Penn State Dept. of Astronomy & Astrophysics
Image Catalog

Soft X-ray Shadows 

(by David Burrows, Gordon Garmire

 Soft X-ray shadows provide unique information on the distribution in space of the emission sources responsible for the soft X-ray diffuse background. Although this diffuse background was discovered nearly 25 years ago, it is only within the past few years that definitive observational evidence for the locations of emitting regions has been obtained by the Position Sensitive Proportional Counter on the US/German ROSAT satellite (shown in upper right). Penn State is one of the centers for analysis of these data.  Several examples are available for display.

Soft X-ray shadows are the X-ray analog of optical dark nebulae such as the Horsehead nebula. In optical light these are produced by light from a bright nebula being absorbed in an intervening dark cloud. In X-rays, the background source is typically the diffuse X-ray background (although shadows are also seen against the bright X-ray emission from old supernova remnants and stellar wind superbubbles). Unlike the optical night sky, the X-ray sky is bright from X-rays produced by hot gas in our Galaxy and from the combined emission from distant extragalactic sources such as active galactic nuclei. In fact, the X-ray sky somewhat resembles the night sky as seen from the downtown of a large city, where light from street lamps scattered in the atmosphere produces a bright haze that obscures all but the brightest stars. The X-ray group at Penn State is one of the leading groups around the world studying this diffuse background to learn more about its origins and what it can tell us about hot gas in our galaxy and X-ray sources at cosmological distances. Soft X-ray shadows have become an important tool in this investigation, because they provide the only way of isolating the X-ray emission in distance. Observation of X-ray absorption by a cloud at known distance permits a measurement of the amount of X-ray emission coming from behind the cloud. These measurements have permitted the first unambiguous observational proof of the existence of million degree gas in the galactic halo, and are providing important clues to the distribution of hot gas in the galactic disk as well.

Studies of soft X-ray shadows are being pursued by Evan Pugh Professor Gordon Garmire and by Dr. David Burrows. Several graduate students are also involved in this work.

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For more information on X-ray shadows research, contact David Burrows ( ) .

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Last update:  December 16, 1997