On the Origin of Cosmic Rays

Abstract

The problem in identifying the sites of origin of Galactic Cosmic Rays (CRs) is reviewed. Recent observational evidence from very-high energy (VHE, energies above 100 GeV) γ-ray measurements is in contradiction with the surmise that synchrotron radiation from relativistic electrons is indicative for hadron acceleration. It rather points to a CR-acceleration efficiency of supernova remnants (SNRs) below one percent, much less than the value required if these objects were to be the main sources of Galactic cosmic rays (about 30%). Observations of CR anisotropy and the emission of lowenergy (energy < 10 GeV) γ-rays from the Galactic disk indicate that the sources of low-energy cosmic rays are distributed with a Galactocentric radial scale length in the order of 25 ± 10 kpc, much larger than expected if SNRs are the main sources of CRs. These two facts together with the body of evidence from CR isotope abundances strongly suggest that a new class of astrophysical objects distinct from SNRs and located manly in the outer reaches of our Galaxy is the major source of hadronic CRs in our Galaxy. The basic observational features of ultra high-energy (UHE, energy > 10 eV) CRs are most naturally understood if the same CR sources accelerate CRs up to the highest observed CR energies. Proposals for the nature of a new source class are mentioned. The origin of CRs is still as much shrouded in mystery as it was in 1957, when Philip Morrison wrote a seminal review about CR origin. The potential for discoveries is thus great. ∗email:plaga@mppmu.mpg.de 1 The term “SNR” is below always used in the sense: “the well known non-relativistic ejecta of supernova with a typical kinetic energy of 10 ergs”

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@inproceedings{Plaga2008OnTO, title={On the Origin of Cosmic Rays}, author={Rainer Plaga}, year={2008} }