Detection of proton acceleration in supernova remnants

The cosmic-ray (CR) flux in our Milky Way galaxy is dominated by protons presumably accelerated by sources within the galaxy to energies as high as 10⁶ GeV. For decades, the best guess as to the acceleration mechanism has been shock acceleration at the expanding margins of supernova remnants (SNRs). X-ray data showed that such shock fronts do indeed accelerate electrons to CR energies. But evidence of proton acceleration by SNR shocks has remained frustratingly indirect. Now, however, four years of observing by the Fermi Gamma-Ray Space Telescope , shown in the drawing, has revealed unambiguous evidence of protons accelerated to CR energies in two SNRs. That evidence is a signature imposed on the SNR gamma-ray spectra by the decay of neutral pions created when sufficiently accelerated protons collide with low-energy ambient nuclei. In earlier observations, that telltale spectral structure had been obscured by gamma radiation from the scattering of high-energy electrons and poor detector sensitivity to the gamma spectrum below 500 MeV. Looking at more SNR spectra over the next five years, Fermi and new ground-based gamma telescopes will address two important questions: What fraction of the CR proton flux originates in SNR shock fronts? And what’s the highest proton energy such shock acceleration can achieve? (M. Ackermann et al., Science 339, 807, 2013 .)—Bertram Schwarzschild