Rapid fluctuations of water maser emission in VY Canis Majoris

We report the observational results of short timescale monitoring of the 22 GHz water maser emission in VY CMa. A quasi-sinusoidal fluctuation has bee n detected with the relative flux intensity change of 20%-25% and a period of 10.3 day for two dominant features. This detected variability appears to be superimposed on the normal maser lines. We cannot easily explain the ra pid fluctuation with the variation of the radiative input or the strong int erstellar scintillation along the line of sight. The variation may be cause d by the periodic shock.