Multiwavelength VLBI observations of Sagittarius A

Context. The compact radio, NIR, and X-ray source Sagittarius A* (Sgr A*), associated with the super massive black hole at the center of the Galaxy, has been studied with Very Long Baseline Interferometry (VLBI) observations performed on 10 consecutive days and at mm-wavelength. Aims. SgrA* varies in the radio through X-ray bands and occasionally shows rapid flux density outbursts. We monitor Sgr A* with VLBI, aiming at the detection of related structural variations on the submilliarcsecond scale and variations of the flux density occurring after NIR-flares. Methods. We observed SgrA* with the Very Long Baseline Array (VLBA) at 3 frequencies (22, 43, 86GHz) on 10 consecutive days in May 2007 during a global multiwaveband campaign. From this we obtained accurate flux densities and sizes of the VLBI structure, which is partially resolved at mm-wavelength. Results. The total VLBI flux density of SgrA* 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 86GHz. The radio spectrum is inverted between 22 and 86 GHz, suggesting inhomogeneous synchrotron self-absorption with a turnover frequency at or above 86GHz. The radio spectral index correlates with the flux density, which is harder (more inverted spectrum) when the source is brighter. The average source size (FWHM) 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 +/- 0.12), 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 86GHz 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.