  Sagittarius A* harbors the supermassive black hole that lies at the dynamical
center of our Galaxy. Sagittarius A* spends most of its time in a low
luminosity emission state but flares frequently in the infrared and X-ray,
increasing up to a few hundred fold in brightness for up to a few hours at a
time. The physical processes giving rise to the X-ray flares are uncertain.
Here we report the detection with the NuSTAR observatory in Summer and Fall
2012 of four low to medium amplitude X-ray flares to energies up to 79 keV. For
the first time, we clearly see that the power-law spectrum of Sagittarius A*
X-ray flares extends to high energy, with no evidence for a cut off. Although
the photon index of the absorbed power-law fits are in agreement with past
observations, we find a difference between the photon index of two of the
flares (significant at the 95% confidence level). The spectra of the two
brightest flares (~55 times quiescence in the 2-10 keV band) are compared to
simple physical models in an attempt to identify the main X-ray emission
mechanism, but the data do not allow us to significantly discriminate between
them. However, we confirm the previous finding that the parameters obtained
with synchrotron models are, for the X-ray emission, physically more reasonable
than those obtained with inverse-Compton models. One flare exhibits large and
rapid (< 100 s) variability, which, considering the total energy radiated,
constrains the location of the flaring region to be within ~10 Schwarzschild
radii of the black hole.
