Multiwavelength VLBI observations of Sagittarius A*
  The compact radio source Sgr\,A*, associated with the super massive black
hole at the center of the Galaxy, has been studied with VLBA observations at 3
frequencies (22, 43, 86\,GHz) performed on 10 consecutive days in May 2007. The
total VLBI flux density of Sgr\,A* varies from day to day. The variability is
correlated at the 3 observing frequencies with higher variability amplitudes
appearing at the higher frequencies. For the modulation indices, we find 8.4\,%
at 22\,GHz, 9.3\,% at 43\,GHz, and 15.5\,% at 86\,GHz. The radio spectrum is
inverted between 22 and 86\,GHz, suggesting inhomogeneous synchrotron
self-absorption with a turnover frequency at or above 86\,GHz. The radio
spectral index correlates with the flux density, which is harder (more inverted
spectrum) when the source is brighter. The average source size does not appear
to be variable over the 10-day observing interval. However, we see a tendency
for the sizes of the minor axis to increase with increasing total flux, whereas
the major axis remains constant. Towards higher frequencies, the position angle
of the elliptical Gaussian increases, indicative of intrinsic structure, which
begins to dominate the scatter broadening. At cm-wavelength, the source size
varies with wavelength as $\lambda^{2.12\pm0.12}$, which is interpreted as the
result of 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 or point-like 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 non-variable VLBI structure. This agrees with the
velocity range derived from the radiation transport modeling of the flares from
the radio to NIR wavelengths.}
