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Hard X-ray Morphological and Spectral Studies of The Galactic Center Molecular Cloud Sgr B2: Constraining Past Sgr A* Flaring Activity

Galactic Center (GC) molecular cloud Sgr B2 is the best manifestation of an X-ray reflection nebula (XRN) reprocessing a past giant outburst from the supermassive black hole Sgr A*. Alternatively, Sgr B2 could be illuminated by low-energy cosmic ray electrons (LECRe) or protons (LECRp). In 2013, NuSTAR for the first time resolved Sgr B2 hard X-ray emission on sub-arcminute scales. Two prominent features are detected above 10 keV - a newly emerging cloud G0.66-0.13 and the central 90" radius region containing two compact cores Sgr B2(M) and Sgr B2(N) surrounded by diffuse emission. It is inconclusive whether the remaining level of Sgr B2 emission is still decreasing or has reached a constant background level. A decreasing Fe K$α$ emission can be best explained by XRN while a constant background emission can be best explained by LECRp. In the XRN scenario, the 3-79 keV Sgr B2 spectrum can well constrain the past Sgr A* outburst, resulting in an outburst spectrum with a peak luminosity of $L_{3-79\rm~keV} \sim 5\times10^{38} \rm~erg~s^{-1}$ derived from the maximum Compton-scattered continuum and the Fe K$α$ emission consistently. The XRN scenario is preferred by the fast variability of G0.66-0.13, which could be a molecular clump located in the Sgr B2 envelope reflecting the same Sgr A* outburst. In the LECRp scenario, we derived the required CR ion power $dW/dt=(1-4)\times10^{39}\rm~erg~s^{-1}$ and the CR ionization rate $ζ_{H}=(6-10)\times 10^{-15}\rm~H^{-1}~s^{-1}$. The Sgr B2 background level X-ray emission will be a powerful tool to constrain GC CR population.