The variability of Sagittarius A* at 3-millimeters

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We have performed monitoring observations of the 3-millimeter flux density toward the Galactic center compact radio source Sgr A* with the Australia Telescope Compact Array (ATCA) since 2005 October. Careful calibrations of both elevation-dependent and time-dependent gains have been done to establish the variability behavior of Sgr A*. It has been found that during several observing epochs Sgr A* was quite active, showing significant intraday variation (IDV). Here we report a detected IDV event. On 2006 August 13, Sgr A* exhibits a maximum variation of 22% within 2 hrs, and two peaks with a separation of 4 hrs are seen. We discuss two possible scenarios, the orbiting hotspot model and the expanding plasmon model.