Gamma astronomy

Gamma astronomy or gamma-ray astronomy is the exploration of space using gamma telescopes . Due to the much higher energy range of the gamma quanta (> 500 keV ) compared to visible light (~ 1 eV) and thus also the z. Sometimes completely different causes, the gamma astronomy allows insights into new phenomena in the universe , especially huge explosions and collisions of stars and other celestial bodies. Gamma astronomy thus opened a window into completely different areas of astronomy .

Basics

Space-based gamma astronomy

This branch of astronomy is still relatively young, as it is not possible on earth to collect gamma rays from space , as these are absorbed by the earth's atmosphere . Scientists who want to study gamma ray sources in space therefore have to rely on appropriate observatories that orbit the earth on satellites . However, it is also not possible outside the earth's atmosphere to observe gamma ray sources like in visible light by means of a lens or mirror telescope , since these high-energy rays are not refracted by lenses and are not reflected by mirrors. Scintillation counters that are sandwiched one on top of the other are used, in which flashes of light are generated when a gamma photon passes through a certain material: The flashes of light are measured by semiconductor photomultipliers , and their track through the detector stack enables a rough estimate of the direction of the incident gamma photon to within a few degrees .

Gamma astronomy on the ground

MAGIC, Cherenkov telescope on La Palma; Image: MAGIC collaboration

With Cherenkov imaging telescopes, it has been possible since the beginning of the 2000s to observe gamma rays indirectly from the earth by observing the interaction of cosmic gamma rays with the earth's atmosphere . When the gamma photons collide with the molecules of the high atmosphere, secondary particle showers arise , which in turn emit Cherenkov light when they fly through the atmosphere . The resulting conical flash of light in the direction of flight of the particles (i.e. towards the ground) can be measured with Cherenkov telescopes.

history

Beginnings

Even if it was already suspected in the 1940s and 1950s that there might be gamma rays in space, it was only the Explorer 11 satellite (launched on April 27, 1961), which was built for this purpose, that was able to detect gamma rays. During his 4 month long mission he discovered 22 gamma ray events.

Gamma satellites

This was the first of a series of satellites that from now on regularly observe gamma rays in orbit :

• OSO-3 discovered sources of gamma rays along our galaxy , the Milky Way , concentrated around the halo in 1967 .
• The Vela satellites , actually American spy satellites that nuclear weapons tests should track, discovered the so-called between July 1969-April 1979 for the first time gamma-ray bursts .
• SAS-2 ( NASA ) and COS-B ( ESA ) were able to provide detailed maps of the gamma spectrum in space for the first time in the 1970s .
• CGRO , a 17-ton satellite of superlatives, delivered enormous amounts of data about gamma-ray sources in the 1990s and enormously expanded our knowledge in this area. However, it had to be brought down in 2000.
• INTEGRAL , a satellite with an even more precise resolution that ESA put into orbit on October 17, 2002.
• Fermi Gamma-ray Space Telescope , a wide-angle gamma-ray space telescope (formerly Gamma-ray Large Area Space Telescope , GLAST) was launched into orbit on June 11, 2008.

Gamma telescopes on the ground

Two telescopes of the HESS telescope array

In the case of the earth-based observation of gamma rays, after a series of smaller test projects, two groundbreaking projects that are currently in operation should be mentioned:

• HESS (High Energy Stereoscopic System) in Namibia , which consists of 4 individual telescopes with a diameter of 13 meters each and a large telescope with 614 m² mirror surface in the middle of the square array. The mirrors of the individual telescopes consist of 400 round (60 cm diameter) or 875 hexagonal (90 cm edge-to-edge) segments.
• MAGIC (Major Atmospheric Gamma-Ray Imaging Cherenkov Telescope) on La Palma , Canary Islands . The telescope has a 17 meter segment mirror made of 1000 individual aluminum plates and, due to its mobility, can be used in particular to observe short-lived gamma-ray bursts. It is the successor to the HEGRA Atmospheric Cherenkov Telescope System in the same place.

Research objects of gamma astronomy

Due to the already mentioned high energy of gamma radiation (over 10 ⁵  eV compared to light with ~ 1.5… 3 eV) the mechanisms of formation of this radiation must be completely different from those of light. The majority of these are dramatic explosions and collisions in space:

• Gamma-ray flashes that last for a few seconds and during this time outshine all other gamma sources in the universe. According to the latest theories, they are in most cases a special form of the supernova explosion of a massive star at the end of its life.
• Remnants of supernova explosions, such as neutron stars and black holes , also emit gamma radiation when matter is captured (in black holes this radiation is also called the death scream of matter , since it is the last thing you see of it).
• Shock waves in the scuffed gas envelopes of stellar explosions that occur when the with nearly the speed of light expanding gas collides with slower gas.
• Hot gas clouds that are constantly stimulated by various processes. E.g. intergalactic gas in galaxy clusters .
• Active galaxies , i.e. galaxies in which a lot of energy is converted. This includes starburst galaxies (extremely high star formation rate , mainly caused by galaxy collisions), active galactic nuclei (the central area and especially the central black hole are extremely active, i.e. a quasar ).
• Finally, one looks for traces of the annihilation (pairwise annihilation) of dark matter in order to be able to directly detect this mysterious matter, the effects of which can be seen indirectly through its gravitational effect in the universe, and to find out which particles it consists of.

The highest photon energy of 16 TeV observed so far  , observed with the HEGRA telescope, had its source in the Blazar Markarjan 501 .

Gamma radiation sources found by HESS (assembly)

See also

Astronomy , satellite , telescope , gamma radiation , Cherenkov radiation , X-ray astronomy

literature

• Felix A. Aharonian: High energy gamma-ray astronomy. American Inst. Of Physics, Melville 2009, ISBN 978-0-7354-0616-2
• Poolla V. Ramana Murthy, AW Wolfendale: Gamma-ray astronomy. Cambridge Univ. Press, Cambridge 1993, ISBN 0-521-42081-4
• Johannes A. Bleeker, W. Hermsen: X-ray and gamma-ray astronomy. Pergamon Pr. Oxford 1989, ISBN 0-08-040158-9

Web links

• Further information on gamma astronomy ( Memento from March 4, 2012 in the Internet Archive )
• Max Planck Institute for Nuclear Physics - The HEGRA Atmospheric Cherenkov Telescope System
• The MAGIC Telescope Project
• Max Planck Institute for Nuclear Physics - The HESS Project
• Gamma astronomy at the Max Planck Institute for Extraterrestrial Physics, Garching
• The History of Gamma-ray Astronomy NASA Goddard Space Flight Center (accessed June 18, 2011)