Draco Shadow
Credit: D. Burrows/PSU

Silhouettes on the X-ray Sky

The entire sky is lit by X-ray emission. Some of this emission, known as the soft X-ray background, is believed to be produced by 1 million degree gas that has been heated by supernova explosions at some time in the distant past (perhaps as much as a million years ago). In order to determine the origin of this emission, astronomers need to know how far away it is. One way to measure the distance is to look for the silhouette of a nearby object on the X-ray background; if an object is between us and the source of the X-ray background, the object will block some of the background X-ray emission. Finding this type of "X-ray shadow" would provide at least a lower limit to the distance of the hot gas producing the X-ray background. However, more that 20 years of X-ray observations failed to find any such shadows, leaving the origin of the soft X-ray background a mystery.

This situation changed after the launch of the ROSAT X-ray observatory. ROSAT's exquisite X-ray optics and sensitivity enabled it to detect X-ray silhouettes which were undetectable before. The image above shows the first X-ray silhouette ever detected. This picture is a false-color image showing the X-ray shadow (the blue-black region) of an interstellar cloud (called the Draco nebula) on the soft X-ray background. The boundaries of the Draco nebula are indicated by the contour lines. This clearly shows that the X-ray brightness is lower at the location of the cloud, and thus that the cloud is blocking some of the X-ray emission. These data were obtained by Penn State scientists Dave Burrows & Jeff Mendenhall, (Nature, Vol. 351, p. 629, 1991) and provide the first direct observational evidence for the existence of million degree gas in the halo of the Milky Way. For more information about silhouettes on the X-ray background, see Dave Burrows' catalog of X-ray shadow images.

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Page Author: Dr. Michael F. Corcoran
Last modified May 11, 2000