ON THE ENERGY SPECTRA OF GeV/TeV COSMIC RAY LEPTONS

Abstract

Recent observations of cosmic ray electrons from several instruments have revealed various degrees of deviation in the measured electron energy distribution from a simple power-law, in a form of an excess around TeV energies. An even more prominent deviation has been observed in the fraction of cosmic ray positrons around 100 GeV energies. In this paper we show that the observed excesses in the electron spectrum may be easily re-produced without invoking any unusual sources other than the general diffuse Galactic components of cosmic rays. The primary physical effect involved is the Klein-Nishina suppression of the electron cooling rate around TeV energies. With a very reasonable choice of the model parameters characterizing the local interstellar medium, we can reproduce the most recent observations by Fermi and HESS experiments. We also find that high positron fraction increasing with energy, as claimed by the PAMELA experiment, cannot be explained in our model with the conservative set of the model parameters. We are able, however, to reproduce the PAMELA results assuming high values of the starlight and interstellar gas densities, which would be more appropriate for vicinities of supernova remnants. A possible solution to this problem may be that cosmic rays undergo most of their interactions near their sources due to the efficient trapping in the far upstream of supernova shocks by self-generated, cosmic ray-driven turbulence.