Sagittarius ${\rm{A}}* $ High-energy X-Ray Flare Properties during NuStar Monitoring of the Galactic Center from 2012 to 2015

International audienceUnderstanding the origin of the flaring activity from the Galactic center supermassive black hole Sagittarius A* is a major scientific goal of the NuSTAR Galactic plane survey campaign. We report on the data obtained between 2012 July and 2015 April, including 27 observations on Sgr A*, with a total exposure of ≃ 1 Ms. We found a total of 10 X-ray flares detected in the NuSTAR observation window, with luminosities in the range of L3-79 keV ∼ (0.2-4.0)× 1035 erg s-1. With this largest hard X-ray Sgr A* flare data set to date, we studied the flare spectral properties. Seven flares are detected above 5σ significance, showing a range of photon indices (Γ ∼ 2.0-2.8) with typical uncertainties of ±0.5 (90% confidence level). We found no significant spectral hardening for brighter flares, as indicated by a smaller sample. The accumulation of all of the flare spectra in 1-79 keV can be well fit with an absorbed power-law model with Γ =2.2±0.1, and does not require the existence of a spectral break. The lack of variation in the 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*, and derive an upper limit on the quiescent luminosity of Sgr A* above 10 keV to be LXq,10-79 keV ≤ (2.9± 0.2)× 1034 erg s-1