The interstellar cosmic-ray flux is dominated by high-energy protons presumably accelerated by sources within the Milky Way. For decades the best guess as to those intragalactic acceleration sites has been the shock fronts of supernova remnants (SNRs). They’re thought to be capable of accelerating ambient protons to energies as high as 106 GeV, a million times the proton mass. But the evidence has remained frustratingly indirect.
Now, at last, the collaboration that runs the Large Area Telescope (LAT) aboard the Fermi Gamma-Ray Space Telescope has found convincingly direct evidence of proton acceleration to cosmic-ray (CR) energies in two 10 000-year-old SNRs a few thousand light-years away.1 After four years of data taking and analysis, the gamma-ray spectra of the two SNRs revealed a long-sought signature of abundant pion production, which can’t be happening unless the remnants are indeed accelerating protons to CR energies.
Cosmic-ray protons represent a mean...
