A decade of unchanged 1.3 cm VLBI structure of Sgr A

We observed Sgr A* at 1.3 cm on 1992 November 8 with a 14 station VLBI array and, using data from 10 of these stations, obtained a high-dynamic range image with a beam of 3.35×1.7 milliarcseconds in position angle 14°. The observed surface brightness distribution of Sgr A* is best modelled by an elliptically shaped gaussian component with parameter values: flux density 1.05 ± 0.10Jy, major axis of FWHM 2.67±0.15 milliarcseconds in position angle 79°±10°, and axial ratio 0.61±0.12, which are almost the same as the estimates of Alberdi et al. (1993). We found no evidence of changes in the source structure of Sgr A* among four sets of 1.3 cm VLBI observations spanning almost a decade. It seems reasonable to conclude that both the emission of Sgr A* itself and the surrounding medium acting as a refractive screen have reasonably constant relative orientation, emission rates and structures.