New Source of Cosmic Rays Discovered Near Arches Cluster at Center of Milky Way

Using the European X-ray astronomy satellite XMM-Newton, researchers from CNRS and CEA have discovered a new source of cosmic rays in the vicinity of the Arches cluster, near the center of the Milky Way. The particles are accelerated in the shock wave generated by tens of thousands of young stars moving at a speed of around 700,000 km/h. These cosmic rays produce a characteristic X-ray emission by interacting with the atoms in the surrounding gas. Their origin differs from that of the cosmic rays discovered exactly a hundred years ago by Victor Hess, which originate in the explosions of supernovae. In this particular region, the energy density of the accelerated ions is around a thousand times greater than that of cosmic rays in the neighborhood of the Solar System.

A hundred years ago, the Austrian physicist Victor Franz Hess discovered the existence of ionizing radiation of extraterrestrial origin, which he called cosmic rays. Today, their nature is well understood. 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 Earth's atmosphere.

However, low-energy cosmic rays are not detected in the region of our planet, since the solar wind prevents them from entering the heliosphere. Little is therefore known about their chemical composition and flux outside the Solar System, although everything suggests they play a significant role in the Galaxy. For instance, by ionizing and heating the densest interstellar clouds, they probably regulate the formation of stars.

The authors of the article 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 theoretical emission in X-ray data collected by XMM-Newton since its launch in 1999. By analyzing 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 one hundred light-years from the center of the Milky Way. The stars in this cluster are traveling together at a speed of 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.  

For more information: Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keVline emission from the Arches cluster region, V. Tatischeff, A. Decourchelle, and G. Maurin, A&A, 2012, 546, A88, dx.doi.org/10.1051/0004-6361/201219016 Journal reference: Astronomy & Astrophysics

The composite image at top of page shows an envelope of 60-million-degree gas around the Arches cluster. The Chandra data, shown as the diffuse blue emission in the inset box, overlays aHubble Space Telescope infrared image of the same region, in which some of the individual stars in the cluster can be seen as point-like sources. Both the X-ray and infrared observations are shown in context of the spectacular filamentary structures that appear in radio wavelengths displayed in red. Radio observations were obtained using the Very Large Array (VLA) of radio telescopes.

The Arches cluster contains about 150 hot, young stars concentrated within a radius of about one light year, making it the most compact cluster of stars in our Galaxy. Many of these stars are 20 times as massive as the Sun and live short, furious lives that last only a few million years. During this period, gas evaporates from these stars in the form of intense stellar winds. The envelope of hot gas observed by Chandra is thought to be due to collisions of the winds from numerous stars.

Source: The Daily Galaxy via Centre National de la Recherche Scientifique