Variations in the spectral slope of sagittarius A* during a near-infrared flare

We have observed a bright flare of Sgr A* in the near-infrared with the adaptive optics-assisted integral-field spectrometer SINFONI. Within the uncertainties, the observed spectrum is featureless and can be described by a power law. Our data suggest that the spectral index is correlated with the instantaneous flux and that both quantities experience significant changes within less than 1 hour. We argue that the near-infrared flares from Sgr A* are due to synchrotron emission of transiently heated electrons, the emission being affected by orbital dynamics and synchrotron cooling, both acting on timescales of ≈20 minutes.