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

Murakami, H.1, Senda, A.2, Maeda, Y.1, and Koyama, K.2

1 ISAS, Sagamihara, Kanagawa, Japan
2 Kyoto University, Sakyo-ku, Kyoto, Japan

E-mail contact: hiro@astro.isas.ac.jp

See: ApJ, 558, 687 (2001); ApJ, 550, 297 (2001)

We 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 distribution of the 6.4-keV line emissions are indeed coincident with clouds, and shifted towards the Galactic center. X-ray spectra exhibit very strong 6.4-keV line with the equivalent width of  1 keV, and larger column densities than interstellar absorption. These features imply that heavy molecular clouds are irradiated by an external X-ray source in the direction of the Galactic center and emit fluorescent and scattered X-rays. Thus we conclude that these clouds are new category of X-ray source: ``X-ray Reflection Nebula''.

These reflected X-ray flux show the past luminosities of the primary source, which would be the massive black hole Sgr A*, corresponding to the light traveling time to each cloud. Combined with the radio observations of the cloud mass, we revealed that Sgr A* had been as luminous as 1039 erg s-1 - 2000 years ago, and has gradually decreased to present value.