Simultaneous NIR/sub-mm observation of flare emission from Sagittarius A*

Context. We report on a successful, simultaneous observation and modeling of the sub-millimeter to near-infrared flare emission of the Sgr A* counterpart associated with the super-massive (4 x 10(6) M-circle dot) black hole at the Galactic center. Aims. We study and model the physical processes giving rise to the variable emission of Sgr A*. Methods. Our non-relativistic modeling is based on simultaneous observations that have been carried out on 03 June, 2008. We used the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the LABOCA bolometer at the Atacama Pathfinder Experiment (APEX). We emphasize the importance of a multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare modeling. Results. The observations reveal strong flare activity in the 0.87 mm (345 GHz) sub-mm domain and in the 3.8 mu/2.2 mu m NIR. Inspection and modeling of the light curves show that the sub-mm follows the NIR emission with a delay of 1.5 +/- 0.5 h. We explain the flare emission delay by an adiabatic expansion of the source components. The derived physical quantities that describe the flare emission give a source component expansion speed of nu(exp) similar to 0.005c, source sizes around one Schwarzschild radius with flux densities of a few Janskys, and spectral indices of alpha = 0.8 to 1.8, corresponding to particle spectral indices similar to 2.6 to 4.6. At the start of the flare the spectra of these components peak at frequencies of a few THz. Conclusions. These parameters suggest that the adiabatically expanding source components either have a bulk motion greater than nu(exp) or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*.