A hundred years ago, the Austrian physicist Victor Franz Hess discovered the existence of a ionizing radiation of extraterrestrial origin, the "cosmic rays". When some stars explode at the end of their life and become supernova, their matter is ejected at a supersonic speed and generates shockwaves that accelerate the particles. Some atomic nuclei thus acquire a very high kinetic energy and reach the Earth. They are the high energy cosmic rays.
But cosmic rays which kinetic energy is lower than half a billion electron volts are not detected near our planet, because the solar wind prevent them to penetrate the heliosphere. So we don't know much about the chemical composition of low energy cosmic rays or their flux outside the Solar System, but everything indicates that they play a major role in the Galaxy. By ionizing and heating the denser interstellar clouds, they probably regulate the formation of stars.
From theory to observation
The authors of the publication began by studying in theory the X emission that low energy cosmic rays interacting with the interstellar medium should generate. Then they searched the trace of this theoretical emission in X-ray data accumulated by XMM-Newton since its launch in 1999.
By analysing the properties of the interstellar iron X emission recorded by the satellite, they found signatures of a high population of fast ions near the Arches cluster, at about one hundred light-years from the center of the Milky Way. The stars of this cluster move together at a speed of about 700,000 km/h.
The cosmic rays are very likely produced by the collision at high speed of the star cluster with a gas cloud in their way. In this particular region, the energy density of the accelerated ions is about a thousand times higher to the energy of the cosmic rays near the Solar System.
"These results show the power of combining imagery-spectral analysis and XMM-Newton performances. They also result of a remarkable modelling and scientific analysis work", explains Olivier La Marle, CNES astrophysics thematic responsible. The CNES funded part of the study and have been supporting the processing and the distribution of XMM-Newton data since the beginning.
It is the first discovery of a major source of low energy cosmic rays outside the Solar System. It shows that supernova shockwave are not the only objects that can accelerate the mass of the atomic nuclei in the Galaxy. These results should allow to identify new sources of ions in the interstellar medium and perhaps to improve our knowledge of the effects of these electromagnetic particles on the formation of stars.
V. Tatischeff1, A. Decourchelle2, and G. Maurin2,3 (2012), Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keV line emission from the Arches cluster region, A&A, 546, A88.
1 Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, IN2P3/CNRS and Univ Paris-Sud, 91405 Orsay, France
2 Service d’Astrophysique (SAp)/IRFU/DSM/CEA Saclay, Bt. 709, 91191 Gif-sur-Yvette Cedex, Laboratoire AIM, CEA-IRFU/CNRS/Univ Paris Diderot, CEA Saclay, 91191 Gif-sur-Yvette, France
3 Laboratoire d’Annecy le Vieux de Physique des Particules, Univ. de Savoie, CNRS, BP 110, 74941 Annecy-le-Vieux Cedex, France
- Scientific contacts: Vincent Tatischeff, Anne Decourchelle, Gilles Maurin
- Astrophysics program scientist: Olivier La Marle