The Development of CTA for the Study of Extreme Extragalactic Particle Acceleration

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The Earth is being bombarded by charged particles from outer space. These particles are known as 'cosmic rays' and have been observed across an astonishingly large range of energies. The highest energy, or ultra-high energy, cosmic rays (UHECRs) are so energetic that they cannot be contained within our galaxy. An UHECR can have an energy equivalent to a 100 mph tennis ball in single proton. How such particles are produced in celestial objects is still a mystery. We know from experiments on Earth that accelerating particles to even a fraction of these energies is very difficult - it requires huge machines such as the LHC, at CERN, costing billions of pounds. However, recent breakthroughs by the Pierre Auger Observatory, built to measure these extreme particles, suggest that UHECRs are extragalactic in origin with arrival directions that correlate with the locations of active galaxies powered by super-massive black holes. Unfortunately, due to the limited angular resolution of the detector and the deflection of these charged particles in magnetic fields, direct measurements of UHECRs are not sufficient to unequivocally reveal their origin. But all is not lost! The acceleration of cosmic rays is accompanied by the production of very high energy (VHE) gamma rays. VHE gamma rays are simply photons, like star-light, but much more energetic (each possesses the energy of a trillion photons of visible light). Unlike charged cosmic rays, gamma rays wing their way directly to us without having their paths bent by galactic and extragalactic magnetic fields. 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 several times the size of Wembley Stadium. Imaging Atmospheric Cherenkov Telescopes offer just that. When a VHE gamma-ray enters the Earth's atmosphere, it generates a shower of charged particles which cause a flash of blue Cherenkov light. Each flash lasts for just ten thousand millionths of a second, so it is impossible for our eyes to register the constantly glittering sky. To detect the flashes 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 images pointing to the position of an astronomical object in the camera's field of view. This method of 'Hillas parameterisation', from the work of Professor Michael Hillas at Leeds, was used in the first detection of a VHE gamma-ray source, the Crab Nebula, in 1989, and has since been adopted world-wide. The current generation of VHE gamma-ray telescopes, such as the High Energy Stereoscopic System, HESS (in Namibia) has revealed a sky scattered with bright gamma-ray sources. HESS. has produced the first hard evidence that lower energy cosmic rays are accelerated in the shells of supernovae remnants within our galaxy, but falls short of the sensitivity required to resolve the mystery of UHECRs. The proposed Cherenkov Telescope Array (CTA), currently under design by a team of European astronomers, will provide 10x the sensitivity of HESS and will operate on high-energy frontier of photon astronomy. I will help to develop CTA for the specific purpose of studying extreme particle acceleration outside our galaxy. The optimisation of CTA for UHECR studies requires the use of cutting-edge technology, numerical simulations and HESS data. So far the sky in gamma rays is a bit like the sky in visible light seen from a big city - only the brightest objects are visible. With CTA the window on the VHE universe will truly be opened and the detection of some 1000 sources is expected. With the appropriate preparation accelerators of the most energetic particles in the Universe, the UHECRs, will be amongst them!