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 ) \gamma -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 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 ^ { 51 } ergs ” ) 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 low-energy ( energy < 10 GeV ) \gamma -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 \pm 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 ^ { 19 } 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 .