  There are several lines of evidence that the super-massive black hole at the
Galactic center had higher activities in the past than directly observed at
present. Here I show that these lines of evidence can quantitatively and
consistently be explained if the mean accretion rate during the past ~10^7 yrs
has been ~10^{3-4} times higher than the current rate, by the picture of
radiatively inefficient accretion flow (RIAF) and associated outflow that has
been successfully applied to Sgr A*. I argue that this increased rate and its
duration are theoretically reasonable in the Galactic center environment, while
the accretion rate suddenly dropped about 300 years ago most likely because of
the shell passage of the supernova remnant Sgr A East. The chance probability
of witnessing Sgr A* in such a low state is not extremely small (~0.5%). The
outflow energetics is sufficient to keep the hot (~8 keV) diffuse gas observed
in the Galactic center region. Then, I show that a significant amount of
positrons should have been created around the event horizon during the higher
activity phase, and injected into interstellar medium by the outflow. The
predicted positron production rate and propagation distance are close to those
required to explain the observed 511 keV annihilation line emission from the
Galactic bulge, giving a natural explanation for the large bulge-to-disk ratio
of the emission. The expected injection energy into interstellar medium is
about 1 MeV, which is also favorable as an explanation of the 511 keV line
emission.
