NuSTAR Detection of High-energy Emission and Fast Variability from a Sagittarius A* X-ray Flare

Sagittarius A* (Sgr A*) is a supermassive 4×10₆ M^⊙ black hole that lies at the dynamic center of our Galaxy. Although Sgr A* spends most of its time in a low luminosity emission state (bolometric luminosity ~300 L^⊙), it exhibits frequent flares in the near infrared and in the X-ray band. The radiation processes giving rise to these flares have yet to be identified. Here we report the detection of several X-ray flares with the recently launched NuSTAR mission. The brighter flares have a spectrum extending to at least 40 keV, and one flare exhibits variability on a 100s timescale. These spectra favor the synchrotron (with a cooling break) emission mechanism, implying that hard X-ray light curves trace the acceleration of energetic particles.