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 suzakusgrb2.tex  PASJ, 2008, in press
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%astro-ph/0711.2853

\documentclass{pasj00}
%\draft
\begin{document}
\SetRunningHead{K. Koyama}{A Time Variable X-Ray Echo}
\Received{2007/06/20}%{yyyy/mm/dd}
\Accepted{2007/08/24}%{yyyy/mm/dd}
\title{A Time-Variable X-Ray Echo: Indications of a Past Flare of the Galactic-Center Black Hole }
\author{Katsuji \textsc{Koyama}, Tatsuya \textsc{Inui}, Hironori \textsc{Matsumoto}, and Takeshi Go \textsc{Tsuru} }
\affil{Department of Physics, Graduate school of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502}
\email{koyama@cr.scphys.kyoto-u.ac.jp}
\KeyWords{ISM: clouds---ISM: H~\emissiontype{II} regions---ISM: supernova remnants}

\maketitle

\begin{abstract}
A time-variability study of the neutral iron line flux at 6.40~keV in the Sgr B2 region from data of
Suzaku and  Chandra is presented.
The highly ionized iron line at 6.68~keV is due to Galactic Center Diffuse X-rays (GCDX), and is thus  time invariable.
By comparing the 6.68 keV and 6.40 keV line fluxes, we found
that the 6.40 keV flux from the Sgr~B2 complex region is time variable;
particularly the giant molecular cloud
M~0.66$-$0.02, known as ``Sgr B2 cloud'' is highly variable.
The variability of the 6.40~keV line in intensity and spatial distribution strongly supports the scenario that
the molecular clouds in the Sgr B2 region are X-ray Reflection Nebulae irradiated by the Galactic Center (GC)
black hole Sgr A$^*$.
\end{abstract}

\end{document}