Multi-Epoch VERA Observations of Sagittarius A*: I. Images and
  Structural Variability
  We report 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 flux at 11 % level and intrinsic size at
19 %. In addition, comparisons with previous Very Long Baseline Array (VLBA)
results shows that Sgr A* underwent the flaring event at least longer than 10
days in May 2007. The intrinsic size of Sgr A* remained unchanged within 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
the simple interstellar scattering model. Hence, the flaring event is most
likely associated with the changes in 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.
