Sagittarius A* High-energy X-Ray Flare Properties during NuStar Monitoring of the Galactic Center from 2012 to 2015
ASTROPHYSICAL JOURNAL(2017)
摘要
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 similar or equal to 1 Ms. We found a total of 10 X-ray flares detected in the NuSTAR observation window, with luminosities in the range of L3-79keV similar to (0.2-4.0) X 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 sigma significance, showing a range of photon indices (Gamma similar to 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 Gamma = 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) X 10(34) erg s(-1).
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关键词
accretion, accretion disks,quasars: supermassive black holes,radiation mechanisms: non-thermal,X-rays: individual (sgra)
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