Cosmic rays extragalactic

The primary cosmic rays propagate through the interstellar medium (ISM) until they either escape into extragalactic space, or are removed by interaction or energy losses in the ISM. Their interstellar equilibrium intensity may be recorded with a detector which is usually carried above the earth s atmosphere on spacecraft or balloon. Secondary cosmic rays are those that are generated as products from interactions of the primaries in the ISM positrons and antiprotons mostly come from interactions of primary protons, while the secondary nuclei such as Li, Be, B, and the elements just below iron, which cannot be produced by primary nucleosynthesis, are the products of spallation reactions of heavier primaries in the ISM. The overall arriving cosmic-ray intensity represents a mix of primary and secondary particles. 

Obviously, if we could understand the sky distribution of events at these extreme energies, then the option of using pulsars would become very attractive. For particles with a single charge as protons, the anisotropies in arrival directions would be severe, but for iron nuclei this constraint would be much alleviated. Therefore, the option of considering pulsars entails an interpretation as iron particles, inconsistent at the transition from galactic to extragalactic cosmic rays at about 3 EeV, but conceivable at a possible transition from one source population to another at the expected GZK cutoff. 

As far as we can see into the Universe, we don t observe any primordial antimatter. Within the limits of our present observational horizon the Universe is seen to contain only matter and no antimatter. The presence of cosmic antimatter would lead to observable traces of annihilation however the measurements of the extragalactic 7 ray flux indicate an absence of annihilation radiation, and the microwave background spectrum lacks a corresponding distortion. These findings preclude the existence of a significant amount of antimatter within tens of Megaparsecs, which is the scale of super-clusters of galaxies. 

There is a longstanding debate on the nature of the extragalactic X ray background between 1 and 100 keV whether it is the integrated effect of faint discrete sources, or whether there is a truly diffuse radiator, e.g. a hot intergalactic gas. The latter would fit the observed spectrum quite well, but would have a substantial effect on the cosmic microwave background, which has already been excluded by the recent COBE measurements (Mather et al. 1990). 

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