H.E.S.S. is a system of Imaging Atmospheric Cherenkov Telescopes that investigates cosmic gamma rays in the energy range from 10s of GeV to 10s of TeV. The name H.E.S.S. stands for High Energy Stereoscopic System, and is also intended to pay homage to Victor Hess, who received the Nobel Prize in Physics in 1936 for his discovery of cosmic radiation. The instrument allows scientists to explore gamma-ray sources with intensities at a level of a few thousandths of the flux of the Crab nebula (the brightest steady source of gamma rays in the sky). H.E.S.S. is located in Namibia, near the Gamsberg mountain, an area well known for its excellent optical quality. The first of the four telescopes of Phase I of the H.E.S.S. project went into operation in Summer 2002; all four were operational in December 2003, and were officially inaugurated on September 28, 2004. A much larger fifth telescope – H.E.S.S. II – is operational since July 2012, extending the energy coverage towards lower energies and further improving sensitivity.
The H.E.S.S. observatory is operated by the collaboration of more than 170 scientists, from 32 scientific institutions and 12 different countries: Namibia and South Africa, Germany, France, the UK, Ireland, Austria, Poland, the Czech Republic, Sweden, Armenia, and Australia. To date, the H.E.S.S. Collaboration has published over 100 articles in high-impact scientific journals, including the top-ranked ‘Nature’ and ‘Science’ journals.
In a survey in 2006, H.E.S.S. was ranked the 10th most influential observatory worldwide, joining the ranks with the Hubble Space Telescope or the telescopes of the European Southern Observatory ESO in Chile.
Named HESS II, the giant telescope’s 600-tonne bulk and 28-metre mirror will survey the southern hemisphere, hunting for violent, high-energy cosmic sources such as supermassive black holes, supernovae and pulsars.
Cherenkov telescopes search for signs of very-high-energy gamma rays by watching for Cherenkov radiation – a scatter of charged particles produced from gamma-ray interactions in our atmosphere and captured as faint flashes of blue light. HESS II captures these flashes with a camera around a million times as fast as one you or I might own.
There are currently two other operating Cherenkov systems – MAGIC, in the Canary Islands and VERITAS, in Arizona. The HESS array includes four smaller telescopes, each with a 12-metre mirror. HESS II has an “unprecedented” resolving ability, says the team, enabling it to capture a sharper picture of the skies.
Gamma-ray observations from HESS will be used in combination with data from the Square Kilometre Array (SKA), the world’s largest radio telescope system, expected to begin observing in 2020. Comprising telescopes scattered across South Africa, Australia and New Zealand, the SKA will be sensitive enough to detect the equivalent of an airport radar on a planet 50 light years away.