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

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 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 quiescentstate 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) × 10³⁴ erg s^-1.