Almost all primary cosmic rays are charged particles—mostly protons. But the one primary in a thousand that’s a gamma has special value for astrophysicists. The arrival direction of a charged particle of energy less than 1018 eV reveals nothing about where it came from. That’s because the random microgauss magnetic field pervading the galaxy thoroughly scrambles the trajectories of all but the most ultrahigh-energy charged particles (which are presumed to originate somewhere beyond the galaxy). But gammas, being impervious to magnetic fields, are superb astrophysical pointers.

What is the astrophysical mechanism that accelerates charged particles to the TeV (1012 eV) energies typical of high-energy cosmic rays that originate in our galaxy? The general presumption is that the particles are accelerated by the moving magnetic fields caught up in the remnant shock fronts created by supernova explosions. But until now, the evidence for that mechanism has only been indirect....

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