Arrival directions and chemical composition of ultrahigh-energy cosmic rays
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Galactic plane
Physicists have known since 1912 that the Earth is constantly bombarded by a cosmic rain of high-energy particles – mostly protons and heavier nuclei – but until recently little progress had been made in understanding where these particles might come from. The reason we cannot do astronomy with cosmic rays is simple: they are charged particles that are deflected by magnetic fields, which means that we cannot determine where they originate from. Even the galactic magnetic field, which is only a few microgauss, can redirect cosmic rays that have energies up to 109 gigaelectron-volts (1018eV) so that they appear to come uniformly from all directions.
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In this chapter origin of ultra high energy cosmic rays is discussed, namely possible sources, processes in which particles are accelerated, and energetic spectra. The chapter includes five sections. In Section 1 different hypotheses of ultra high energy cosmic rays origin are reported and the list of arrays for their detection is given. From our point of view, cosmic ray sources may be active galactic nuclei. Then it is possible to identify cosmic ray sources directly. The identification procedure is described and results of identification are presented in Section 2. It appears that potential sources are active galactic nuclei with red shifts z 1 and it is unclear if particles accelerated in these objects can reach the Earth at energies 31020 eV, that is the maximal energy detected in cosmic rays. Can our model explain the measured spectrum? Cosmic ray spectra at ultra high energies are considered in Section 4. Spectra of cosmic protons at the Earth are calculated and are compared with the measured one. In addition the limit on maximal cosmic ray energy is derived. In Section 5 main results are listed and predictions of different models are compared.
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We will review the main physical aspects of Ultra High Energy Cosmic Rays. We will discuss in particular their propagation through astrophysical backgrounds, focusing on the latest experimental observations of HiRes, Telescope Array and Auger. We will also review the issue of the transition between galactic and extra-galactic cosmic rays.
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The world data on extensive air showers (EAS) produced by cosmic rays of energy above 1019 eV have been examined in detail, the individual shower characteristics being re-analysed where the basic data are available. The authors consider that their previous claim for an enhancement of intensity in the galactic plane is strengthened, at least in the outer galaxy; in particular they find new evidence for the presence of specific clusters of EAS both near the galactic plane and elsewhere. The clusters may correspond to the presence of specific sources or groups of sources.
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The physics of cosmic rays was intensively developed during the last century. The subject of ultra high energy cosmic rays has received special attention during the last partly due to the controversy between the results of the HiRes and AGASA Observatories and the puzzle caused by super‐GZK observed events. In this article we review the subject of cosmic rays, first from a historical point of view, then we give examples of methods of measuring cosmic ray properties, including those used in HiRes and AGASA..
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In this talk I discuss two problems in high energy particle astrophysics, the flux of cosmic ray antiprotons and the chemical composition in the region of the ‘‘knee’’ of the cosmic ray energy spectrum (∼1015 eV). Both are rather poorly known at present, yet they have great potential for informing us about the sources, acceleration and propagation of high energy cosmic radiation.
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We discuss the most recent developments in our understanding of the acceleration and propagation of cosmic rays up to the highest energies. In particular we specialize our discussion to three issues: (a) developments in the theory of particle acceleration at shock waves; (b) the transition from galactic to extragalactic cosmic rays; (c) implications of up-to-date observations for the origin of ultra high energy cosmic rays (UHECRs).
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