The past year, 2012 has been the hundredth anniversary of the discovery of the cosmic-rays by Viktor Hess. It was also the tenth anniversary of the High Energy Stereoscopic System, in short H.E.S.S., named after him and last but not least 2012 also marked the start of the second phase of the experiment with the inauguration in September of a fifth and much larger telescope.
It was 1912, when Viktor Hess performed several flights in air balloon, and discovered the extra-terrestrial origin of ionizing particles bombarding the Earth. The cosmic-rays were discovered. Since then, they have attract attention of a large community of scientists: physicists, astrophysicists and astronomers. A new field of research, astro-particle physics, emerged, spanning several orders of magnitude both in energy and particle rate, with the aim of studying their origin and properties.
At lower energies, where fluxes are high, charged particles are deflected by different electromagnetic fields before they can reach our detectors – on ground or in space – making it impossible to trace back their sites of emission. At ultra high energies, 1 billion times the energy of the particles accelerated at Large Hadron Collider (LHC), the events recorded by the Pierre Auger experiment in Argentina, are less affected by magnetic fields and point at their acceleration site. However, at such energies, rates are very low (1 particle per km square per century!), and even if Auger is one of the largest experiment on Earth, the number of events detected does not allow to study their acceleration mechanisms.
Charged cosmic-rays however can produce gamma-rays, most commonly by inverse Compton scattering of accelerated electrons on low energy photon fields or by decay of neutral pions, for instance arising from the collisions of very energetic hadrons on dense molecular clouds. These gamma-rays travel in straight line from their sites of production and their spectra can also serve to determine the nature and acceleration mechanisms of their charged cosmic-rays progenitors. Observations of gamma-rays have been so far the only way to locate high-energy cosmic-rays accelerators and they have been used successfully to establish catalogs of astrophysical objects with extraordinary acceleration power by experiments such as Fermi and H.E.S.S.
The Large Area Telescope (LAT) aboard the Fermi satellite, had provide us a tremendous amount of information in the Giga-electron-Volt (GeV) domain, with more than 2000 sources contained in its second year catalog, and a spectacular map of the entire sky in this energy range. But the LAT, launched via a space rocket in 2008, is size limited and does not have the possibility to study the sky in the Tera-electron-Volt (TeV) domain, due to very low rate of events at these energies.
Study of TeV emitters are therefore performed from the ground. This adventure started already a long time ago, with the detection of the first source, the Crab Nebula, by the Whipple collaboration in 1989. Few other sources were discovered afterwards, either by the same collaboration, or by other experiments (Hegra, CAT, Celeste, …) built in different countries of the Northern hemisphere. In the 90s, German groups involved in the Hegra experiment and French institutes from the former CAT experiments, decided to join their efforts and established the H.E.S.S. array of imaging Atmospheric Cherenkov telescopes in Namibia, where the Galactic centre and its surroundings are observable in ideal conditions.
H.E.S.S. design was a new step with respect to the previous existing ones. The array is formed by several telescopes providing a larger effective collection area. It also provides multiple views of the same event and improves the background rejection as well as the overall instrument response functions (angular resolution, energy resolution…). The telescopes have large reflective area, to collect more Cherenkov light per showers and lower the energy threshold. Finally, they are equipped with fine pixellised camera coupled with fast acquisition electronics. The fine pixellisation improves the determination of the nature and origin of the incident primary particles, whereas the fast electronic reduces night sky background noise and dead-time.
Here you can find the characteristics of the four telescopes of H.E.S.S. phase I.
Continue reading A great year for H.E.S.S.