Unveiling the high energy Milky WayIn a recent publication in Science Magazine, the H.E.S.S. team of international astrophysicists reported results of a first sensitive survey of the central part of our galaxy in very high energy gamma-rays. This survey reveals eight new sources of very high energy gamma-rays in the disk of our Galaxy, thereby essentially doubling the number of known at these energies. The results have pushed astronomy into a previously unknown domain, extending our knowledge of the Milky Way in a novel wavelength regime thereby opening a new window on our galaxy. Gamma-rays are produced in extreme cosmic particle accelerators such as supernova explosions and provide a unique view of the high energy processes at work in the Milky Way.
The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes is currently the most sensitive detector of very high energy gamma-rays, radiation a million million times more energetic than the visible light. These high energy gamma rays are quite rare – even for relatively strong sources, only about one gamma ray per month hits a square meter at the top of the earth's atmosphere. However, since they are absorbed in the atmosphere, a direct detection of a significant number of the rare gamma rays would require a satellite of huge size. The H.E.S.S. telescope employ a trick – they use the atmosphere as detector medium. When gamma rays are absorbed in the air, they emit short flashes of blue light, named Cherenkov light, lasting a few billionths of a second. This light is collected by the H.E.S.S. telescopes with big mirrors and extremely sensitive cameras and can be used to create images of astronomical objects as they appear in gamma-rays.
The H.E.S.S. telescopes represent a multi-year construction effort by an international team of more than 100 scientists and engineers from Germany, France, the UK, Ireland, the Czech Republic, Armenia, South Africa and the host country Namibia. The instrument was inaugurated in September 2004 by the Namibian Prime Minister, Theo-Ben Guirab, and its first data have already resulted in a number of important discoveries, including the first astronomical image of a supernova shock wave at the highest gamma-ray energies.
Particularly stunning is that many of these new sources discovered by H.E.S.S. have no obvious counterparts in more conventional wavelength bands such as optical and X-ray astronomy. The discovery of very high energy gamma-rays from such sources suggest that they may be `dark accelerators', as Stefan Funk from the Max-Planck Institut in Heidelberg affirms: "These objects seems to only emit radiation in the highest energy bands". Indeed, the new sources tend to show `hard' photon spectra, in contrast to most previously known emitters of very-high-energy gamma rays, where spectra decrease more rapidly with increasing energy.
Another important discovery is that the new sources appear with a typical size of the order of a tenth of a degree; the H.E.S.S. instrument for the first time provides sufficient resolution and sensitivity to see such structures. Since the objects cluster within a fraction of a degree from the plane of our Galaxy, they are most likely located at a significant distance – several 1000 light years from the sun – which implies that these cosmic particle accelerators extend over a size of light years.
Notes for Editors
The H.E.S.S. array
Dr. Jim Hinton
Sources: Optical image: A. Mellinger, Infrared image: S.L. Wheelock, et al. 1994, IRAS Sky Survey Atlas Explanatory Supplement, JPL Publication 94-11(High Resolution)