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

Abstract Understanding 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 L 3 – 79 keV ∼ ( 0.2 – 4.0 ) × 10 35 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 5 σ 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 L Xq , 10 – 79 keV ≤ ( 2.9 ± 0.2 ) × 10 34 erg s−1.