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

In 2013, NuSTAR observed the Sgr B2 region and for the first time resolved its hard X-ray emission on subarcminute 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 X-ray emission can be best explained by the X-ray reflection nebula scenario, where the cloud reprocesses a past giant outburst from Sgr A{}^\star . In the X-ray reflection nebula (XRN) scenario, the 3-79 keV Sgr B2 spectrum allows us to self-consistently test the XRN model using both the Fe Kα line and the continuum emission. The peak luminosity of the past Sgr A{}^\star outburst is constrained to {L}_3-79{keV}∼ 5× {10}³⁸ {erg} {{{s}}}^-1. A newly discovered cloud feature, G0.66-0.13, shows different timing variability. We suggest that it could be a molecular clump located in the Sgr B2 envelope reflecting the same Sgr A{}^\star outburst. In contrast, if the Sgr B2 X-ray emission has reached a constant background level, it would imply an origin of low-energy cosmic-ray (CR) proton bombardment. In this scenario, from the NuSTAR measurements we infer a CR ion power of {dW}/{dt}=(1-4)× {10}³⁹ {erg} {{{s}}}^-1 and a CR ionization rate of {\zeta }_{{H}}=(6-10)× {10}^-15 {{{H}}}^-1\quad {{{s}}}^-1. These measurements can become powerful tools to constrain the GC CR population.