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A cosmic accelerator in the Milky Way

An international team of scientists, including researchers from CNRS and CEA,1 have discovered a new source of very high-energy cosmic rays at the center of our galaxy.(2) These results were obtained using the High Energy Stereoscopic System (HESS) array of telescopes in Namibia.(3)

Cosmic rays are high-energy particles from outer space that continually bombard the Earth from all directions. Although first detected by the Austrian physicist Victor Hess in 1912, scientists still know very little about where they come from and how they are accelerated to such high energies.



© HESS Collaboration

HESS telescopes in the Khomas Highland of Namibia.

Very high-energy gamma rays, which have energies as high as a million million electronvolts (eV) or more, are rare and strike the Earth's atmosphere only about once per month per square meter. HESS measures the short flashes of blue light that are produced when the gamma rays are absorbed in the atmosphere. This light is collected by four 13-meter diameter telescopes and recorded by ultrasensitive cameras. Each image gives the position in the sky of a gamma photon, as well as its energy. Building up the images photon by photon allows HESS to create maps of astronomical objects as they appear in gamma rays.

Using this method, the researchers detected very high-energy gamma rays coming from several hydrogen gas clouds located near the center of the Milky Way. These clouds are huge, with masses 50 million times that of the Sun. The team believes that the gamma ray emission is caused by the collision of galactic cosmic rays–highly energetic particles that are either atomic nuclei or protons. They calculated how these cosmic rays were distributed in space by comparing the total mass of the clouds, and discovered that their density around the center of the galaxy is about five times higher than in the vicinity of the Earth. Moreover, the cosmic rays at the galactic center are more energetic than those in the solar system. Both these findings suggest that there is an active cosmic ray 'accelerator' located near the center of the Milky Way. This is the first clear detection of such an accelerator.

The team indicates that the accelerator could be due to a supernova explosion that occurred some 10,000 years ago in the center of our galaxy.4 Another possibility is that the cosmic rays were accelerated as they swung around the supermassive black hole Sagittarius A*, thought to lurk at the center of the Milky Way.5 “Further observations in gamma rays, as well as at other wavelengths, will allow us to pinpoint this galactic accelerator and determine its nature,” explains team member Regis Terrier, from the Astroparticles and Cosmology Lab in Paris.6 “Observations of other galactic clouds in different places in the Milky Way will also help us understand the origins, distribution and propagation of cosmic rays throughout our galaxy.”


Isabelle Dumé

Notes :

1. CEA: commissariat à l'énergie atomique (the Atomic Energy Commission).
2. F. Aharonian et al., “Discovery of very-high-energy rays from the Galactic Centre ridge,” Nature. 439: 695-698. 2006.
3. France accounts for one third of the funding for HESS, a collaboration of over 100 international scientists. Since its inception three and a half years ago, HESS has multiplied the number of known gamma ray sources in our galaxy by five and produced the first astronomical image of a supernova shockwave in high-energy gamma rays.
4. A supernova occurs when a star explodes at the end of its life.
5. A black hole is a celestial body so dense that light cannot escape its gravitational pull. It is formed when a star's gravity causes it to collapse into itself.
6. Laboratoire d'Astroparticule et Cosmologie (CNRS / Université Paris-VII / Observatoire de
Paris / CEA joint lab). View web site

Contacts :

> Régis Terrier
Astroparticule et Cosmologie, Paris.

> Loïc Rolland
CEA, Saclay.


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