Galaxies that resemble enormous whirlpools, or intergalactic clouds of dust shaped like a horse’s head – the field of astronomy is known for spectacular images. But there are also telescopes that detect light outside the visible spectrum, such as gamma rays. Gamma radiation is produced during cosmic events of tremendous violence, such as supernova explosions. The planned Cherenkov Telescope Array (CTA) gamma observatory will detect this high-energy radiation with unprecedented sensitivity. DESY is a major contributor to this large-scale international project.
Gamma rays are the most energetic electromagnetic waves. A gamma quantum can contain many trillions of times more energy than a quantum of visible light. The processes during which this radiation is created in the universe are correspondingly violent. For instance, gamma rays are produced when matter particles are accelerated to incredible energies in the vicinity of black holes, or when a neutron star rotates about its axis at mind-boggling speed. Special-purpose telescopes that detect gamma radiation can provide valuable details about these distant, formidable events.
Gamma telescope of the future
Gamma telescopes work as follows: When high-energy gamma rays strike the atmosphere of the Earth, they trigger a cascade of particles, which, in turn, generates characteristic flashes of blue light. Huge mirror systems focus this atmospheric light onto very fast cameras. The data can then be used to reconstruct where a gamma quantum came from and how much energy it contained.
Today’s gamma telescopes consist of at most five individual telescopes, but CTA is designed to have approximately 100. To keep the entire sky in view, two locations are planned: a smaller one in the northern hemisphere with 20 to 30 telescopes distributed on one square kilometre, and a larger one in the southern hemisphere with 70 to 100 telescopes on about ten square kilometres. The CTA team includes 1000 experts from 25 countries – a global consortium. DESY makes up the biggest group in this international project. Among other tasks, the DESY physicists are responsible for the design and construction of the medium-sized telescopes with a mirror diameter of around 12 metres, and they are playing a leading role in the development of the telescope control system.
The sky in the light of gamma radiation
The universe is full of natural particle accelerators, such as supernova explosions, binary star systems or active galactic nuclei. The CTA gamma-ray telescope will observe thousands of these cosmic accelerators with unprecedented sensitivity. This will allow researchers to decode what mechanisms in cosmic sources millions of light years away are capable of accelerating particles to create such high-energy light. CTA would make it possible to study the spatial structure and temporal changes of a large number of sources in detail and so obtain a complete astronomical picture over the entire electromagnetic spectrum.
Enigmatic “dark” gamma-ray sources that cannot be seen in any other spectral region could offer the researchers valuable information on the origin of cosmic rays. In addition, CTA will search for signs of dark matter and perhaps also help us to better understand the nature of the mysterious dark energy in the cosmos.