Two sources of diffuse X-ray emission from the Galactic Centre

Abstract

THE weak extended X-ray source at the Galactic Centre has so far lacked a reasonable explanation. Measurements of this roughly elliptical source from the Ginga satellite revealed¹ emission in the 6.7-keV line of ionized iron, indicating that the X-rays originate in an optically thin plasma. But to account for the hard X-ray spectrum, this plasma needs to be very hot — too hot, in fact, to be confined by the gravitational potential of the Galactic Centre². We have recently shown³ that the morphology of the emitting region changes at energies above 11 keV: the source becomes extended in the galactic plane, resembling the distribution of the molecular gas clouds in this region. Here we report the detection of a pronounced absorption feature in the emission spectrum in the energy range 8–11 keV. This result, combined with the changing 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.