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

Lead Research Organisation: University of Leicester
Department Name: Physics and Astronomy


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!


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{Abramowski}, A. And {Acero}, F. And {Aharonian}, F. And {Akhperjanian}, A.~G. And {Anton}, G. And {Balzer}, A. And {Barnacka}, A. And {Barres De Almeida}, U. And {Becherini}, Y. And {Becker}, J. And {White} And {Punch} (2012) The 2010 Very High Energy {$\gamma$}-Ray Flare and 10 Years of Multi-wavelength Observations of M 87 in \apj

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{Abramowski}, A. And {Acero}, F. And {Aharonian}, F. And {Akhperjanian}, A.~G. And {Anton}, G. And {Balzer}, A. And {Barnacka}, A. And {Becherini}, Y. And {Becker}, J. And {Bernl{\"o}hr}, K. And {Birsin}, E. And {Biteau}, J. And {Bochow}, A. And {Boisson}, C. And {Bolmont}, J. And {Bordas}, P. And {Brucker}, J. And {Brun}, F. And {Brun}, P. And {Bulik}, T. And {B{\"u}sching}, I. And {Carrigan}, S. And {Casanova}, S. And {Cerruti}, M. And {Chadwick}, P.~M. And {Charbonnier}, A. And {Chaves}, R.~ (2012) Spectral Analysis and Interpretation of the {$\gamma$}-Ray Emission from the Starburst Galaxy NGC 253 in \apj

Related Projects

Project Reference Relationship Related To Start End Award Value
ST/G00790X/1 30/09/2009 29/09/2010 £233,691
ST/G00790X/2 Transfer ST/G00790X/1 30/09/2010 29/09/2012 £157,450
Description CTA 
Organisation Heidelberg University
Country Germany 
Sector Academic/University 
PI Contribution I am a member of the CTA Consortium. During the period of this grant CTA was in the Preparatory Phase, with the Project Office hosted by the University of Heidelberg. CTA consists of ~1000 scientists and engineers across ~30 countries. During the period of this fellowship I was the CTA SST Camera Coordinator, responsible for the organisation of work for ~1/3 of the telescope cameras. I also played a leading role in the UK effort to prototype cameras for the SST, which began in the last 6 months of this fellowship.
Collaborator Contribution CTA members across the world are working together to design and prototype technology for the upcoming telescope array.
Impact This collaboration is ongoing, and could result in the UK playing a major role in the design, build and operation of CTA. During this fellowship, the first CTA official publication was published (10.1007/s10686-011-9247-0). Work done in this fellowship also contributed to the bid for a successful grant from STFC (£800k) to develop a prototype camera for CTA (I was CO-I). Funding as CO-I for a second camera prototype was obtained from a University of Leicester infrastructure bid.
Start Year 2009
Description H.E.S.S. 
Organisation Max Planck Society
Department Max Planck Institute for Nuclear Physics
Country Germany 
Sector Academic/University 
PI Contribution As part of the H.E.S.S. collaboration I contributed to the writing of journal papers, observing proposals and the upgrade of camera hardware.
Collaborator Contribution The H.E.S.S. collaboration provides access to data for all observations taken with the telescopes to all members of the collaboration.
Impact This collaboration resulted in 16 journal papers over the course of this fellowship and 2 successful observing proposals.
Start Year 2011
Description CTA Consortium Meeting Organisation 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? Yes
Type Of Presentation Workshop Facilitator
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact 200 members of the CTA consortium travelled to RAL for 5 days, hundreds of talks were given across 5 rooms, as a result the development of CTA was pushed forward a step.

Members of the wider particle physics and astronomy community became interested in CTA.
Year(s) Of Engagement Activity 2011
Description H.E.S.S. Collaboration Meeting Organisation 
Form Of Engagement Activity Scientific meeting (conference/symposium etc.)
Part Of Official Scheme? No
Type Of Presentation Workshop Facilitator
Geographic Reach International
Primary Audience Participants in your research and patient groups
Results and Impact 100 members of the H.E.S.S. Collaboration attended a 5 day meeting at the University of Leicester. Significant progress was made on several important research papers, and local University of Leicester colleagues became aware of H.E.S.S. and very high energy gamma-ray astronomy.

Senior staff members in the Physics department at the University of Leicester have taken an ongoing interest in very-high energy gamma-ray astronomy following this meeting.
Year(s) Of Engagement Activity 2012
Description Wrote the CTA-UK Website 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Type Of Presentation Paper Presentation
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact I designed, setup and wrote the CTA-UK website, resulting in a single point of access to CTA for anyone interested in the experiment from the UK. This is useful as many universities in the UK are involved in CTA, and having a single point of reference brings clarity to the public.

Several members of the wider scientific community have noted that the website has been useful to find out more about CTA and upcoming meetings.
Year(s) Of Engagement Activity 2012