Inauguration of the gamma experiment HESS on 3rd September / Four Cherenkov telescopes to detect radiation from far off galaxies or supernovae using the highest known level of sensitivity

On 3rd September 2002, a new window on the universe will be opened in Namibia. This date marks the official inauguration of the first of four telescopes of the HESS experiment located on Goellschau farm, which is at an altitude of 1800 meters. In two years time, this “High Energy Stereoscopic System” should be completed and able to explore high-energy radiation from galaxies or the remnants of supernovae. The mirrors, which are arranged in the shape of a honeycomb, detect weak light flashes that develop when cosmic gamma quanta penetrate the earth’s atmosphere. More than seventy scientists from Germany, France, England, Ireland, the Czech Republic, Armenia, Namibia, and the Republic of South Africa are involved in the HESS project. The Federal Republic of Germany is represented by the Max Planck Institute for Nuclear Physics in Heidelberg, the Humboldt University in Berlin, the Ruhr University in Bochum, the University in Hamburg, the University in Kiel, and the Heidelberg Observatory. The Max Planck Society and the German Federal Ministry for Education and Research have spent a total of 6 million euros on the project, a sum that amounts to roughly three quarters of the total costs of 7.6 million euros.

Given that the history of astronomy goes back centuries, the observations in the gamma spectrum are really among the newest areas in celestial research. The high-energy light is swallowed by the earth’s atmosphere yet the light cannot be captured with conventional lenses or mirrors. Special detectors in satellites and high altitude research rockets register gamma rays with energies of up to around ten billion electron volts. These instruments are, however, not suitable to detect radiation from the cores of active galaxies or from the remnants of supernovae having even higher energies of up to a trillion electron volts.

Up until now, this interesting spectral range has not been investigated in detail. To perform research, the astronomers use a trick enabling them to make observations from the ground. Fortunately for life on earth, a gamma particle from the universe does not penetrate to the earth’s surface, but if it flies past an atomic nucleus within the earth’s atmosphere, the gamma particle can transform itself into an electron und its antiparticle, a positron. During its journey through the air, this pair comes across more atomic nuclei and a gamma quantum is generated which then once again hits atomic nuclei. Thus, a single cosmic gamma particle creates roughly a thousand secondary particles in a cascade-like process.

Cherenkov light develops within this air shower. Because the particle moves faster than the speed of light in air, there is a sonic boom or shock wave, which sends out a flash of blue light in the direction of the primary gamma quantum and lasts a few billionths of a second. This happens about ten kilometers above the earth’s surface. Such a Cherenkov flash illuminates an area of roughly 250 meters in diameter. The blue light is similar to a meteor track and is much too faint for the naked eye to see; the retina would have to be a million times more sensitive.

This is where HESS comes into play. Each telescope has a diameter of twelve meters and 380 individual round mirrors that make up a light-collecting surface area of 108 square meters. The focus of the telescope has a French-built electronic camera with 960 photo tube detectors, which are mounted on an area of about 1.4 meters in diameter. The camera enables exposure times of a mere one hundred millionth of a second. The HESS acronym alludes to the Austrian physicist Viktor Franz Hess (1883-1964) who discovered cosmic rays during ten balloon flights between 1911 and 1913. In 1936, he was awarded the Nobel Prize for Physics.

Unlike a conventional telescope, the Cherenkov telescope does not produce images of celestial objects. Instead, it only detects the air shower in the earth’s atmosphere. In order to obtain the image of the gamma’s origin, a computer on the HESS site combines up to four images and determines the position as well as the energy of the air shower. The result is plotted as a point on a map of the sky. Only a large number of points produced in this way show a galaxy or the remnants of a supernova. Contrary to visible light, gamma rays are non-thermal meaning that they are not produced in hot celestial bodies like the sun. Gamma rays occur in exceptional circumstances such as in the aftermath of a stellar explosion, in the vicinity of black holes, or at the core of active galaxies.

Searching for gamma quanta is tedious because they hit the earth with much less frequency than optical photons. Therefore, the telescopes have to be directed at the same location in the sky for many hours. The astronomers reckon that they will collect up to one thousand observation hours per year. In order to detect the weak blue light, the HESS telescopes operate during nights when the moon is not visible. Every night, the astronomers can sight up to a dozen different objects. If all four telescopes detect a flash simultaneously, a stereoscopic observation can be made. Moreover, the instruments are weatherproof and are not protected by any sort of roofing or building. The four telescopes are positioned to form a square; the distance between each telescope is 120 meters.

The HESS telescopes are ten times more sensitive than earlier Cherenkov telescopes. Peter Gruss, President of the Max Planck Society, emphasized that the HESS telescopes are “in friendly competition with similar facilities in Australia, the USA, and on the Canary Islands”. HESS is also a good example of “efficient international scientific collaboration”. The concept for the project crystallized in 1996 at the Max Planck Institute for Nuclear Physics in Heidelberg. The parameters for international cooperation were set in January 1998 and a year and a half later, the Goellschau farm located in the Khomas Highland in Namibia was chosen as the location. Construction began in August 2000. The year 2002 marked the completion of the first of four telescopes to be officially inaugurated today. The remaining three are still under construction.

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