Researchers using the European X-ray astronomy satellite XMM-Newton have discovered a new source of low-energy cosmic rays in the vicinity of the Arches cluster, near the centre of the Milky Way. Their origin differs from that of higher-energy cosmic rays that originate in the explosions of supernovae.

Low-energy cosmic rays with kinetic energy less than half a billion electronvolts are not detected at Earth, since the solar wind prevents them from entering the heliosphere. Therefore little is known about their chemical composition and flux outside the solar system.

V Tatischeff, A Decourchelle and G Maurin, from the institutes of CNRS and CEA in France began by studying the X-ray emission that should theoretically be generated by low-energy cosmic rays in the interstellar medium. They then looked for signs of this in X-ray data collected by XMM-Newton since its launch in 1999. By analysing the properties of the X-ray emission of interstellar iron recorded by the satellite, they found the signature of a large population of fast-moving ions in the vicinity of the Arches cluster, about 100 light-years from the centre of the Milky Way. The stars in this cluster are travelling together at approximately 700,000 km/h. The cosmic rays are in all likelihood produced in the high-speed collision of the star cluster with a gas cloud in its path.

This is the first time that a major source of low-energy cosmic rays has been discovered outside the solar system. It shows that the shock waves of supernovae are not the only objects that can cause mass acceleration of atomic nuclei in the galaxy. These findings should make it possible to identify new sources of ions in the interstellar medium, and may lead to a better understanding of the effects of these energetic particles on star formation.