DEBRIS from explosions that shook our galaxy around a million years ago may be pelting our atmosphere. This might explain a mysterious 鈥渟pike鈥� in the numbers of cosmic rays bombarding Earth.
Cosmic-ray detectors in Japan and Australia have noticed an unexpectedly large number of particles with an energy of about 1018 electronvolts coming from the centre of our galaxy. Peter Biermann of the Max-Planck Institute for Radio Astronomy in Bonn wondered if they might come from gamma-ray bursts 鈥� powerful explosions thought to mark the violent deaths of the most massive stars in the universe.
Biermann鈥檚 team calculated the likely effect of one or more gamma-ray bursts in the bustling centre of the Milky Way about a million years ago. Shock waves from the explosions would accelerate protons, which would zoom out into the galaxy and undergo a series of interactions on their journey, many of them ending up as neutrons. Biermann says the upshot on Earth today would be a shower of several thousand neutrons a year 鈥� all with energies of about 1018 electronvolts, matching the spike astronomers see ().
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Until now, astronomers have only observed gamma radiation and visible light from recent bursts. Studies of the cosmic rays they fling out could help explain what triggers the explosions, and also help build up a 鈥渇ossil record鈥� of gamma-ray bursts in the Milky Way long ago. 鈥淲e can see the past,鈥� says Biermann.