High-Frequency Inter-day VLBI Monitoring of Sgr A*

We present results of a multi-frequency inter-day VLBA monitoring of Sgr A*. On daily timescales, Sgr A* showed correlated flux density variations with increasing variability amplitudes towards higher frequencies. A positive correlation was found between the spectral index and the flux density. The sizes of the minor axis tend to vary with time and total flux, whereas the major axis remains constant. Our data indicate that the position angle slightly increased towards higher frequency, suggesting that the intrinsic structure becomes dominant. At cm-wavelength, the apparent source size scales as lambda 2.12 +/- 0.12 due to interstellar scatter broadening. After removal of this scatter broadening, the intrinsic source size varies as lambda(1.4 ... 1.5). The VLBI closure phases at 22, 43, and 86 GHz are zero within a few degrees, indicating a symmetric source structure. In the context of an expanding plasmon model, we obtain an upper limit of the expansion velocity of about 0.1 c from the observed norm-variable VLBI structure.