Variations in the spectral slope of Sgr A* during a NIR 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. The associated power law index is subject to systematic effects, namely the determination of the background level. We explore these effects and can show that while the absolute value of the spectral power law index is relatively uncertain, our data nevertheless suggest that the spectral index is correlated with the instantaneous flix. Both quantities experience signifbant changes within less than one 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.

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