UK Participation in the Preparatory Phase of the Cherenkov Telescope Array 2012-2015

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The Universe is full of particles with such high-energies that they are travelling at very close to the speed of light. These particles play a significant role in many areas of astrophysics, from the life-cycles of stars to the evolution of galaxies. These particles are hard to trace, but can reveal their presence by producing gamma rays. Like their lower-energy cousins, X-rays, gamma rays do not penetrate the Earth's atmosphere and usually satellite-based telescopes are used to detect them. However, at very high energies (VHE) there are very few gamma rays, and detecting them from spacecraft becomes impossible. Luckily, it is possible to detect them from the ground via the flashes of blue light, Cherenov radiation, produced by the cascades they initiate in the atmosphere. The glow from Cherenkov radiation in the atmosphere is 10,000 times fainter than starlight, so large mirrors are required to collect it, and because the flashes last only a few billionths of a second, ultra-fast cameras are needed to record them. We know from current ground-based gamma-ray telescopes such as HESS that there is a wealth of phenomena to be studied. VHE gamma ray telescopes have detected the remains of supernova explosions, binary star systems, highly energetic jets produced by black holes in distant galaxies, star formation regions, and many other objects. These observations can help us to understand not only what is going on inside these objects, but also answer fundamental physics questions relating to the nature of dark matter and of space-time itself. However, we have now reached the limit of what can be done with current instruments, and so ~800 scientists from 25 countries around the world have come together to build a new instrument - the Cherenkov Telescope Array (CTA). CTA will offer a dramatic increase in sensitivity over current instruments, and extends the energy range of the gamma rays observed to both lower and higher energies. It is predicted that the catalogue of known VHE emitting objects will expand from the 130 known to over 1000, and we can expect many new discoveries in key areas of astrophysics and fundamental physics research. To achieve the wide energy range we require of CTA, it is necessary to build telescopes of three different sizes: ~5 m diameter small-sized telescopes (SSTs), ~12 m medium-sized telescopes (MSTs) and ~23 m large-sized telescope (LST). CTA will consist of two arrays of telescopes, one in the northern hemisphere and one in the southern hemisphere. The northern array will likely consist of 4 or so LSTs, and around 20 MSTs. The southern array will contain similar numbers of large and medium telescopes, but add to them an extensive array of ~50 SSTs, specifically to investigate the highest-energy phenomena which can be observed preferentially from the southern hemisphere. The SSTs will detect the highest energy photons ever seen, with energies approach a petaelectronvolt, each a thousand billion times more energetic than an X-ray. CTA is currently in its preparatory phase, and we expect construction to start in 2015.There are currently 11 UK universities involved in CTA. The UK groups are concentrating their efforts on the construction of the SSTs. We have already produced an innovative dual-mirror SST design, which will be built in sight of the Eiffel Tower in Paris. In this proposal we request funds to do several things. Firstly, we want to build and test a prototype dual-mirror SST camera, together with a calibration system. Secondly, we will use our expertise in computer simulations to optimise the design of the SSTs and the overall array layout. Thirdly, we will develop data analysis techniques for CTA, to ensure that UK scientists are able to analyse the data from CTA as soon as the first telescopes start operation. Finally, several UK team members have leadership roles in CTA and we are requesting funds to support these people and enable us to travel to the relevant meetings.