Observations of the bursting activity of the 6.7 GHz methanol maser in G33.641-0.228

We have observed bursting variability of the 6.7 GHz methanol maser of G33.641-0.228. Five bursts were detected in the observation period of 294 days from 2009 to 2012. The typical burst is a large flux density rise in about one day followed by a slow fall. A non-typical burst observed in 2010 showed a large and rapid flux density enhancement from the stable state, but the rise and fall of the flux density were temporally symmetric and a fast fluctuation continued for 12 days. On average, the bursts occurred once every 59 days, although bursting was not periodic. Since the average power required for causing a burst of order of 10(21) Js(-1) is far smaller than the luminosity of G33.641-0.228, a very small fraction of the source's power would be sufficient to cause the burst occasionally. The burst can be explained as a solar-flare-like event in which the energy is accumulated in the magnetic field of the circumstellar disk, and is released for a short time. However, the mechanism of the energy release and the dust heating process are still unknown.