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 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 $L_{3-79keV}$~$(0.2-4.0)\times 10^{35}erg{s}^{-1}$. With this largest hard X-ray Sgr A* flare dataset to date, we studied the flare spectral properties. Seven flares are detected above 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 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 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 $L_{Xq,10-79keV}$ < $(2.9\pm 0.2)\times 10^{34}erg{s}^{-1}$.

11 pages, 4 figures, Accepted for publication in ApJ

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• The Curious Case of ASASSN-20hx: A Slowly-Evolving, UV and X-ray Luminous, Ambiguous Nuclear Transient
• General relativistic MHD simulations of non-thermal flaring in Sagittarius A*
• A Possible Model for the Long-Term Flares of Sgr A*
• A "coronal-mass-ejection'' model for flares in Sagittarius A*
• Multi-wavelength Variability of Sagittarius A* in July 2019
• A Systematic Chandra study of Sgr A${}^{\star }$: II. X-ray flare statistics
• Sgr B2 hard X-ray emission with INTEGRAL after 2009: still detectable?
• Possible evidence from the flaring activity of Sgr A* for a star at a distance of ~3.3 Schwarzscild radii from the blackhole
• A 149 minute periodicity underlies the X-ray flaring of Sgr A*
• The X-ray variation of M81* resolved by Chandra and NuSTAR
• High Energy Emission from the Galactic Center