Cosmic-ray accelerator in the Galactic Center

1 February 2006

A group of astrophysicists (and among them, researchers from Paris Observatory and CNRS) has announced the detection with the H.E.S.S. gamma-ray telescopes (in Namibia) of very-high-energy gamma rays from huge gas clouds known to pervade the centre of our Galaxy. These gamma rays are expected to result from the even more energetic cosmic-ray particles, which permeate our entire Galaxy, crashing into the clouds. However, precise measurements of the intensity and energies of these gamma rays, thanks to this most sensitive instrument in the world in this energy range, further show that in the central region of our Galaxy these cosmic-ray particles are typically more energetic than those we measure falling on the Earth’s atmosphere. Possible explanations for this energisation of the cosmic rays near the heart of our Galaxy include the echo of a Supernova which exploded some hundred centuries beforehand or a burst of particle acceleration by the supermassive black hole at the very centre of our galaxy.

In a recent publication in Nature magazine (9 February 2006), the international H.E.S.S. collaboration report the discovery of gamma-ray emission from a complex of gas clouds near the centre of our own Milky-Way Galaxy. These giant clouds of hydrogen gas encompass an amount of gas equivalent to 50 million times the mass of the sun. With the highly sensitive H.E.S.S. gamma-ray telescopes, it is possible for the first time to show that these clouds are glowing in very-high-energy gamma rays (see Figure 1). At low energies, around 100 Million electronVolts (man-made accelerators reach energies up to 1,000,000 Million electronVolts), this technique has been used by the EGRET satellite to map cosmic rays in our Galaxy. At really high energies - the true domain of cosmic-ray accelerators - no instrument was so far sensitive enough to "see" interstellar gas clouds shining in very-high-energy gamma rays. H.E.S.S. has for the first time demonstrated the presence of cosmic rays in this central region of our Galaxy. The surprise from H.E.S.S. data is that the density of cosmic rays exceeds that in the solar neighbourhood by a significant factor. Interestingly, this difference increases as we go up in energy, which implies that the cosmic rays have been recently accelerated. So, these data hint that the clouds are illuminated by a nearby cosmic-ray accelerator, which was active over the last ten thousand years. Candidates for such accelerators are a gigantic stellar explosion which apparently went off near the heart of our Galaxy in "recent" history (Chandra press release), another possible acceleration site is the super-massive black hole at the centre of the Galaxy. Jim Hinton (Max-Planck Institute for Nuclear Physics, Heidelberg, Germany), one of the scientists involved in the discovery, concludes "This is only the first step. We are of course continuing to point our telescopes at the centre of the Galaxy, and will work hard to pinpoint the exact acceleration site - I’m sure that there are further exciting discoveries to come"

Last update on 22 October 2013

Also in this section

Know more

Reference

Contacts