X-ray Diagnostics of Giant Molecular Clouds in the Galactic Center
  Region and Past Activity of Sgr A*
  Strong iron fluorescence at 6.4 keV and hard-X-ray emissions from giant
molecular clouds in the Galactic center region have been interpreted as
reflections of a past outburst of the Sgr A* supermassive black hole. Careful
treatment of multiple interactions of photons in a complicated geometry is
essential to modeling the reprocessed emissions from the dense clouds. We
develop a new calculation framework of X-ray reflection from molecular clouds
based on Monte Carlo simulations for accurate interpretation of high-quality
observational data. By utilizing this simulation framework, we present the
first calculations of morphologies and spectra of the reflected X-ray emission
for several realistic models of Sgr B2, which is the most massive molecular
cloud in our Galaxy. The morphology of scattered hard X-rays above 20 keV is
significantly different from that of iron fluorescence due to their large
penetrating power into dense regions of the cloud, probing the structure of the
cloud. High-resolution spectra provide quantitative evaluation of the iron line
including its Compton shoulder to constrain the mass and the chemical
composition of the cloud as well as the luminosity of the illuminating source.
These predictions can be checked in the near future with future X-ray missions
such as NuStar (hard X-rays) and ASTRO-H (both iron lines and hard X-rays).
