Gamma-ray Afterglow from Galactic Centre Gas Clouds reveals Pre-Historic Particle Accelerator

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Astrophysicists using the H.E.S.S. gamma-ray telescopes, in Namibia, have announced the detection 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.

Gamma rays: Gamma rays resemble normal light or X-rays, but are much more energetic. Visible light has an energy of about one electronVolt (1 eV) of energy in physicist's terms. X-rays are thousands to millions of eV. H.E.S.S. detects very-high energy gamma-ray photons with an energy of a million million eVs, or Tera-electronVolt energies. These high energy gamma rays are quite rare; even for relatively strong astrophysical sources, only about one gamma ray per month hits a square metre at the top of the Earth's atmosphere.

Cosmic rays: High-energy particles from space which continuously bombard the Earth's atmosphere from all directions. Their energies exceed by far those that can be reached using man-made particle accelerators. Cosmic rays were discovered in 1912 by Victor Hess, and while they have been extensively studied for almost a century, their origin - often declared as one of the key themes of astrophysics - is still not completely understood. One important early result of the H.E.S.S. experiment was to reveal a supernova explosion shock-wave as a site of intense particle acceleration (Nature 432, p75)


A cartoon of the production of gamma rays via cosmic ray interactions in gas clouds (larger image)


Dr. Jim Hinton
Max-Planck-Institut fuer Kernphysik
Saupfercheckweg 1
69117 Heidelberg
Tel +49 6221 516279
Koenigstuhl 12
University of Heidelberg
Tel +49 6221 541737

Prof. Werner Hofmann
Max-Planck-Institut fuer Kernphysik
Saupfercheckweg 1
69117 Heidelberg
Tel +49 6221 516330

The discovery: In a recent publication in Nature magazine, the international H.E.S.S. collaboration reported 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.

The issue: One key issue in our understanding of cosmic rays is their distribution in space. Do they permeate the entire Galaxy uniformly, or do their density and distribution in energy vary depending on one's location in the Galaxy (for example due to the proximity of cosmic particle accelerators)? We can only take direct measurements of cosmic rays within our solar system, located about 25,000 light years from the centre of the Galaxy. However, a subterfuge allows astrophysicists to investigate cosmic rays elsewhere in the Galaxy: when a cosmic-ray particle collides with an interstellar gas particle, gamma rays are produced.

The gamma-ray / cosmic-ray link: The central part of our Galaxy is a complex zoo, containing examples of every type of exotic object known to astronomers, such as the remnants of supernova explosions and a super-massive black hole. It also contains huge quantities of interstellar gas, which tends to clump in clouds. If gamma rays are detected from the direction of such a gas cloud, scientists can infer the density of cosmic rays at the location of the cloud. The intensity and distribution in energy of these gamma rays reflects that of the cosmic rays.

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: The H.E.S.S. data show 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, 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"

HESS view of the Galactic Centre

The H.E.S.S. view of the galactic centre region. The top panel shows the gamma-ray image of the Galactic Centre region taken by H.E.S.S. Two bright sources dominate the view: HESS J1745-290, a mysterious source right at the centre of the Galaxy; and, about 1 degree away, the gamma-ray supernova remnant G 0.9+0.1. The lower panel shows the same image with the bright sources subtracted. In this image gamma-ray emission extending along the plane is visible as well as another mysterious source: HESS J1745-303. The dashed lines show the position of the Galactic Plane. The white circles show the positions from which the two sources were removed. (larger image)

Notes on H.E.S.S.

The collaboration: The High Energy Stereoscopic System (H.E.S.S.) team consists of scientists from Germany, France, the UK, the Czech Republic, Ireland, Armenia, South Africa and Namibia.

The detector: The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes is currently the most sensitive detector of very high energy gamma rays. These are absorbed in the atmosphere, where they give a short-lived shower of particles. The H.E.S.S. telescopes detect the faint, short flashes of blueish light which these particles emit (named Cherenkov light, lasting a few billionths of a second), collecting the light with big mirrors which reflect onto extremely sensitive cameras. Each image gives the position on the sky of a single gamma-ray photon, and the amount of light collected gives the energy of the initial gamma ray. Building up the images photon by photon allows H.E.S.S. to create maps of astronomical objects as they appear in gamma rays.

The H.E.S.S. telescope array represent a multi-year construction effort by an international team of more than 100 scientists and engineers. The instrument was inaugurated in September 2004 by the Namibian Prime Minister, Theo-Ben Guirab, and its first data have already resulted in a number of important discoveries, including the first astronomical image of a supernova shock wave at the highest gamma-ray energies.

Photograph of the H.E.S.S. telescopes