Cosmologists puzzle over mystery in the cosmos

摘要

It may not be the ethereal spirit that it looks like, but to cosmologists it is still rather surprising: two "hot spots" (yellow/red) of cosmic rays. The hot spots were observed using the Milagro telescope in the Jemez mountains near Los Alamos in the US – the first observatory capable of continuously monitoring the northern sky for cosmic rays. As cosmic rays bombard nuclei in the atmosphere they produce showers of secondary particles. Milagro detects these particles as they fall into a huge pool of water and create flashes of Cerenkov radiation, which can be picked up by photomultipliers. The cosmic-ray hot spots above are unusual because of their relatively low energy: at about 10 TeV one would expect galactic magnetic fields to still be able to scramble the paths of cosmic rays, so that they would appear with a random distribution across the sky. Physicists in the Milagro team are unsure where the hot spots come from, but suggest that they could be the result of a special magnetic-field structure that guides the cosmic rays to Earth. Another possibility is that the cosmic rays are neutral particles such as neutrons, although these have lifetimes too short to make this likely (Phys. Rev. Lett. 101 221101). "We are nearing the centenary of the discovery of cosmic rays and we still do not have proof of where they come from," says Milagro teammember John Pretz. "If these hot spots are due to a source of cosmic rays, then it would be the solution to a 100-year-old riddle. It is also exciting because, for the first time, we are starting to see isolated features in the cosmic-ray sky, and that can help us understand their origin."