An Implication for a very long-term AGN variability 
--- What can we learn from our Galactic Center? --- 

Y. Maeda,$^1$ F. K. Baganoff,$^2$ E.
D. Feigelson,$^1$ M. W. Bautz,$^2$ W. N.
Brandt,$^1$ D. N. Burrows,$^1$
J. P. Doty,$^2$
G. P. Garmire,$^1$ M. Morris,$^3$,
S. H. Pravdo,$^4$,
G. R. Ricker,$^2$ L. K. Townsley$^1$

$^1$ Department of Astronomy and Astrophysics, 525 Davey
Laboratory, The Pennsylvania State University, University Park, PA
16802-6305, U.S.A.\newline
$^2$ Massachusetts Institute of Technology, Center for
Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139-4307,
U.S.A.\newline
$^3$ Division of Astronomy, Box 951562, UCLA, Los Angeles,
CA 90095-1562, U.S.A.\newline
$^4$ Jet Propulsion Laboratory, MS 306-438, 4800 Oak Grove
Drive, Pasadena, CA 91109, U.S.A.


With Chandra, we studied the X-ray emission from the shell-like,
non-thermal radio source Sgr~A East (SNR~! 000.0$ $00.0) and found that
Sgr~A East is an old supernova remnant (SNR), most likely to be
located very near the central massive blackhole at Sgr A*.  A
comparison with the radio observations requires the dense ambient
medium to be ionized, but a luminous X-ray irradiator with an expected
X-ray luminosity of $\sim 10^{40}$ ergs~s$^{-1}$ is not currently
present.  The presence of the ionized gas may be explained by the past
AGN activity scenario by Koyama et al. (1996) that Sgr~A* was bright
in X-rays about three hundred years ago, but is presently dim.  It is
possible that the dust/molecular ridge compressed by the forward shock
of Sgr~A East hit Sgr~A* in the past, and the passage of the ridge may
have supplied material to accrete onto the black~hole in the past, and
may have removed material from the black hole vicinity, leading to its
present quiescent state. This may be a specific example to indicate
the recurrecy of AGN activities. In this paper, we briefly discuss the
possibility to detect the AGN variability in galaxies regulated by a SNR
using an all-sky monitor detector such as MAXI.