H.E.S.S.

H.E.S.S. is a system of several Cherenkov imaging telescopes investigating gamma radiation in the energy range of 100 GeV. The name H.E.S.S. is an acronym for High Energy Stereoscopic System and was also given in memory of Victor Hess, who received the Nobel Prize in Physics in 1936 for the discovery of cosmic radiation.

In 2004, when H.E.S.S. went into operation, it was one of the first instruments in the world to spatially resolve sources of cosmic gamma radiation. It is located at an altitude of 2000m near the Gamsberg in Namibia, one of the driest regions on earth, which is particularly suitable for observing the night sky. 

H.E.S.S. was built in two phases. During the first phase of the project, H.E.S.S. consisted of four 12m Cherenkov telescopes, which went into operation in December 2003. The project was officially inaugurated September 28, 2004.

The second phase, also known as H.E.S.S. II, began in July 2012 with the extension of the system by a significantly larger 28m telescope (CT5). As a result, the energy range, the angular resolution and the sensitivity of the instrument have been considerably increased.

H.E.S.S. observes gamma radiation. When this very high-energy electromagnetic radiation hits the Earth's atmosphere, it triggers a so-called particle shower with a large number of charged particles. These particles are still so energetic that they can move faster than light in the atmosphere (light propagates more slowly in air than in vacuum). Charged particles also excite other atoms and molecules to emit light waves as they pass.

What happens then is very similar to what is observed in airplanes when their speed exceeds the speed of sound: the individual waves superimpose themselves on a front, which leads to a supersonic bang in airplanes. Particle showers are referred to as Cherenkov radiation, a flash of light that lasts a few billionths of a second, too short to be detected by the human eye.

H.E.S.S.'s mirrors and their high-speed cameras, on the other hand, can image this radiation. An image is taken which shows the direction of the air showers and thus also the direction to the source of the gamma photon.

The intensity of the image is a measure for the energy of the photons. If only one telescope observes a particle shower, it is difficult to determine the geometry of the shower and the exact direction of origin. Therefore, several telescopes are usually combined and these allow a stereoscopic determination of the air shower geometry.

In the case of H.E.S.S., four 12 m diameter telescopes are located in a square with 120 m side length and a 28 m diameter telescope is in its center. Each telescope can be rotated about two axes so that objects in the night sky can be tracked automatically. A video from the Helmholtz Alliance For Astroparticle Physics demonstrates the mobility of the telescopes.

The 12 m telescopes are each composed of 382 mirrors with a diameter of 60 cm each and achieve a total mirror area of 108 m². The 28 m telescope, on the other hand, consists of 875 hexagonal mirrors with a diameter of 90 cm each and a total mirror surface of 614 m². Two so-called actuators are attached to each mirror, motors that allow remote control of the mirrors. Before each observation, the mirrors are automatically aligned to optimize the image of the observed object.

The H.E.S.S. II system has a significantly lower energy threshold (< 0.1 TeV) for the detection of TeV photons compared to the H.E.S.S.I array consisting of four 12 m telescopes. In the energy range of ~0.1 to 1 TeV the sensitivity is about a factor two higher than with H.E.S.S. I. These properties make H.E.S.S.II particularly interesting for time-variable sources whose discovery in the TeV range or detailed investigation was not sufficient for the sensitivity of the previous instruments (Cherenkov telescopes and the Fermi gamma satellite).

Relevant (known) object classes for such investigations are active galactic nuclei, gamma-ray bursts, and galactic binary systems. Furthermore, the reduced energy threshold allows both to extend the extragalactic horizon for TeV observations to larger redshifts and to investigate sources whose spectral cutoff lies in the ~50 GeV range (e.g. magnetospheric emission of pulsars). At higher energies (0.1-1 TeV), the sensitivity allows detailed morphological studies of selected sources that were previously not feasible due to the observation time required, such as supernova remnants (SNR).

The IAAT is involved in the data analysis and simulations of H.E.S.S. Mainly SNRs and binary star systems are studied.

In recent years, for example, it has been studied whether two H.E.S.S.-sources that are close together may also be causally related (Cui et al. 2016). One source, the supernova remnant HESS J1731-347, produces strong gamma and X-ray radiation, whereas the larger and still unknown source HESS J1729-345 emits much weaker radiation. The second source is located at the same place as molecular clouds known from radio observations. IAAT researchers have shown through simulations that the supernova remnant is very likely still expanding, and that the weaker source can be generated by the interaction of high-energy cosmic radiation emitted from the supernova remnant with the neighbouring molecular clouds.

Further analyses of H.E.S.S. data at the IAAT in recent years have allowed a better understanding of the origin of TeV radiation (Capasso et al. 2016). The morphology of the TeV radiation emission region was examined with unprecedented accuracy. Thus the TeV radiation region could be compared with the distribution of the gas in the molecular clouds, and the scenario of the interaction of cosmic radiation with molecular clouds could be further confirmed.

Another example of the analysis of H.E.S.S. data at the IAAT is the development of a new method to detect new SNR shells (Gottschall et al. 2016). This method, tested against four known sources, has led to the discovery of 3 SNR-shell candidates.