Since their detection in 1912, cosmic rays--streams of energetic particles that bombard the earth from all directions--have confounded scientists trying to determine just where these jets are produced and how they achieve such high accelerations. Based on theoretical calculations, some researchers had predicted that supernovae, the cataclysmic explosions that mark the deaths of stars, could be the source of this radiation. And for the electronic component of cosmic rays, observations bore this out. Now a report published today in the journal Nature provides strong empirical evidence that supernovae could also supply the protons that form the bulk of these particles.

Ryoji Enomoto of the University of Tokyo and colleagues used the CANGAROO telescope in Australia to observe the supernova remnant known as RX J1713.7-3946. They detected the decay of particles called pions, which are generated when protons accelerated by the shock wave of a supernova collide with the material in an interstellar cloud. The resulting gamma rays have very high energies of several TeV (one TeV is equivalent to 1012 electron volts). Though other mechanisms can produce such so-called hadronic gamma rays, they cannot explain the spectrum obtained by CANGAROO, the scientists report. As Felix Aharonian of the Max Planck Institute for Nuclear Physics in Germany notes in an accompanying commentary, "when earlier x-ray data are also taken into account, it seems that the [gamma] rays must have been created as a result of protons being accelerated by the supernova remnant."

The data collected so far are insufficient to pinpoint the exact location of the high-energy gamma rays, but the number detected suggests that their production must occur in a very dense region. Future observations with other telescopes should help narrow the search. According to Aharonian, "these telescopes will make a crucial contribution to our efforts to trace the origin of galactic cosmic rays."