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The Power of HESS

Cosmic gamma rays are one of the most puzzling phenomena in the Universe. Indeed, so mysterious that it has taken nearly seven years for a hundred researchers to develop an entirely new kind of instrument capable of piercing their secrets.


© HESS coll.

The Hess instrument in Namibia.


The instrument is the HESS detector (High Energy Stereoscopic System)–composed of four 12-meter telescopes located at the four corners of a square with 120-meter sides. Set up at the Khomas Highland site in Namibia, its job is to investigate one of nature's strangest phenomena: very-high-energy gamma rays (VHEGR).

Slightly more than two years after its first light in December 2004, HESS has gathered such a wealth of information with its stereoscopic system1 that the international collaboration of nine countries (including IN2P3 and INSU at CNRS) which designed, built, and runs it, has recently been honored with the Descartes Prize, one of the most prestigious awards in European science.

By definition, gamma rays can be found at the far end of the electromagnetic spectrum, with wavelengths even shorter than ultraviolet and X-ray radiation. Ever since the 1950s, scientists have known that photons (light particles) with such high energy arrive from space at a rate of one per square meter per month. But the “hardest” gamma rays–which can reach energies of several million millions electronvolts–have physicists perplexed. If, theoretically, most radiation reaching Earth was originally produced by “hot objects” like stars, calculations show that no bodies in the Universe are hot enough to have generated VHEGR. So where do they come from? Scientists believe that they were produced by huge particle accelerators rather than by heat sources. If so, what exotic cosmic objects could emit such highly energetic streams of particles?

HESS's goal is to provide the answer to these questions. Located in Namibia at a latitude ideal for observing the Milky Way, its main objective is to add to the catalog of known gamma-ray sources. Technologically speaking, HESS combines instruments that preceded it, in particular the ultrafast very high-resolution camera on the French CAT telescope (Cherenkov Array at Thémis, CNRS / CEA) at Font-Romeu, and the Spanish-German HEGRA (High Energy Gamma Ray Astronomy) stereoscopic telescope in the Canary Islands. The detector, which was designed as early as 1997, is not intended to detect gamma rays directly, since they don't reach the ground. Instead, it spots the showers of particles that gamma rays trigger when they enter the atmosphere.




Gamma emission as large as 100 light-years around a pulsar. On the left, X-ray closeup of an emission close the pulsar.


By year-round analysis of the weak light trails produced in the night sky by particle showers, HESS first calculates whether some of them are caused by gamma-ray photons. It then reconstructs the path of the photons to identify the area of the sky where they originated. This is a key piece of information when it comes to identifying the radiation's sources.

HESS, a €10 million project, was co-funded by the Max Planck Institute (75%) and by CNRS (25%). Yet this does not make it an exclusively Franco-German facility. As well as Namibian scientists, technicians, and companies, the project brings together teams from the Czech Republic, Armenia, the UK, Ireland, Poland, and South Africa. And much of the significant advances made since it began operation can be directly linked to this international collaboration. In fact, since HESS's launch, the number of gamma sources listed both inside and outside our galaxy has gone from five to forty in a little over two years, giving rise to around forty publications. Among HESS's numerous achievements, there are historical firsts. These include the image of the remains of a supernova (seen in gamma radiation), the complete mapping of the gamma sources in the central region of our galaxy, and the discovery of gamma emissions from a binary system consisting of a massive star and a black hole or a neutron star. The results are so impressive that on March 7th of this year, the jury of the European Commission awarded the Descartes Prize, bestowed every year in recognition of transnational research, to the hundred or so scientists involved. This will undoubtedly encourage the researchers and warrant their earlier decision to add, in 2008, a fifth 28-meter telescope to further increase the instrument's sensitivy. Following that, the final phase in 2010 will see the beginning of the Cherenkov Telescope Array (CTA), an array of several dozen telescopes entirely dedicated to the search for gamma rays.


Vahé Ter Minassian

Notes :

1. A system that makes it possible to reconstruct the 3D relief or the position of objects in space from at least two 2-dimensional images taken from different angles.

Contacts :

Michael Punch
APC, Paris.


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