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 ^ { \star } .
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 ^ { \prime \prime }  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 ^ { \star } outburst , resulting in an outburst spectrum with a peak luminosity of L _ { 3 - 79 ~ { } keV } \sim 5 \times 10 ^ { 38 } ~ { } 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 ^ { \star } outburst .
In the LECRp scenario , we derived the required CR ion power dW / dt = ( 1 - 4 ) \times 10 ^ { 39 } ~ { } erg~ { } s ^ { -1 } and the CR ionization rate \zeta _ { H } = ( 6 - 10 ) \times 10 ^ { -15 } ~ { } H ^ { -1 } ~ { } s ^ { -1 } .
The Sgr B2 background level X-ray emission will be a powerful tool to constrain GC CR population .