NuSTAR Observation of Sgr B2: Reflection of Past Sgr A* X-ray Outburst, Cosmic Ray Illumination or Both?

The Galactic Center (GC) supermassive black hole Sgr A* radiates at a level about 9 orders of magnitude lower than its Eddington luminosity. But indication of its glorious past has come from X-ray observations of surrounding giant molecular clouds. Their varying neutral iron lines and continuum emission could be due to reflection of Sgr A* X-ray outbursts hundreds of years ago. However, such observational facts can also be explained by low energy cosmic ray electron (LECRe) illumination. It has remained a long-standing question which scenario gives rise to the molecular cloud emission: X-ray reflection, LECRe or both. In this talk, I present the NuSTAR observation of Sgr B2, the densest and most massive GC molecular clouds. Its substructure at energies greater than 10 keV is revealed for the first time. The direct comparison with iron line emission morphology is consistent with X-ray reflection from a source in the direction of Sgr A*. We also applied physical models of X-ray reflection and LECRe to the broadband X-ray spectra. Results show that the X-ray reflection model is preferred over the LECRe model. We derived that both the iron fluorescence line emission and the Compton scattered continuum emission point to a faraway primary source with photon index of 2 and a luminosity of ~10^39 erg/s, probably due to a past Sgr A* outburst. Though probably not a dominant process, the LECRe contribution can be constrained by the data. Besides Sgr B2, I will also briefly talk about the NuSTAR observation of other GC molecular clouds and the nature of their X-ray emission.