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Dr Stella Bradbury,
Dr Jim Hinton,
Dr Johannes Knapp,
Dr Jeremy Lloyd-Evans and
Dr Joachim Rose.

Very High Energy Gamma-Ray Astronomy

Artists impression of a microquasar. Microquasars are accreting binary systems which emit very-high-energy gamma-rays.

Very High Energy gamma-rays do not occur naturally on Earth. They are associated with the acceleration and interaction of relativistic particles at energies beyond those achievable in man-made accelerators. Gamma-ray astronomy is the astronomy of the "non-thermal universe", tracing the most violent and energetic phenomena at work inside our galaxy and beyond. These phenomena include the explosive deaths of stars (supernovae or hypernovae), the particle winds and shocks driven by neutron stars spinning on their axes millions of times faster than the sun, and the superluminal jets of active galaxies powered by super-massive black holes. Unlike charged cosmic ray particles, gamma-ray photons wing their way directly to us without having their paths bent by galactic and extra-galactic magnetic fields. They are also more penetrating than light of longer wavelengths, so less easily absorbed by the gas and dust surrounding feeding black holes. VHE gamma-rays therefore offer a unique insight into some of the most extreme regions of our universe. The only trouble is, there are very few of these exceptionally energetic photons, so satellite detectors simply aren't big enough to catch them. We need instruments which can collect gamma-rays from an area the size of a couple of football pitches.

The prototype 12m diameter telescope for
VERITAS: the Very Energetic Radiation Imaging
Telescope Array System

The Imaging Atmospheric Cherenkov Technique for VHE gamma-ray astronomy was pioneered by the Whipple Collaboration, operating a 10 m diameter optical reflector near Tucson, Arizona. When a VHE gamma-ray enters the Earth's atmosphere, it generates a shower of secondary charged particles which in turn cause a flash of blue Cherenkov light. The Cherenkov light flashes due to incoming gamma-rays and charged cosmic rays are estimated to contribute about 0.1% of the brightness of the night sky. However, each flash lasts for just ten thousand millionths of a second, so it is impossible for our eyes to register the fact that the atmosphere is scintillating. To detect the flashes of Cherenkov light we use photomultiplier tube cameras at the focus of large optical reflectors. Gamma-rays are extracted from the huge background of charged cosmic rays by searching for narrow Cherenkov images pointing to the position of an astronomical object in the camera's field of view. This method of "Hillas parameterisation", which stemmed from the work of Professor Michael Hillas at Leeds, was used in the first detection of a steady VHE gamma-ray source - the Crab Nebula - in 1989, and has since been adopted world-wide.

Members of the School of Physics & Astronomy are now involved in two major VHE gamma-ray projects. These instruments, VERITAS in the Northern Hemisphere and H.E.S.S. in the South, together provide a view of the whole very high energy sky.

VERITAS

Together with collaborators in the U.S., Ireland and Canada, we are constructing the Very Energetic Radiation Imaging Telescope Array - VERITAS. By the end of 2006 we will have four 12 m diameter Imaging Atmospheric Cherenkov Telescopes operating on Kitt Peak in Arizona. Leeds' contributions include a nuclear instrumentation-type high speed trigger system, data analysis software and the acronym (thanks to Alisdair Rodgers, PhD student 1997)!

The H.E.S.S. telescopes in the Khomas Highlands of Namibia

H.E.S.S.

The High Energy Stereoscopic System (H.E.S.S.) is a similar array of four Cherenkov telescopes located in the Khomas Highlands of Namibia in Southern Africa. Fully operational since the beginning of 2004, H.E.S.S. has produced a string of important new results, including: the first true astronomical gamma-ray image, the first detection of diffuse VHE gamma-ray emission from molecular clouds, the first detection of periodic VHE emission from a microquasar, and the discovery of the most distant known VHE emitting active galaxy. The H.E.S.S. is a collaboration of around 100 scientists from ten different European and African countries.

Views of the high energy universe: the Vela region seen in X-rays (left and centre) and VHE gamma-rays (right). See here for more details.