UK participation in the pre-production phase of CTA extension 2021

The Universe is full of particles with energies so great that they are travelling at very close to the speed of light. They affect the Universe in many ways, influencing the life cycles of stars and 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 so few gamma rays that detecting them using spacecraft becomes impossible. Luckily, it is possible to observe them from the ground via the flashes of blue light, Cherenkov radiation, produced when they interact 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 reached the limit of what can be done with current instruments, and so over 1640 scientists and engineers from 31 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 extend the energy range of the gamma rays observed to both lower and higher values. It is predicted that the catalogue of known VHE emitting objects will expand from the roughly 200 known now to over 1000, and we can expect many new discoveries in key areas of astrophysics and fundamental physics. To achieve the energy coverage of CTA, telescopes of three different sizes are needed: Small (~4 m diameter), Medium (12 m) and Large (23 m) Sized Telescopes (SSTs, MSTs and LSTs, respectively). CTA will have arrays in the northern and southern hemispheres. Both arrays will include LSTs and MSTs, and the SSTs will be situated only in the souths as these are designed to investigate the highest energy phenomena, which are visible mainly
in the southern sky. We expect construction of the first telescopes on the CTA southern site to begin in 2022.

There are currently 12 UK universities and Laboratories involved in CTA. The four UK groups developing the hardware are concentrating their efforts on the construction of the SSTs for which we previously developed the Compact High Energy Camera (CHEC). CHEC was selected along with the Italian ASTRI telescope structure, from the three competing SST designs, as the basis for the final SST design. During the 2021 funding period we will use the lessons learned from CHEC to design the final production-ready cameras for SST. We will also prepare AIV facilities in preparation for production. We will expand outreach activities to include a planetarium show and UK science meeting and enhance project management and product assurance in readiness for production.