Searching for Structural Variability in Sgr A*
Z.-Q. Shen1,2, M. C. Liang2, K. Y. Lo2
1 Institute of Space and Astronautical Science (ISAS), Yoshinodai 3-1-1, Sagamihara, Kanagawa 229-8510, JAPAN
2 Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), PO Box 23-141, Taipei 106, Taiwan, CHINA

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The Chandra discovery of an X-ray flaring from the direction of Sgr A* and the discovery of the quasi-periodical variation at radio to sub-millimeter wavelengths are consistent with the intensity variations arising from the instabilities in an accretion disk. Thus, it would also seem inevitable that the variability in the flux density of Sgr A* would be accompanied by structural changes in Sgr A*. The existing VLBI images made at short wavelengths (13.5, 7.0 and 3.5 mm) have shown a possible variation in the observed source size of Sgr A* up to 6 sigma. These changes have so far been ascribed to the uncertainties in the calibration of the VLBI data. Thus, it is essential to improve on the calibration of such data. We will report on our ongoing effort to search for such a structure variability in Sgr A* from multi-epoch 7mm VLBA observations. To minimize the calibration errors and thus to improve the accuracy of the measurements, we have developed a model fitting program by using (implicitly) the amplitude closure relationship. We are examining more than six epochs of data (including some archive data). The preliminary results seem to show the sign of change to the source apparent structure in at least one epoch over the period of 7 years (1994-2001). Any discernible structural variability in the scattering image itself will have a direct bearing on the intrinsic source structural variation. A correlation analysis of the source size with the radio flaring is underway.