We present Chandra observations of giant molecular clouds in the Galactic center region. We observed three molecular clouds of Sgr B2, Sgr C, and M0.11-0.08 (at the Radio Arc region). All clouds emit a strong 6.4-keV line with the equivalent width of >= 1 keV, and exhibit large column densities. The distributions of the 6.4-keV line emissions are coincident with the clouds. 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. These clouds are a new category of X-ray source: ``X-ray Reflection Nebula''.X-ray reflection nebulae can be probes for investigating the past activities of our Galactic center. The observed fluorescent and scattered X-rays from the molecular clouds reflect the past luminosity of the primary source, which would be the massive black hole Sgr A*. It was as luminous as 1039 erg s-1 a few hundreds years ago. Then the luminosity decreased gradually to the present value.
8th Asian-Pacific Regional Meeting, Volume II
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