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Windows on the Universe

The HESS Experiment in High Energy Gamma-Ray Astronomy

A French-built camera 1.5 meters in diameter is placed at the focal point of the telescope making it possible to digitalise light signals with a temporal resolution of one nanosecond.

The largest part of our knowledge of the universe comes from observing the electromagnetic radiation emitted by celestial bodies, such as star light. The HESS experiment (High Energy Stereoscopic System) was born of the idea of looking instead at high energy gamma radiation, especially for understanding the most violent events in the Universe. The year 2003 marked a milestone in the HESS project, as its four-telescope network came on line.

High energy gamma-ray detection is accomplished by recording the cascade of secondary particles known as an air shower which occurs as each gamma particle interacts with earth's atmosphere. The Cherenkov effect means that the air shower in turn gives off a brief and weak signal in visible light which can be captured by mirror telescopes on moonless nights and transformed into an image in a specially designed camera. By analyzing the image produced, astrophysicists1 can establish the properties of the original gamma radiation, such as energy and direction.

telescope Cherenkov

© Source : H.E.S.S. collaboration

The first Cherenkov telescope of the HESS project with its 12-meter composite mirror, on site at Gamsberg in Namibia.

HESS represents the third generation of earthbound gamma-ray astronomy, following the French telescope CAT, the German-Spanish HEGRA project, and the American Whipple observatory. For the moment HESS comprises four telescopes whose construction was begun in Namibia in 2001 in the region of Gamsberg with its excellent atmospheric conditions and good line of sight to the center of our galaxy. HESS will function as a southern hemisphere  complement to VERITAS, a similar American project to be built in Arizona.

The HESS' scientific community is composed of both particle physicists and astrophysicists. In France, laboratories associated with HESS include three labs from the CNRS' National Institute for Particle and Nuclear Physics (IN2P3) – the Leprince-Ringuet laboratory, the Laboratory for Particle Physics and Cosmology, and the Laboratory for Nuclear and High Energy Physics – as well as the CEA/DSM/DAPNIA/SAp, and three laboratories from the CNRS' National Institute for Sciences of the Universe (INSU), namely the Observatories of Meudon and Grenoble, and Toulouse's Center for Radiation and Space Studies. In Germany, the Max-Plank Institut für Kernphysik in Heidelberg, the universities of Berlin, Bochum, Hamburg, and Kiel, and the Heidelberg Observatory are all involved in HESS.
The project also includes researchers from the UK (Durham), Ireland (Dublin), the Czech Republic (Prague), Armenia (Erevan), Namibia (Windhoek), and South Africa (Potchefstroom).

Objects known to emit gamma radiation are those which produce highly accelerated charged particles. These include:


Pulsars: rapidly rotating neutron stars which are the remains of a supernova;

Supernova remnants: when a star collapses at the end of its life, it sends its outer layers into interstellar space in a shock wave which is apt to accelerate charged particles;

Plerions: A particular class of supernova remnants, whereby the central pulsar is surrounded by a synchrotron nebula. The pulsar emits an intense wind of relativistic electrons, and the shock wave produced in the nebula is capable of re-accelerating the electrons (the Crab nebula is an example);

Blazars: Certain galaxies are characterised by a core which emits more radiation than the all of the rest of the galaxy and are known as a active galactic nuclei (AGN). It is currently believed that the core of these objects is composed of a super-massive black hole surrounded by an accretion disk. For some AGN, jets of relativistic matter shooting out from the disk have been detected by radio. A blazar is an AGN whose jet is directed toward earth and is found to emit gamma radiation, which suggests that the origin of the gamma-ray energy is tied to these jets.

• Other objects may well be gamma-ray sources also: Microquasars for example, or the annihilation of supersymmetrical particles (the so-called WIMPS or weakly interacting massive particles, which are a possible form of dark matter accumulated at the center of our galaxy).

Many of these sources and especially the rapidly varying ones require simultaneous observation in several widely separated wavelengths covering radio, visible, X, and gamma radiation.




Gérard Fontaine
Laboratoire Leprince-Ringuet (LLR)
CNRS-École polytechnique

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