Sagittarius A High Energy X-ray Flare Properties During NuSTAR Monitoring of the Galactic Center from 2012 to 2015

• Published in: arXiv.org 2017-05
• Main Authors: Zhang, Shuo, Baganoff, Frederick K, Ponti, Gabriele, Neilsen, Joseph, Tomsick, John A, Dexter, Jason, Clavel, Maïca, Markoff, Sera, Hailey, Charles J, Mori, Kaya, Barrière, Nicolas M, Nowak, Michael A, Boggs, Steven E, Christensen, Finn E, Craig, William W, Grefenstette, Brian W, Harrison, Fiona A, Madsen, Kristin K, Stern, Daniel, Zhang, William W
• Format: Article
• Language: English
• Subjects: Confidence intervals; Flares; Luminosity; X ray spectra
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1705.08002

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 July 2012 and April 2015, including 27 observations on Sgr A* with a total exposure of ~ 1 Ms. We found a total of ten X-ray flares detected in the NuSTAR observation window, with luminosities in the range of \(L_{3-79~keV}\)~\((0.2-4.0) \times 10^{35}~erg~s^{-1}\). With this largest hard X-ray Sgr A* 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} ~ 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 the flare spectra in 1-79 keV can be well fit with an absorbed power-law model with {\Gamma}=2.2+/-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*, and derived an upper limit on the quiescent luminosity of Sgr A* above 10 keV to be \(L_{Xq, 10-79 keV}\) < \((2.9{\pm}0.2) \times 10^{34}~erg~s^{-1}\).