The DESY scientists also conduct research in astroparticle physics. Various kinds of particle from the cosmos constantly reach the Earth – particles that can provide insights into events happening in the depths of the universe. The DESY researchers use two of these cosmic messengers, neutrinos and high-energy gamma rays, to uncover the secrets of stellar explosions, cosmic particle accelerators – such as the surroundings of black holes – or dark matter.
The gamma-ray telescope CTA
The complete map of the high-energy heavens will not be revealed until we make consistent use of all the information supplied by the cosmos. The possible messengers from the universe include electromagnetic radiation – referred to as gamma radiation at high energies – and charged cosmic particles, neutrinos and, in future, perhaps also gravitational waves. In line with the principle of multi-messenger astronomy, the DESY researchers are relying on several of these heavenly messengers.
The first image of a supernova as seen in the light of high-energy gamma radiation, recorded by the gamma-ray telescope H.E.S.S. The colour scale specifies the intensity of the gamma radiation, the lines indicate the intensity of the X-ray light. The image proves that the ring-shaped shock waves of such supernovae can act as cosmic particle accelerators.
For example, a young scientists’ group in Zeuthen is not only participating in the neutrino telescope IceCube, but also in the gamma radiation project MAGIC on the island of La Palma in the Canaries. Gamma-ray telescopes register the characteristic light from particle showers triggered by high-energy cosmic gamma radiation in the Earth’s atmosphere. The telescopes consist of huge systems of mirrors that focus the atmospheric light from these air showers onto fast cameras capable of resolving events on the scale of billionths of a second. This enables the direction of the shower – and thus the direction of origin of the gamma ray that caused it – to be determined. Telescopes of this type are built on high mountains, as far as possible from sources of light that would interfere with the results, for example on La Palma in the Atlantic Ocean (MAGIC) or in Namibia (H.E.S.S.).
The sky in the light of gamma radiation
In recent years, the gamma-ray telescopes have opened up previously unimagined insights into the far reaches of the universe. Most of the gamma-ray sources discovered so far coincide with known objects, which are also visible in other wavelength regions. In this way, it was first possible to demonstrate that stellar supernova explosions really do act as cosmic accelerators, boosting particles to high energies in their shock waves. The gigantic magnetic and electric fields of pulsars – fast-rotating neutron stars – are also obviously cosmic particle accelerators, as are the regions around the black holes at the heart of active galaxies.
The researchers also have discovered a series of “dark” gamma-ray sources which have not been observed to date in any other spectral region. In particular, these sources emit neither X-rays nor radio waves, both of which arise when electrons are accelerated to high energies. It is possible that these sources represent a previously unknown type of heavenly body which only accelerates protons. As protons and nuclei make up 99 per cent of the charged cosmic radiation which constantly bombards the Earth, these mysterious gamma-ray sources could offer the researchers valuable information on the origin of cosmic rays.
Next-generation gamma-ray telescopes
The sources published so far on the gamma-ray map of the sky are probably only the tip of the iceberg. Totally new phenomena could be discovered here, using telescopes with ten times the sensitivity of today’s facilities. With such instruments, the researchers could also decode what mechanism in cosmic sources millions of light years away is capable of accelerating particles to create such high-energy light. Telescopes like these 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.
To achieve a tenfold increase in gamma-ray sensitivity over a wide energy range requires more than 50 telescopes with diameters of between six and 25 metres arranged over an area of at least one square kilometre. An observatory of this type is currently in preparation. Starting in 2012, the gamma-ray telescope CTA (Cherenkov Telescope Array) is to be constructed by an international consortium in order to look for cosmic high-energy accelerators with previously unavailable sensitivity. CTA will be able to record around 1000 sources and thus raise the field of gamma-ray astronomy to the level of astronomy with radio waves or X-rays. What’s more, CTA will also search for signs of dark matter and perhaps also help us to better understand the nature of the mysterious dark energy in the cosmos. DESY physicists are participating in optimization calculations, the design of the gigantic reflecting telescopes and the conception of an operations and data centre as part of the prototype study for CTA.