Cosmic Ray Composition Around the Knee: Where are we?

Abstract

One of the most intriguing feature of the Cosmic Ray (CR) spectrum is the presence of the “knee”. This is a sharp change of slope in the CR energy spectrum around 3 × 10¹⁵ eV. If we exclude the “ankle” (which is another, but less evident, change of slope in the highest energy region) the knee is the only remarkable feature of a spectrum which extends smoothly for more than 30 orders of magnitude. Discovered in 1958 [1], the existence of the “knee” still waits for an explanation. At present, there are several models that try to explain its presence. One of the most convincing models is that of diffusive shock acceleration in Supernova Remnants (SNR), since it can account for the total energy stored by the bulk of CR in the Galaxy (with an energy density ε ≃ 1 eV/cm³). Nevertheless, normal supernovas are expected to have neither high enough magnetic fields nor long enough shock lifetimes to accelerate particles to energies higher than 10¹⁵ eV. The coincidence of these values with the energy region of the knee is a strong indication that the change in the power law index may be due to a change in the acceleration mechanism. A direct check of this scenario is the observation of TeV Υ-rays from Supernovas, produced by the collision of TeV nucleons with the surrounding interstellar medium. Some experiments reported on the observations of TeV Υ-rays from Supernovas, but in all cases it has not been possible to assure that this emission is due to hadro-production processes.