First Simultaneous NIR/X-ray Detection of a Flare from SgrA*

We report on the first simultaneous near-infrared/X-ray detection of the SgrA* counterpart which is associated with the massive 3--4$\times10^6$\solm \~black hole at the center of the Milky Way. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope and the ACIS-I instrument aboard the Chandra X-ray observatory. A flare was detected in the X-ray domain with an excess 2 - 10 keV luminosity of about 6$\times10^{33}$ erg/s. The NIR observations began near the peak of the X-ray flare and detected a fading NIR flare of Sgr~A$^*$ with $>2$ times the interim-quiescent flux. We find that the flaring state can be conveniently explained with a synchrotron self-Compton model involving up-scattered sub-millimeter photons from a compact source component showing bulk relativistic motion. The size of that component is assumed to be of the order of the Schwarzschild radius. The overall spectral indices $\alpha_{NIR/X-ray}$ ($S_{\nu}$$\propto$$\nu^{-\alpha}$) of both states are consistent with a value of $\sim$1.3. Since the interim-quiescent X-ray emission is extended, the spectral index for the interim-quiescent state is only a lower limit for the compact source Sgr~A*. A conservative estimate of the upper limit of the time lag between the end of the NIR and X-ray flare is of the order of 15 minutes.