New Constraints on Cosmic Particle Populations at the Galactic Center Using X-Ray Observations of the Molecular Cloud Sagittarius B2

The Sagittarius B2 (Sgr B2) molecular cloud complex is an X-ray reflection nebula whose nonthermal X-ray emissions have continued to decrease since 2001 as it reprocesses one or more past energetic outbursts from the supermassive black hole Sagittarius A* at the Galactic Center. The X-ray reflection model explains the observed time variability of X-ray emission from Sgr B2 and provides a window into the luminous evolutionary history of our nearest supermassive black hole. In light of evidence of elevated cosmic particle populations in the Galactic Center, X-rays from Sgr B2 are also of interest as a probe of low-energy (sub-GeV) cosmic rays, which may be responsible for an increasing relative fraction of the nonthermal emission as the contribution from X-ray reflection decreases. Here, we present the most recent NuSTAR and XMM-Newton observations of Sgr B2, which were jointly recorded in 2018, and we emphasize the Kα fluorescence line of neutral Fe. These 2018 observations reveal small-scale variations within lower-density portions of the complex, including brightening features, yet still enable upper limits on X-rays from low-energy cosmic-ray interactions in Sgr B2. We present Fe Kα line fluxes from cloud regions of different densities, facilitating comparison with models of ambient low-energy cosmic-ray interactions throughout the cloud.