Talk Title: Simultaneous NIR and X-Ray Observations of Sagittarius A*
Abstract: We report on the first simultaneous near-infrared/X-ray observations of the massive 3--4$\times$10$^6$\solm black hole at the center of the Milky Way using the NACO adaptive optics instrument at the European Southern Observatories Very Large Telescope and the ACIS-I aboard the Chandra X-ray observatory. On one occasion, a fading flare of Sgr~A* with $>$2 times the quiescent flux was detected at the beginning of the NIR observations. This flare was also detected in the X-domain. Our data therefore represent the first simultaneous NIR/X-ray measurement of the quiescent and flaring state of SgrA*. 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. With a value of about 1.3 the spectral overall indices of both states are quite comparable. Since the X-ray flux density is extended, the spectral index for the quiescent state is probably only a limit for the compact source Sgr~A*.
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