Biggest ever Gamma Ray search starts in Namibia

by PPARC

The world's most sensitive Gamma Ray telescopes are being inaugurated in Namibia (in Southwest Africa) on September 3rd. The High Energy Stereoscopic System (H.E.S.S.), a European/African collaboration in which the UK is a partner, will look for Gamma Rays produced by the most energetic particles in the Universe. The array initially consists of four telescopes, the first of which will become operational next week. This one telescope alone is more sensitive than any other existing ground-based array or telescope working in this particular area of the electromagnetic spectrum.

Once all four telescopes are operational in late 2003, researchers from the University of Durham will use H.E.S.S. to investigate a range of extreme cosmic environments such as the supernova remnants formed when a star dies. A major goal is to see if these are a source of cosmic rays - charged particles that constantly bombard the Earth from space. The origin of cosmic rays is difficult to determine as they are influenced by the magnetic field of our Galaxy. However, the Gamma Rays they emit travel in a straight line, so they may reveal the primary source of the cosmic rays. H.E.S.S. will also be probing the structure of pulsars (rapidly rotating stars formed when a massive star explodes at the end of its life, which emit pulses across the range of the electromagnetic spectrum) and active galactic nuclei to find the source of their energy.

Dr Paula Chadwick, of the Durham team, explains: "H.E.S.S. is set to give us unique insights into some of the most extreme environments in the universe. We have some expectations about what we will be able learn more about - supernova remnants, active galaxies and so on - but experience tells us that when you improve the sensitivity of your telescope, you see things you never expected as well. It's going to be very exciting!"

When Gamma Rays are absorbed by the Earth's atmosphere, pairs of electrons and positrons are created and emit tiny flashes of light in a process known as Cherenkov radiation. Telescopes such as H.E.S.S can detect these tiny flashes of light. By using the Earth's atmosphere as part of the detector, the telescopes have much greater sensitivity than an equivalent space based device, and can detect far fainter Gamma Ray sources than previously possible.

Gamma rays are usually produced by particles moving very rapidly. The study of Gamma Rays enables astronomers to learn more about systems that accelerate these particles, such as active galactic nuclei where supermassive black holes produce jets of particles travelling near the speed of light. These are strong and highly variable sources of gamma rays. Gamma Rays can also be produced by the annihilation of massive particles that may be the source of the 'missing mass' in the universe.

The University of Durham' s role in the design and manufacture of HESS has been in calibrating the camera that will record the Cherenkov radiation and in developing systems that will measure the atmospheric conditions. This is critically important as variations in the atmosphere, such as cloud cover, can dramatically reduce the amount of light reaching the telescopes. The Durham scientists are now working on various refinements to calibration systems, and a more efficient mirror making technique that they hope to use when the array is extended from the current 4 telescopes to the planned 12 or 16.

Namibia is an excellent site scientifically, one of the best in the world for ground-based optical astronomy and with ideal atmospheric conditions for the techniques used by H.E.S.S. However, practically it has represented a huge challenge with limited road access to the site and water, power and computing connections having to be put in place specially.

Gamma Rays are a type of electromagnetic radiation, like optical light or radio waves. Electromagnetic (em) waves have a wide range of wavelengths and they can be used to see different views of objects. Visible light only shows us what is on the surface, but other types of em waves can reveal more, like the x-rays used by doctors to see broken bones. In their quest to understand our universe, astronomers long ago branched out from visible light to examine objects that radiate in the microwave and x-ray wavelengths. However, gamma rays, the most energetic kind of EM wave have remained difficult to use until now, as there are very few strong sources for astronomers to view.

The H.E.S.S. array is named after Viktor Hess who received the 1936 Nobel Prize in Ph

ysics for his discovery of cosmic radiation. All four telescopes should be operational by late 2003, at which point a decision will be made on whether to extend the project with additional telescopes.

More details about the H.E.S.S. project can be found at: http://www.mpi-hd.mpg.de/hfm/HESS/HESS.html