elementary particles with energies exceeding 10{sup 20}thinspeV poses a puzzle to the physics and astrophysics of cosmic rays which is still unresolved. Explaining the origin and nature of these particles is a challenge. In order to constrain production mechanisms and sites, one has to account for the processing of particle spectra by interactions with radiation backgrounds and magnetic fields on the way to the observer. In this paper, I report on an extensive study on the propagation of extragalactic nucleons, {gamma}-rays, and electrons in the energy range between 10{sup 8}thinspeV and 10{sup 25}thinspeV. We have devised an efficient numerical method to solve the transport equations for cosmic ray spectral evolution. The universal radiation background spectrum in the energy range between {approx_equal}10{sup {minus}9}thinspeV and {approx_equal}10thinspeV is considered in the numerical code, including the diffuse radio background, the cosmic microwave background, and the infrared-optical background, as well as a possible extragalactic magnetic field. We apply the code to compute the particle spectra predicted by several models of ultrahigh energy cosmic ray origin. A comparison with the observed fluxes, especially the diffuse {gamma}-ray background in several energy ranges, allows one to constrain certain classes of models. {copyright}more » {ital 1998} {ital The American Physical Society}« less

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On the Propagation of Extragalactic High Energy Cosmic and Gamma-Rays

Introducing a simple Galactic wind model patterned after the solar wind we show that back-tracing the orbits of the highest energy cosmic events suggests that they may all come from the Virgo cluster, and so probably from the active radio galaxy M87. This confirms a long standing prediction, as well as a theoretical model for the radio galaxy jet emission of M87. With this picture in hand, one clear expectation is that those powerful radio galaxies that have their relativistic jets stuck in the interstellar medium of the host galaxy, such as 3C147, will yield limits on the production of any new kind of particle, expected in some extensions of the standard model in particle physics.

The origin of the highest energy cosmic rays Do all roads lead back to Virgo

KASCADE and KASCADE-Grande were multi-detector installations to measure individual air showers of cosmic rays at ultra-high energy. Based on data sets measured by KASCADE and KASCADE-Grande, 90% C.L. upper limits to the flux of gamma-rays in the primary cosmic ray flux are determined in an energy range of ${10}^{14} - {10}^{18}$ eV. The analysis is performed by selecting air showers with a low muon content as expected for gamma-ray-induced showers compared to air showers induced by energetic nuclei. The best upper limit of the fraction of gamma-rays to the total cosmic ray flux is obtained at $3.7 \times {10}^{15}$ eV with $1.1 \times {10}^{-5}$. Translated to an absolute gamma-ray flux this sets constraints on some fundamental astrophysical models, such as the distance of sources for at least one of the IceCube neutrino excess models.

KASCADE-Grande Limits on the Isotropic Diffuse Gamma-Ray Flux between 100 TeV and 1 EeV

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Search for isotropic γ radiation in the cosmological window between 65 and 200 TeV