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

Kavli Affiliate: Melania Nynka

| First 5 Authors: Shuo Zhang, Charles J. Hailey, Kaya Mori, Maïca Clavel, Régis Terrier

| Summary:

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$alpha$ 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 $L_{3-79rm~keV} sim 5times10^{38} rm~erg~s^{-1}$ derived from the
maximum Compton-scattered continuum and the Fe K$alpha$ 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
$zeta_{H}=(6-10)times 10^{-15}rm~H^{-1}~s^{-1}$. The Sgr B2 background level
X-ray emission will be a powerful tool to constrain GC CR population.

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