Remarks on the origin of the cosmic radiation

Abstract

In this article the present knowledge of the origin and acceleration of cosmic ray particles is summarized. This knowledge is obtained from investigations of the composition, the energy spectra, and the anisotropy of the radiation. Galactic objects known to accelerate particles and the process of shock acceleration are discussed. The extent of the energy spectra to 1020 eV combined with near isotropy of the flux forces one to contemplate an extragalactic origin for the cosmic rays of highest energy. At the present time one can only speculate on the sources of these particles. M ore than 20 years had passed since the discovery of cosmic radiation in 1911 when Zwicky was said to suggest that supernova explosions might be the source of this remarkable radiation. Zwicky, who investigated the supernova phenomenon, was aware that these objects produce the largest sudden energy release observed in our and similar galaxies and hence are candidates for the sources of high-energy cosmic ray particles. We know today that this was a remarkably accurate prediction, made at a time when little or nothing was known about the nature of the cosmic radiation, the interstellar medium, or the interplanetary medium through which the radiation must travel. Cosmic radiation encompasses all particles of high energy that are of astrophysical origin. It contains a wide range of species, and covers an enormous span of energies. The dominant species are protons and helium nuclei but, importantly, it also contains small quantities of all elements of the periodic system and their isotopes. In addition, electrons and the two antiparticles, positrons and antiprotons, are also observed. Missing are nuclides that decay radioactively with a lifetime too short to survive the journey between source and observer: neutrons, and many radioactive isotopes. The energies of cosmic ray particles reach 102o eV, by far the highest energy of any known radiation. The particle flux at these high energies is very small. The bulk of the cosmic rays is found around 108 and 109 eV, still high compared to laboratory standards. Particles over such a wide energy range need not necessarily be produced by the same type of source. One should not be surprised if several origins were to contribute to the observed radiation. Composition, energy spectra, and anisotropies in the arrival direction represent the most important evidence for the particle's origin and for the mechanisms leading to their acceleration. Additional evidence from gamma-ray astronomy, X-ray astronomy, and radioastronomy discloses the presence of cosmic rays in distant objects. Some of these features are now reasonably well understood, others are the subject of speculations.