SAGITTARIUS A⋆ HIGH ENERGY X-RAY FLARE PROPERTIES DURING NuSTAR MONITORING OF THE GALACTIC CENTER FROM 2012 TO 2015

Understanding the origin of the aring activity from the Galactic center supermassive black hole, Sagittarius A?, is a major scientic goal of the NuSTAR Galactic plane survey campaign. We report on the data obtained between July 2012 and April 2015, including 28 observations with Sgr A? in the NuSTAR 130 130 eld of view, resulting in a total exposure of ' 1 Ms. Part of the NuS- TAR Sgr A? observations were jointly performed with Chandra, XMM-Newton Spitzer, VLA, VLT, etc. By applying Bayesian block analysis to the X-ray light curves obtained by NuSTAR (and also Chandra/XMM-Newton for joint observations), we found a total of ten X-ray ares 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? are dataset to date, we studied the spectral properties of the ares. Seven bright ares are detected above 5 signicance, showing a range of photon indices (􀀀 2:0{2.8) with typical uncertainties of 0:5 (90% condence level). We found no signicant correlation between the are spectral shape and the are average luminosity. Our result also shows that the accumulation of all the are spectra in 1{79 keV can be well t with an absorbed power-law model with 􀀀 = 2:2 0:1, and does not require the existence of a spectral break in X-rays. The lack of variation in X-ray spectral index with luminosity would point to a single mechanism for the ares and is consistent with the synchrotron scenario. Lastly, we present the spectrum of the inner 3000 of Sgr A? when it is in the X-ray quiescent state, and derived an upper limit on the quiescent luminosity of Sgr A? above 10 keV to be LXq;10􀀀79 keV (2:9 0:2) 1034 erg s􀀀1.