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G.C. Bower (UC-Berkeley), R.S. McGary, J.-H. Zhao (CfA), W.M. Goss (NRAO-Socorro)
It is generally accepted that a 2.6x106 Msun supermassive black hole is located at the dynamical center of the Milky Way Galaxy. At radio wavelengths, synchrotron emission from just outside the black hole is observed as the bright, point-like source, Sagittarius A* (Sgr A*). Previous analysis of Very Large Array (VLA) archival data have indicated a variability in the flux density of Sgr A* with a period of ~100 days and an amplitude increasing towards shorter wavelengths (Zhao et al. 2001). However, further observations with better sampling are necessary in order to confirm whether the observed cycle is a persistent phenomenon and whether it is intrinsically quasi-periodic. High precision, better sampled data will allow us to search for fast modes in variability related to the fast rotation and large convective gradients in the very inner part of the disk as suggested in the convective-ADAF theory.
Since June 2000, we have made weekly observations of Sgr A* at 2.0 cm, 1.3 cm and 7 mm with the VLA. These observations result in a well-sampled dataset covering more than 500 days and sensitive to periods as short as 20 days. Systematic offsets in flux calibration are removed by comparison to two calibrator sources. At 7 mm, we also remove effects due to opacity and obtain errors of ~10%. Errors of 5-10% are obtained for the longer wavelengths. In this poster, we present our monitoring data to date showing the intrinsic variability in the flux density of Sgr A* and outlining our data reduction method in detail. A statistically significant variability in the flux density of ~30% is observed at both 2.0 cm and 1.3 cm and all three bands are sensitive to large flares of Sgr A*. A preliminary power spectral density (PSD) analysis of the data will be presented showing evidence for a quasi-periodic variability in the flux density. In addition, we will compare our data to results at other wavelengths including sub-millimeter and x-ray.