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09th February 2016

Major milestone for research by Dr Garret Cotter, Fellow in Physics

Scientists developing the world’s next-generation high-energy gamma-ray observatory, including Exeter’s Fellow in Physics Dr Garret Cotter and Exeter DPhil student Andrea De Franco (2013, Astrophysics), achieved a major milestone recently when a prototype telescope recorded its first observations of light from high-energy cosmic rays during testing at l’Observatoire de Paris in Meudon, France.

The Cherenkov Telescope Array (CTA) is a global initiative to build the world’s largest and most sensitive high-energy gamma-ray observatory. CTA uses technologies from both astronomy and particle physics to detect the intense, but extremely short, bursts of light released when extremely high energy cosmic rays and gamma-rays from space collide with the top of the Earth’s atmosphere. These air-showers consist of what is known as Cherenkov light: bursts of very blue light lasting only a few billionths of a second.

CTA will serve as an open observatory to a wide astrophysics community and will provide a deep insight into the non-thermal high-energy universe. The aims of the CTA project include improving our understanding of the origin of cosmic rays and their role in the universe; helping us to comprehend the nature and variety of particle acceleration around black holes; and searching for the ultimate nature of matter and physics beyond the Standard Model.

CTA includes the creation of at least three types of telescope – small-, medium-, and large-size telescopes – distributed over two observatories – one in the northern hemisphere and one in the southern hemisphere. These will enable the CTA observatory to detect high-energy radiation with unprecedented accuracy and approximately 10 times the sensitivity of current instruments, providing novel insights into some of the most extreme and violent events in the universe.

The GCT prototype. Credit: Akira Okumura

It is an ambitious, multi-million pound project involving scientists and engineers from 32 countries and over 170 research institutes. In the UK, the universities of Durham, Leicester, Liverpool and Oxford are participating in the design and construction, and the Oxford group is led by Exeter physics Fellow Dr Garret Cotter.  Dr Cotter’s team have been working on the software and electronics for the camera for one of the small-size telescope prototypes, the Gamma-ray Cherenkov Telescope (GCT).

During two weeks in November 2015 the GCT team battled poor weather to install and begin testing the GCT camera on the telescope structure in Paris. On the evening of Thursday, 26 November, they turned the telescope away from a nearly full moon and the bright lights of Paris towards a clear patch of sky. After 20 seconds, a single event triggered the camera, then another – in just over 300 seconds 12 events were captured. It was instantly clear that they were what the team was looking for – images of air showers created in the atmosphere by cosmic rays.

The GCT camera. Credit: Akira Okumura


The image below is one of the events captured by the team. It shows the maximum amount of light captured in each of the camera’s 2048 pixels over 100 frames. CTA astronomers will use images like this to determine the incoming direction and energy of the particle that created the air shower.

Image of an event captured by the GCT. Credit: CTA

In order to detect the short flashes of light produced by cosmic rays and gamma rays as they hit the Earth’s atmosphere, the telescope’s camera has to be about a million times faster than a DSLR camera. To do this, it uses high-speed digitisation and triggering technology capable of recording images at a rate of one billion frames per second and sensitive enough to resolve single photons.

These first images are just the beginning for the GCT. The prototype telescope and camera will undergo rigorous testing over the next year, then the team intends to build 35 cameras and telescopes for the CTA Observatory based on the results of the testing process.

Dr Cotter said: “We are delighted that the hard work put in over the past several years by all the members of the team has resulted in such a clear detection straight away when the telescope and camera were integrated. There’s a huge amount of cutting-edge engineering here – electronic, optical and mechanical. The next few years will continue to be a challenge as we perfect the systems for mass-production and then head to South America to commission 35 telescopes on site. But at that point, we will reap the ultimate reward: a whole new window on high-energy physics and astrophysics.”

Members of the Oxford CTA group at the Observatoire de Paris in December. Left-Right: Tony Bell, Department of Physics; Garret Cotter, Exeter and Department of Physics, Principal Investigator; Laurel Kaye, Department of Physics; Paul Morris, Department of Physics, Andrea De Franco, Exeter DPhil student and Department of Physics; Jason Watson, Department of Physics.


Artist’s impression of the completed Southern Hemisphere CTA site. Credit:CTA


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