ASTRONOMERS are homing in on mysterious cosmic ray guns. They鈥檝e found hints that neutron stars in our own Galaxy might be firing super-energetic particles into space at almost the speed of light.
The particles, known as cosmic rays, are continually bombarding the Earth from space. Most of the relatively low-energy ones probably come from supernovae, explosions of massive stars at the ends of their lives.
The big puzzle surrounding these particles has been the source of the very fastest ones, which can each have energies in excess of 1019 electronvolts 鈥� about the same as a golf ball travelling at 30 kilometres an hour. Detectors have spotted around 400 of them over the past decade, but their origins are still highly uncertain. 鈥淚t鈥檚 remarkable, 90 years after the discovery of cosmic rays we鈥檙e still arguing the toss about where they come from,鈥� says astronomer Arnold Wolfendale of Durham University.
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The only objects thought to be powerful enough to accelerate particles to these huge energies are bright 鈥渁ctive galaxies鈥� fuelled by giant black holes. But these tend to lie very far away, and it鈥檚 likely that the highest-energy cosmic rays originate nearby, because otherwise their energy would be dissipated during the long journey here.
Now Wolfendale and his colleagues have found that many of the most highly energetic particles 鈥� more than expected by chance 鈥� come from the directions of nearby neutron stars, the dense remains of collapsed stars. These particular neutron stars spin around 1000 times a second (Journal of Physics G, vol 28, p 1).
Neutron stars have solid surfaces made of iron nuclei and generate huge magnetic fields. Because of their rotation, they act like giant dynamos and can energise particles, although it was thought that this wouldn鈥檛 be enough to give cosmic rays such super-high energies. But Wolfendale says the evidence suggests we may have underestimated their firepower.