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Survey Finds Mysterious ‘Dark Accelerators’

The central part of the Milky Way has never been surveyed in gamma ray wavelengths with the sensitivity offered by HESS, the High Energy Stereoscopic System. And as announced in the March 25th issue of Science, the HESS team has not only found eight new very high energy (VHE) gamma ray sources in the galactic disk, thus doubling the number of known sources, but has also discovered two ‘dark accelerators,’ objects that emit energetic particles but have no known optical or x-ray counterpart.

It takes a particle accelerator of cosmic proportions to produce gamma rays, such as the explosion of a supernova. But such sources should be visible in other wavelengths. Says Dr. Paula Chadwick of the University of Durham (UK):

“Many of the new objects seem to be known categories of sources, such as supernova remnants and pulsar wind nebulae. Data on these objects will help us to understand particle acceleration in our galaxy in more detail; but finding these ‘dark accelerators’ was a surprise. With no counterpart at other wavelengths, they are, for the moment, a complete mystery.”

From a press release from the Particle Physics and Astronomy Research Council:

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.

The HESS array is located in Namibia, a system of four 13-meter telescopes that comprises the most sensitive detector of VHE gamma rays in the world. Because gamma rays are absorbed by the atmosphere, the HESS detectors work indirectly, collecting the blue ‘Cherenkov light’ the rays emit upon being absorbed. HESS uses this information to construct images of astronomical objects as they appear at these wavelengths.

The paper on the HESS results is Aharonian et al., “A New Population of Very High Energy Gamma-Ray Sources in the Milky Way,” Science 307: 1938-1942 (2005).

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