Observations and implications of the ultrahigh-energy cosmic rays

M. Nagano and A. A. Watson
Rev. Mod. Phys. 72, 689 – Published 1 July 2000
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Abstract

The authors define “ultrahigh-energy cosmic rays” (UHECRs) as those cosmic rays with energies above 1018eV. It had been anticipated that there would be a cutoff in the energy spectrum of primary cosmic rays around 6×1019eV induced by the interaction of the particles with the 2.7-K primordial photons. However, recent experimental data have established that particles exist with energies greatly exceeding this. It follows that the sources of such particles are probably nearby, on a cosmological scale. However, although the trajectories of such energetic particles through the galactic and intergalactic magnetic fields may be nearly rectilinear, no astronomical sources have as yet been identified. This is the enigma of the highest-energy cosmic rays. The paper reviews the history of research in this energy regime and critically assesses the observational results on the energy spectrum, arrival directions, and composition of the primary cosmic rays based on observations made by six experiments. The detection methods currently available are described. Special techniques have been developed as particles of 1020eV or higher occur at a rate of only about 1 per km2 per century. Errors in measurement are given particular attention. The authors also review the theoretical predictions for a number of candidate sources of cosmic rays beyond the predicted cutoff. Finally, the four major projects planned to address the question of the origin of UHECRs are briefly described.

    DOI:https://doi.org/10.1103/RevModPhys.72.689

    ©2000 American Physical Society

    Authors & Affiliations

    M. Nagano

    • Department of Applied Physics and Chemistry, Fukui University of Technology, Fukui 910-8505, Japan

    A. A. Watson

    • Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom

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    Issue

    Vol. 72, Iss. 3 — July - September 2000

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