ROSAT Detection of an X-ray Shadow in the 1/4-keV Diffuse Background in the Draco Nebula

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Science  14 Jun 1991:
Vol. 252, Issue 5012, pp. 1529-1532
DOI: 10.1126/science.252.5012.1529


The detection by the Roentgen satellite (ROSAT) x-ray telescope of a shadow in the 1/4—kiloelectron volt (C band, 0.1 to 0.284 kiloelectron volt) cosmic diffuse background is reported. The location and morphology of the local minimum in x-rays are in clear agreement with a discrete H I cloud. The shadow is very deep with a minimum level at 50 percent of the surrounding emission; therefore, a minimum of 50 percent of the observed off-cloud flux must originate on the far side of the cloud. The analysis of H I velocity components links the cloud with the Draco nebula (distance ∼600 parsecs); it then follows that there is significant 1/4—kiloelectron volt x-ray emission at a large distance (>400 parsecs) from the galactic plane along this line of sight. The extent of the distant emission region is uncertain, and, if it indicates the existence of a hot galactic corona, it must be patchy in nature.