A great year for H.E.S.S.

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.

Complete H.E.S.S. telescope system as in January 2004 (Credit: H.E.S.S. Collaboration, Nukri Komin).
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.

Photo montage showing the gamma-ray sky over Namibia, as measured by the four H.E.S.S. telescopes during the last years, superimposed onto an optical image, with one of the small H.E.S.S. telescopes in the foreground (Credit: H.E.S.S. Collaboration, Fabio Acero and Henning Gast)
A decade after the inauguration of the first telescope and eight years in 4-telescopes operations, the H.E.S.S. collaboration had recorded more than 6 billions of events, during approximately 9000h of data, from which 45% close to the Galactic plane. H.E.S.S. is currently the greatest contributor to the TeVCat catalog of TeV sources, with 80 discoveries for a total of 130, and 3/4 of them being galactic objects. A large variety of Galactic astrophysical objects was found to be TeV emitters, probing against previous thoughts, that cosmic-ray acceleration is a rather common feature in the stellar evolution. Furthermore, most of the galactic sources detected by H.E.S.S. are extended, with for instance resolved emissions in direction of supernovae remnant shells. Apart from these classically considered objects, H.E.S.S. has recorded gamma-rays in direction of pulsar wind nebulae, supernovae remnants in interaction with molecular clouds, star clusters and star forming regions, as well as binary systems in which a large mass star orbiting around a compact object. Mysteries still remain though, since more that a quarter of the Galactic sources detected in the TeV domain has no identified counterparts so far.

View of the full H.E.S.S. array with the four 12 m telescopes and the new 28 m H.E.S.S. II telescope (Credit: H.E.S.S. Collaboration, Frikkie van Greunen)

H.E.S.S. is now entering its second phase. The additional and much larger fifth telescope has been inaugurated at the end of September. This telescope possesses the largest mirror collection area built, with 614 square meters, obtained from an assembly of 875 hexagonal facets of 90 cm size, partly funded by the Wallenberg foundation.

Uncovering of the mirror facets (Credit: H.E.S.S. Collaboration, Clementina Medina).
The camera is equipped with more than two thousands photo-multiplier tubes, and is foreseen to operate with an image recording rate of 3600 images per second, and a very short exposure time (~16 ns). Such short exposure are indeed necessary to identify the very dim Cherenkov light produced by gamma or cosmic-rays on top of the night sky background produced by the stars. H.E.S.S. II will improve performances with respect to the previous phase for energies below few hundreds of GeV and will extend the energy range of the experiment at lower energy, complementing the results obtained by the Fermi-LAT. In particular, in its second phase, H.E.S.S. will have a much better sensitivity than the Fermi-LAT for the studies of transient events such as flares of active galactic nuclei or binary systems. Moreover, even if the new large telescope is equipped with a 2 tons camera located at 28 meters from the mirrors, its drive system allows faster movements than for the four “small” 13 meters diameter ones. H.E.S.S. II might therefore be able to observe the brief but very intense flash of high energies photons released by gamma-ray bursts.
More information on the large size telescope can be found here.

Finally, 2012 has also been marked by a potential hint of a Dark Matter induced line signal in the analysis of the LAT data. Up to now, the strength of the signal remains too low to be able to claim for a detection and analysis of subset of the data recorded by the LAT have cast doubt on the interpretation of the signal, by pointing to a potential instrumental effect. Confirmation of the true nature of the signal by the LAT will take time. H.E.S.S. II on the other hand, thanks to its large collection area, have the capability to confirm or rule out this excess observed in the direction of the Galactic centre in 50 hours of observation (see Bergström et al. 2012 http://arxiv.org/abs/1207.6773). The new instrument is actually in the calibration phase and is expected to be ready for standard data taking by Spring 2103. At that time, the Galactic centre region will become visible in good conditions for few months, hence allowing us to record enough data.

It is clear that H.E.S.S. has now moved to a new and very exciting stage. So stay tuned and you will hopefully heard more about it from us in the near future.

– Christian Farnier – postdoc at the Oskar Klein Centre
(Christian.Farnier@fysik.su.se)

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