Understanding the origin of the flaring activity from the Galactic center supermassive black hole , Sagittarius A ^ { \star } , is a major scientific goal of the NuSTAR Galactic plane survey campaign .
We report on the data obtained between July 2012 and April 2015 , including 27 observations on Sgr A ^ { \star } with a total exposure of \simeq 1 Ms. We found a total of ten X-ray flares detected in the NuSTAR observation window , with luminosities in the range of L _ { 3 - 79 ~ { } keV } \sim ( 0.2 – 4.0 ) \times 10 ^ { 35 } ~ { } erg~ { } s ^ { -1 } .
With this largest hard X-ray Sgr A ^ { \star } flare dataset to date , we studied the flare spectral properties .
Seven flares are detected above 5 \sigma significance , showing a range of photon indices ( \Gamma \sim 2.0 –2.8 ) with typical uncertainties of \pm 0.5 ( 90 % confidence level ) .
We found no significant spectral hardening for brighter flares as indicated by a smaller sample .
The accumulation of all the flare spectra in 1–79 keV can be well fit with an absorbed power-law model with \Gamma = 2.2 \pm 0.1 , and does not require the existence of a spectral break .
The lack of variation in X-ray spectral index with luminosity would point to a single mechanism for the flares and is consistent with the synchrotron scenario .
Lastly , we present the quiescent state spectrum of Sgr A ^ { \star } , and derived an upper limit on the quiescent luminosity of Sgr A ^ { \star } above 10 keV to be L _ { Xq, 10 - 79 ~ { } keV } \leq ( 2.9 \pm 0.2 ) \times 10 ^ { 34 } erg s ^ { -1 } .