2009ApJ...700..417L

We have performed monitoring observations of the 3 mm flux density toward the Galactic center compact radio source Sagittarius A* (Sgr A*) with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevation-dependent and time-dependent gains have enabled us to establish the variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable state in the 2006 June session, and a low and variable state in the 2006 August session. We report the results, with emphasis on two detected intraday variation events during its low states. One is on 2006 August 12 when Sgr A* exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006 August 13 when two peaks separated by about 4 hr, with a maximum variation of 21% within 2 hr, were seen. The observed short timescale variations are discussed in light of two possible scenarios, i.e., the expanding plasmon model and the sub-Keplerian orbiting hot spot model. The fitting results indicate that for the adiabatically expanding plasmon model, the synchrotron cooling cannot be ignored, and a minimum mass-loss rate of 9.7x10^–10 M_☉/yr is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multiwavelength observation is crucial to our understanding of the physical origin of rapid radio variability in Sgr A*.