High-energy cosmic-ray electrons in the Galaxy

The intensity of cosmic-ray electrons is only ∼1% of the protons at 10 GeV, and decreases very rapidly with energy to be ∼0.1% of protons at 1 TeV. Nevertheless, electrons in cosmic-rays have unique features, complementary to all other cosmic-ray nucleonic components, because they enable us to find the origins of cosmic-rays and the properties of their propagation mechanisms in the Galaxy. High-energy electrons lose energy by synchrotron and inverse Compton processes during the propagation in the Galaxy. Since the energy loss rate by these processes is proportional to the square of energy, TeV electrons accelerated in the sources at distances larger than ∼1 kpc, or ages greater than a few 105 yr, cannot reach the solar system. This suggests that some nearby sources leave unique signatures in the form of identifiable structures in the energy spectrum of TeV electrons, and show increases of the flux towards the sources. In this paper, I review the past observations of high-energy cosmic-ray electrons and discuss their astrophysical significance.