Thanks to XMM-Newton, an X-ray astronomy European satellite, scientists from the CNRS and the CEA discovered a new source of cosmic rays which are different from cosmic rays known until now. In the neighbourhood of the Arches cluster, near the Milky Way centre, these particles are accelerated in the shockwave generated by the movement of thousands of young stars at the speed of about 700,000 km/h. These cosmic rays then produce a characteristic X emission while interacting with the atoms of the ambient gas.
The region of the Arches cluster seen in X-rays (outlines) and in the near-infrared (background image). The infrared map is built from observations made by Hubble telescope at 1.875 µm (Data from Dong et al. 2011, Monthly Notices of the Royal Astronomical Society, 417, 114). The outlines mark the intensity of the neutral iron line observed by XMM-Newton around the star cluster (located at the centre of the image). The X emission is produced by cosmic rays accelerated by the shockwave formed in the interstellar medium by the supersonic movement of the cluster. This cluster moves from the right to the left of the image at about 700,000 km/h.
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 whose kinetic energy is lower than half a billion electron volts are not detected near our planet, because the solar wind prevents them from penetrating the heliosphere. We do not 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 star formation.
From theory to observation
The authors of the publication began with a theoretical study of the X emission which low energy cosmic rays interacting with the interstellar medium are expectd to generate. Then they searched the trace of this theoretical emission in X-ray data accumulated by XMM-Newton since its launch in 1999.
By analyzing 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 centre of the Milky Way. The stars of this cluster move together at a speed of about 700,000 km/h.
Cosmic rays are very likely produced by the collision at high speed of the star cluster with a gas cloud. In this particular region, the energy density of the accelerated ions is about a thousand times higher than the energy of cosmic rays near the Solar System.
"These findings show the power of combining imagery-spectral analysis and XMM-Newton performances. They are also the result of a remarkable modelling and scientific analysis work", explains Olivier La Marle, CNES Astrophysics Programme Manager. 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 shockwaves are not the only objects that can accelerate the mass of the atomic nuclei in the Galaxy. These results should contribute to identifying new ion sources in the interstellar medium and perhaps to improving our knowledge of the effects of these electromagnetic particles on star formation.
References of the publication
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