2 SOURCES OF DIFFUSE-X-RAY EMISSION FROM THE GALACTIC-CENTER

THE weak extended X-ray source at the Galactic Centre has so far lacke d a reasonable explanation. Measurements of this roughly elliptical so urce from the Ginga satellite revealed1 emission in the 6.7-keV line o f ionized iron, indicating that the X-rays originate in an optically t hin plasma. But to account for the hard X-ray spectrum, this plasma ne eds to be very hot - too hot, in fact, to be confined by the gravitati onal potential of the Galactic Centre2. We have recently shown3 that t he morphology of the emitting region changes at energies above 11 keV: the source becomes extended in the galactic plane, resembling the dis tribution of the molecular gas clouds in this region. Here we report t he detection of a pronounced absorption feature in the emission spectr um in the energy range 8-11 keV. This result, combined with the changi ng spatial distribution, suggests that the high-energy emissions arise from the scattering of X-rays from the nearby compact sources by the dense molecular clouds. As no comparable absorption feature is seen at lower energies, the softer X-ray emissions may still be understood in terms of thermal emission from a plasma, but the required temperature is no longer unreasonably high.