Exploring the Universe with gamma rays
The last decade has witnessed the birth of a new field of astronomy – Very High Energy (VHE) gamma ray astronomy – expanding wavelength coverage of astronomical instruments by another 10 octaves towards the highest energy radiation. These gamma rays are produced when high energy cosmic rays bump into interstellar gas, creating a bunch of elementary particles. Unlike charged cosmic rays, the gamma rays travel on a straight path and point back to the point in the sky where they were produced. Apart from serving as tracers of cosmic rays, speculation is that some VHE 6.2 Exploring the Universe with gamma rays gamma rays may result from decays of relic particles with have survived since the Big Bang, such as the mysterious dark matter particles; detection of such gamma rays would give first hints towards the nature of dark matter.
Very High Energy gamma rays are absorbed in the Earth’s atmosphere, creating a cascade of secondary elementary particles, most of which never reach the ground. Satellite instruments such as AGILE and Fermi (the former GLAST), now in orbit, detect gamma-rays before they enter the atmosphere, but their size is too small to capture enough of the highest-energy gamma rays.
After long development, a ground-based detection technique pioneered by the American Whipple telescope and perfected by the European-led H.E.S.S. and MAGIC instruments has brought a break-through: Imaging Atmospheric Cherenkov telescopes. These telescopes collect and image the bluish light emitted by the particle cascades created by a VHE gamma ray in the atmosphere. Light from a single VHE gamma ray illuminates a “light pool” of about 150 m radius on the ground, hence a single telescope will detect gamma rays incident upon an area of a few 10000 m2, compared to the sub-m2 area of satellite detectors. Latest generation Cherenkov telescope systems use multiple telescopes to provide stereoscopic viewing of gamma-ray induced particle cascades, for improved determination of impact direction and energy of a gamma-ray.
VHE gamma-ray astronomy is becoming part of mainstream astronomy, with surveys of the Galaxy revealing dozens of VHE gamma-ray emitting cosmic-ray accelerators. Objects discovered include supernova remnants, binary systems, pulsars, stellar associations and different species of active galaxies, hosting super-massive black holes at their centres.