Particle acceleration in our galaxy studied with H.E.S.S.

Most of our knowledge of the universe comes from the observation of electromagnetic radiation from heavenly objects - starlight is the most obvious example of this radiation. Images generated by modern optical telescopes (for example the Hubble Space Telescope) contain a wealth of information for astronomers. However, this visible starlight is only a tiny fraction of the spectrum of radiation incident on the Earth. Modern astrophysics explores the enormous range of frequencies from radio waves to gamma-rays, with each frequency band providing unique information about our own and distant galaxies. The highest frequency (and hence highest energy) radiation observed so far is very-high-energy (VHE) gamma-radiation. This part of the electromagnetic spectrum has been opened-up to observations for only the last ten years (compared to several hundreds of years for optical astronomy) but is already providing us with unique information on some of the most exciting objects in the sky. A single VHE gamma-ray possesses as much energy as one trillion photons of normal visible light. Such extreme frequencies of light can only be generated by very high energy particles - such as electrons or protons. We know from experiments on Earth that to accelerate particles to such energies is very difficult - it requires huge machines such as the LEP collider at CERN which cost billions of pounds. How such particles are produced in celestial objects is still a mystery. We known that our galaxy is filled with so-called cosmic rays - energetic (indeed highly relativistic) protons and nuclei, but their origin is as yet unknown. It is to investigate the origin of this cosmic radiation, and improve our understanding of the high energy universe, that several new gamma-ray observatories are now being built. The recently completed High Energy Stereoscopic System - H.E.S.S. - (located in the Khomas Highlands of Namibia) uses cutting-edge technology to make the best observations so far of the sky in very high energy gamma-rays. Using this new observatory I hope to study some of the most violent places in our galaxy - places where particles are accelerated to huge energies. Observations of such extreme places teach us not only about the objects themselves but also provide an insight into the laws of nature under extreme conditions. Normal stars cannot produce gamma-rays, but stars that are massive enough can explode at the end of their lives as supernovae. It seems likely that it is in such supernovae that most cosmic rays are accelerated, but even after decades of research, we still do not know for sure. Using H.E.S.S. we hope to remedy this situation. Supernova explosions leave behind the dead core of the star that exploded, either as a super-dense neutron star or a black hole. Fast rotating neutron stars (or pulsars) can have a 'day' that is only 1/100 of a second long and produce a 'wind' of high energy particles. We want to use the H.E.S.S. telescopes to better understand such objects as well. So far the sky in gamma-rays is a bit like the sky in visible light seen from a big city - only a few bright objects are visible. In the next few years this picture will change as more and more sources of gamma-rays are discovered and the window on the high energy universe is truly opened by the new generation of telescopes.