We present the ASCA and Chandra observations of diffuse X-ray structures in our Galactic center region. We concentrate on giant molecular clouds, supernova remnants, and other faint structures which are newly discovered with Chandra.
We observed four giant molecular clouds: Sgr B2, Sgr C, M0.11-0.08 (at the Radio Arc region), and Clump 2. All clouds emit strong 6.4-keV line with the equivalent width of >1 keV, and exhibit larger column densities than interstellar absorption. The distributions of the 6.4-keV line are coincident with molecular clouds, and those of Sgr B2 and Sgr C are shifted to the Galactic Center side, in particular. 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. We call this new category of X-ray source "X-ray Reflection Nebula".
Numerical simulations of X-ray reflection nebula model successfully reproduce the observed image and spectrum when the mass of the cloud is smaller than 0.3 times of the mass estimated by the radio observations of molecular lines. This result supports that the molecular clouds in the Galactic center are in pressure equilibrium, not in gravity equilibrium. The abundances of the clouds are also estimated by numerical simulations. The iron abundance and other lighter elements of the Sgr~B2 cloud are estimated to be 1.5-3.0, and 2.0-5.0, respectively in case of cloud mass is 0.2 times of the radio estimated mass. The abundances of other clouds are not constrained in significant region. The lower limit of the iron abundance is 1.0 for all of the three clouds. The abundance of other lighter elements are constrained to be < 1 for M0.11-0.08, and < 3 for Sgr C and Clump 2.
The X-ray reflection nebula model are used to investigate 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 10^39 erg s^-1 about 300--2000 years ago. Then the luminosity decrease gradually to present value. This time variability agrees with the implication obtained by the Chandra observation of Sgr A East. Sgr A East is the 10^4-year-old supernova remnant, and the shock front passed by Sgr A* about 10^3 years ago. Large amount of matters accreted to Sgr A* in the passing time, and then surrounding materials are swept away.
At least three supernova remnants are discovered in the GC region with much higher angular resolution of Chandra than past X-ray observatories. The number of detected supernova remnants are insufficient to explain the total energy of the high temperature plasma in the GC region.
Chandra discovered faint diffuse structures in the GC region. They are candidates of supernova remnants, X-ray reflection nebulae, or something else. Some of them exhibit non-thermal spectrum, and thought to be acceleration sites of high energy cosmic rays.