Using the X-ray astronomy satellite XMM-Newton of the European Space Agency (ESA), French researchers from CNRS (Centre National de la Recherche Scientifique) and CEA (Centre dEtudes Atomiques) have discovered a new source of cosmic rays in the vicinity of the Arches cluster, near the centre of the Milky Way. The shock wave generated by tens of thousands of young stars moving at very high speeds seems to accelerate these cosmic particles. These cosmic rays produce a characteristic X-ray emission by interacting with the atoms in the surrounding gas.
The origin of these cosmic rays differs from that of the cosmic rays discovered exactly a hundred years ago, which originate in the explosions of supernovae. When certain stars at the end of their lives explode and become supernovae, their matter is ejected at supersonic speed, generating shock waves that accelerate the particles. As a result, some atomic nuclei gain very high kinetic energy and enter the earths atmosphere.
However, it is difficult to detect low-energy cosmic rays (energy less than 0.5 GeV) in the earths region since the solar wind prevents them from entering the heliosphere. Therefore, not much is known about the chemical composition and flux of cosmic rays outside the solar system. The scientists involved in this work first studied the nature of X-ray emission that should theoretically be expected if low-energy cosmic rays exist in the interstellar medium. They then searched for signs of this theoretical emission 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 signatures of a large fast ion population 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 a speed of approximately 700,000 km/h. The cosmic rays are in all probability produced in the high-speed collision of the star cluster with a gas cloud in its path.
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