Prof. Dr. Christopher van Eldik
ECAP, Physikalisches Institut
|phone||+49 9131 85-27062|
|fax||+49 9131 85-28774|
Gamma-ray astronomy with H.E.S.S. and CTA
I am working with the H.E.S.S. instrument, the currently leading experiment for very-high-energy gamma-ray astronomy. H.E.S.S. is an array of five imaging atmospheric Cherenkov telescopes and is located in the Khomas Highlands of Namibia. H.E.S.S. detects gamma-rays with an energy of larger than 50 GeV: although the atmosphere of the earth is opaque to photons of such high energy, they can be detected from ground because when absorbed in the atmosphere gamma-rays produce Cherenkov light. With 100 square-metre mirrors on the four small H.E.S.S. telescopes and the 600 square-metre mirror of the large H.E.S.S. telescope, this Cherenkov light is collected and recorded by very sensitive cameras.
Gamma-ray astronomy explores the sky with photons of energies at least a million times higher than the energy of visible light. As opposed to the photons of visible light, which are mainly emitted by the hot surfaces of stars, photons of gamma-ray energies cannot be produced by thermal processes. There is simply no celestial body hot enough to emit photons of gamma-ray energies. Instead, acceleration processes are believed to be taking place in some of the most violent places of the universe, such as in active galactic nuclei or remnants of star explosions. Charged particles like electrons, positrons, protons or atomic nuclei are believed to be accelerated in such sources to very high energies.
Gamma-ray astronomy is a most valuable tool to make the sites of acceleration visible and to study the physics processes that take place in these sources. On their way to earth charged particles get significantly deflected by the galactic magnetic field, and therefore do not point back to their production origin. Instead, neutral gamma-rays, produced in interactions of the accelerated particles with magnetic fields, photons, or intergalactic gas, travel on a straight path to earth. Therefore, their direction of origin points back to where they got produced, which is often close to the accelerators of the charged particles.
The Cherenkov Telescope Array CTA will become the successor of the currently operating ground-based gamma-ray instruments. With about 150 telescopes, it will allow for a 10 times better sensitivity to gamma rays as H.E.S.S. at TeV energies, while extending the energy coverage to a range starting from several 10 GeV to more than 100 TeV.