Multi-Epoch VERA Observations of Sagittarius A*. I. Images and Structural Variability

We report on the results of multi-epoch observations of Sgr A* with VLBI Exploration of Radio Astrometry (VERA) at 43 GHz, carried out from 2004 to 2008. We detected a time variation of the flux at the 11% level and the intrinsic size at 19%. In addition, comparisons with previous Very Long Baseline Array (VLBA) results showed that Sgr A* underwent a flaring event for at least longer than 10 d in 2007 May. The intrinsic size of Sgr A* remained unchanged within the 1 sigma level from the size before/after the flaring event, indicating that the brightness temperature of Sgr A* was increased. The flaring event occurred within 31 d, which is shorter than the refractive time scale. Moreover it is difficult to explain the increase in the spectral index at the flaring event by a simple interstellar scattering model. Hence, the flaring event is most likely associated with changes in the intrinsic properties of Sgr A*. We considered the origin of the brightness temperature variation, and concluded that the flaring event of Sgr A* could be explained by the continuous heating of electrons, such as a standing shock in accretion flow.