Cosmic ray astrophysics in the knee region and implications for gamma ray astronomy

Abstract

Following the discovery of ultra high energy cosmic rays with energies of PeV and higher in the 1950's and detailed studies on the energy spectrum in the TeV-PeV-EeV regions in 1960's, there has been enormous interest in learning about the nature of astrophysical sources capable of accelerating particles to these extremely high energies and about their propagation in the interstelar and intergalactic space. Direct observational studies on cosmic ray sources through detection of PeV energy gamma-rays have yielded very scanty information due to the absence of steady sources above the presently detectable flux limits. Therefore, indirect information on the nature of cosmic ray sources and acceleration processes has to be obtained from measurements on the energy spectra of various nuclear groups. In this context, the presence of the 'knee' at E ~ 3 x 1015 eV is very significant and it is important to obtain accurate information on the chemical composition of cosmic ray flux around the 'knee' to understand the nature of cosmic ray sources contributing to this part of the energy spectrum. We summarise here the present status of UHE ?-ray astronomy and review some of the recent experimental results on energy spectra and composition.