UK participation in the pre-production phase of CTA extension 2020

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 33 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 130 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. The northern array will consist of 4 LSTs and 25 MSTs. The southern array will add to its 4 LSTs and 25 MSTs an extensive array of 70 SSTs, to investigate the highest energy phenomena, visible mainly in the southern sky. We expect construction of the first telescopes on the CTA southern site to begin in 2021.

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 has been recently selected along with the Italian ASTRI telescope structure, from the three competing SST designs, as the basis for the final SST design. During the 2020 funding period we will use the lessons learned from CHEC to design and produce a final production-ready camera for SST. This will involve making essential changes from the CHEC design to improve manufacturability, operation, maintenance and reliability, maximise performance, and cut costs. The research work to be undertaken in 2020 includes: mechanical design modifications, improvements to cooling, a new window and lid design, updated sensors, reduced power consumption, and improvements to electronics components. We will also prepare AIV facilities in preparation for production. This will be supported by ongoing camera software and simulations development. 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.

CTA will have an impact on a wide range of scientific questions, from the nature of gravity to how supernovae accelerate particles and how active galaxies work. In doing so, it will use new and innovative methodologies, combining techniques from both astronomy and particle physics. In the process, many highly skilled researchers will be trained. UK industry stands to gain both from knowledge transfer and by way of contracts for producing electronics, camera housings, and perhaps mirrors and other structural elements. Much of this work will be undertaken in areas of economic deprivation (e.g. North East England, North Wales) thereby contributing to regeneration and economic development.

The atmosphere is an important element of our detectors, and our Monte Carlo simulations already use data from the British Atmospheric Data Centre. Wherever the telescopes are sited, it is likely to be in an area short of detailed weather data for input to climate models. CTA's weather data are therefore likely to be useful to atmospheric modellers.

The public awareness and understanding of science will be enhanced by CTA. Our scientific research covers topics of considerable interest to the public, including black holes, supernova explosions and Dark Matter. We have already had considerable engagement with the public through events such as Stargazing Live and the Royal Society Summer Science Exhibition, as well as giving many talks to local interest groups (Women's Institutes, astronomical societies etc.) and schools. These activities will be continued and enhanced during the next few years as CTA matures as an observatory. These activities will be significantly strengthened by the recent addition of the Armagh Observatory and Planetarium to the CTA-UK consortium.

Early stage researchers will gain a wide range of skills from working on CTA, in areas such as programming, electronics, modelling of complex systems, image analysis etc. In addition, they will gain the "soft skills" which come from working with people from a wide range of cultural backgrounds in the international CTA Consortium. These skills will fit them for non-academic professions. For example, recent Ph.D. graduates who have worked in CTA have already gone on to careers in teaching and software engineering and we expect many more skilled people to move into non-academic careers over the 30-year lifetime of CTA.