We have been performing daily VLBI monitoring of the ux density of Sagittarius (Sgr) A* at 22 GHz from February 2013 to August 2014 using a sub-array of the Japanese VLBI Network (JVN). The purpose of this monitoring is to explore the ux density variability at daily time resolution for a period longer than one year with the G2 cloud approaching. The ux density of Sgr A* is basically stable during the observational period, though there are some small variations. The average and scattering range are consistent with the previously observed values. We have observed no strong are of Sgr A* although it is near the expected peri-center passing.

This paper reviews the progress in the VLBI (Very Long Baseline Interferometry) studies of Sgr A*, the best known supermassive black hole candidates with a dark mass concentration of 4x106 M⊙ at the center of the Milky Way. The emphasis is on the importance of the millimeter and sub-millimeter VLBI observations in the detection of Sgr A*'s intrinsic structure and search for the structural variation.

We observed the thermal transitions of SiO (J=I-0, 2-1) and 29SiO (J=l-O) toward the Sgr A molecular clouds. The distribution and the velocity structure of SiO are very similar to previous results for 'quiet' interstellar molecules. We think· that the SiO has been well mixed with other molecules such as H2 which may indicate that the formation of Sgr A molecular clouds was affected by the activities, such as shock waves or energetic photons, from the Galactic center in large scales. The total column density of SiO is about 4.1×1014cm−2 and the fractional abundance SiO/H2 appears to be about 10 times larger than those of other clouds in the central region of our galaxy. The derived values are thought to be lower limits since the optical depths of the observed SiO lines are not very thin. The formation of SiO has been known to be critically related to shocks, and our results provide informative data on the environment of our Galactic center.