Reflected X-ray Emissions on Giant Molecular Clouds ? Evidence of the Past Activities of Sgr A*

Abstract

We have found strong 6.4-keV line emissions from the giant molecular clouds in the Galactic center region: Sgr B2, Sgr C, and M0.11–0.08 (at the Radio Arc region). The high angular resolution of Chandra reveals that the 6.4-keV line emissions are indeed coincident with the clouds, and shifted towards the Galactic center. The X-ray spectra have very strong 6.4-keV lines with equivalent widths ≥1 keV and are attenuated by larger column densities of interstellar gas. These characteristics imply that the massive molecular clouds are irradiated by an external X-ray source in the direction of the Galactic center and emit fluorescent and scattered X-rays. These clouds are new category of X-ray source: “X-ray Reflection Nebula”. The reflected X-ray flux reveals the recent luminosity history of the primary irradiating source, which may be the massive black hole Sgr A*, according to the light travel time to each cloud. Making use of the radio determinations of the cloud masses, we find that Sgr A* was as luminous as 1039 erg s−1 a few hundred years ago, and has gradually decreased to present value.