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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Cosmos

γ-Astronomy

Hess Orte

Gamma astronomy, also called gamma-ray astronomy, explores the cosmos using gamma-ray detectors.  Gamma-rays are photons with energies higher than X-ray radiation.  By investigating this high energy radiation it is possible to explore a wide range of violent and extreme phenomena in the universe (explosions of stars, excitation of nuclei, ultra-relativistic particle populations) which are not accessible in other bands (e. g. in the optical).

Gamma-rays can only be detected directly by satellites because gamma-ray radiation is absorbed by the Earth's atmosphere.  A direct detection of these photons is possible by using scintillation counters or semiconductors.  A second technique consists of converting the gamma-ray radiation into electron-positron pairs and measuring the secondary particles in a spectrometer.  Above energies of several 10 GeV gamma-rays can only be measured indirectly on the ground. Different technologies have been developed to measure very high energy gamma-rays in ground based installations, but the most successful approach is the one used by so-called Imaging Atmospheric Cherenkov telescopes.  When gamma-ray quanta hit the atmosphere, due to interactions with the molecules in the air, electromagnetic showers are produced consisting of electrons, positrons and further photons.  These showers reach their highest density at a height of about 10 km above the ground.  Whilst flying through the atmosphere, the charged secondary particles produce Cherenkov light which can be detected by Cherenkov telescopes and imaged into a camera composed of photo-detectors (usually photo-multipliers).  The position and shape of the picture in the camera and the total light content of the picture indicate the direction as well as the energy of the primary gamma-ray quantum which had caused the particle shower.

Gamma-ray astronomy with imaging atmospheric Cherenkov telescopes has rapidly advanced within the last 10 years. With the current experiments (H.E.S.S. in Namibia or MAGIC in La Palma) the number of known gamma-ray sources of energies higher than 100 GeV has increased from 10 to more than 100, and different categories of sources (supernova remnants, pulsars, pulsar wind nebulae, gamma-ray binary systems, active galaxies) have been discovered.  The research topics of gamma-ray astronomy focusses on a wide range of different topics, from the astrophysics of radiation sources (like supernova remnants as accelerator of cosmic radiation) and cosmology to questions of fundamental physics (the search for Dark Matter and the Lorentz Invariance violation).  At present, a new next generation gamma-ray observatory is in preparation (Cherenkov Telescope Array - CTA) which is again expected to multiply the number of known sources by ten.


Cosmos | Supernovae | Pulsars | Galaxies | Cosmic Rays | γ-Astronomy