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

Abstract

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.