2006PASJ...58..965T

There are several lines of evidence to show that the supermassive black hole at the Galactic center had higher activities in the past than directly observed at present. It is shown here that these lines of evidence can quantitatively and consistently be explained if the mean accretion rate during the past ∼10⁷yr has been ∼10^3–4 times higher than the current rate, based on the picture of radiatively inefficient accretion flow (RIAF) and associated outflow that has been successfully applied to Sgr A^*. We argue that this increased rate and its duration are theoretically reasonable in the Galactic center environment, while the accretion rate suddenly dropped about 300yr 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. It is then shown 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 ∼MeV, which is also favorable for an explanation of the 511keV line emission.