c 2012. Astronomical Society of Japan. Bursting Activity in a High-Mass Star-Forming Region G33.640.21

We report on the detection of bursts of 6.7 GHz methanol maser emission in a high-mass star-forming region, G33.640.21. One of the spectral components of the maser in this source changed its flux density by 7 times that of the previous day, and it decayed with a timescale of 5 days. The burst occurred repeatedly in the spectral component, and no other components showed such variability. A VLBI observation with the Japanese VLBI Network (JVN) showed that the burst location was at the southwest edge of a spatial distribution, and the bursting phenomenon occurred in a region much smaller than 70 AU. We suggest an impulsive energy release, like a stellar flare, as a possible mechanism for the burst. These results imply that 6.7 GHz methanol masers could be a useful new probe for studying bursting activity in the process of the star formation of high-mass YSOs with a high-resolution of AU scale. Key words: ISM: H II regions — ISM: individual (G33.640.21) — masers: methanol 1