From H.E.S.S. to HESS-II
Standing for High Energy Stereoscopic System, H.E.S.S. is an array of large Cerenkov telescopes, set up in Namibia. Its purpose is to observe cosmic gamma radiation in the Tera electron-volt (TeV).
Originally consisting of four telescopes, the array expanded in 2012, becoming HESS-II with the addition of a fifth very large 28m diameter telescope with a 600 m2 reflector.
This new arrangement is producing its first scientific results : the detection in the Southern sky of the first pulsar, i.e. a neutron star, the collapsed core resulting from the supernova explosion of a massive star.
HESS-II detected thousands of gamma rays with an energy around 30 Giga electron-volts (GeV) coming from the Vela pulsar. The signal is in the form of pulses, at the rate of the period of rotation of the neutron star.
Arache Djannati-Atai, from the Laboratoire Astroparticule et cosmologie (CNRS/Université Paris Diderot/CEA/Observatoire de Paris) notes : ´The detection of these gamma ray photons from the direction of the 11000 year old Vela pulsar, with a period of 89 milliseconds, i.e. at exactly that of the sar, shows that H.E.S.S. is now able to explore the most extreme phenomena in the neighbourhood of pulsars not only within their magnetospheres but also in principle beyond their light cylinders ñ which will help us to better understand the processes by which particles are accelerated and high energy gamma rays are produced by these fascinating objects. ª
This is the second pulsar, after that of the Crab in 2008, which has been seen by a ground based gamma ray telescope, highlighting in a spectacular way the ability of this new instrument.
The detection of periodic gamma ray emission from the Vela pulsar heralds new discoveries and research concerning very high energy galactic and extragalactic gamma ray sources.
A stereoscopic view, the only one in the world
In the Universe, supermassive black holes, clusters of galaxies, supernove, binary sarsand pulsars act as natural accelerators of cosmic particles (electrons, ions, ...). These particules thus acquire very high energies, as witnessed by gamma ray emission. When these rays reach the upper atmosphere of the Earth, they dump their energy and produce a shower of secondary particles, which then emit a very weak flash of blue light.
Gamma ray telescopes are designed to detect this momentary blue light, referred to as "Cerenkov radiation", whose properties enable one to "reach back" to the source ; however, this does involve some considerable modelization work.
To-day, HESS-II is the only telescope in the world with a steroscopic view of atmospheric gamma ray showers, using telescopes of varying sizes.
Over and above the construction work of the new telescope, success was crowned only after two intense years were consecrated to its calibration and adjustment, and to the creation of the data processing system.
"The main challenge was to lower as much as possible the energy threshold, while keeping the signal above the enormous background noise" explains Mathieu de Naurois, from the Leprince Ringuet laboratory, co-director of the HESS collaboration. In the case of the most intense sources, the telescope can register one high energy gamma ray photon per second which constitutes a world record at these energies.
Placed as it is in Namimbia, HESS-II can observe the central regions of our galaxy, in which are found numerous pulsars and other compact sources. The exploration of extragalactic space should follow on, given the current work on active galactic nuclei. The H.E.S.S. data show that Cerenkov telescopes can penetrate the deepest and most energetic mysteries of the Universe.
Many scientists, engineers, technicians and administrtors from the Paris Observatory have contributed to the construction of H.E.S.S.-II or are participants in the Hess experiment, over and above the members of the APC team :
- at the LUTH : C. Boisson, V. Hababou, G. Herpe, O. Hervet, N. Ollivier, C. Rulten, H. Sol, R. Vitry, A. Zech
- at the GEPI : Jean-Michel Huet
- at the DIL : E. Amet, H. Darmalingon.
CTA : the next world instrument
The way is clear for the next world class array, called the Cerenkov Telescope Array (CTA) : starting 2017, this will require the construction of about a hundred telescopes.
The CTA observatory will have two sites, one for each hemisphere, with data processing centres on various continents. Construction of the scientific equipment could start in 2016.
Thanks to the H.E.S.S. experiment, French laboratories have acquired considerable expertise in very high energy astronomy.
With this experience, many teams of French physicists and astrophysicists have grouped themselves around the GATE project in the Ile-de-France, under the scientific leadership of the Paris Observatory, for the construction of various prototypes of the equipment needed for the CTA.
