ar X iv : 0 90 5 . 26 54 v 1 [ as tr o - ph . G A ] 1 6 M ay 2 00 9 The variability of Sagittarius A * at 3 millimeter

We have performed monitoring observations of the 3-mm flux density toward the Galactic Center compact radio source Sgr A* with the Australia Telescope Compact Array since 2005 October. Careful calibrations of both elevationdependent 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 2006 June session, and a low and variable state in 2006 August session. We report the results, with emphasis on two detected intra-day 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 can not be ignored, and a minimum mass-loss rate of 9.7×10M⊙ yr −1 is obtained based on parameters derived for this modified expanding plasmon model. Simultaneous multi-wavelength observation is crucial to our understanding the physical origin of rapid radio variability in Sgr A*. Subject headings: Galaxy: center, techniques: interferometric Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan RD, Shanghai 200030, China; [email protected], [email protected] Graduate School of the Chinese Academy of Sciences, Beijing 100039, China National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan; [email protected], [email protected], [email protected] Key Laboratory for Research in Galaxies and Cosmology, University of Science and Technology of China, Hefei 230026, China; [email protected] University of Melbourne, School of Physics, Parkville, Victoria 3052, Australia; [email protected] Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510, Japan; [email protected]