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

Abstract

Located ∼ 100 pc from the dynamic center of the Milky Way, the molecular cloud Sagittarius B2 (Sgr B2) is the most massive such object in the Galactic Center region. In X-rays, Sgr B2 shows a prominent neutral Fe Kα line at 6.4 keV and continuum emission beyond 10 keV, indicating highenergy, non-thermal processes in the cloud. The Sgr B2 complex is an X-ray reflection nebula whose total emissions have continued to decrease since the year 2001 as it reprocesses what are likely one or more past energetic outbursts from the supermassive black hole Sagittarius A*. The X-ray reflection model explains the observed time-variability of the Fe Kα and hard X-ray emissions, and it 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, recent interest has also focused on X-rays from Sgr B2 as a probe of low-energy (sub-GeV) cosmic particles. In contrast to the timevarying X-ray reflection, in this case we can assume that the X-ray flux contribution from interactions of low-energy cosmic particles is constant in time, such that upper limits on low-energy cosmic particle populations may be obtained using the lowest flux levels observed from the cloud. Here, we present the most recent and correspondingly dimmest NuSTAR and XMM-Newton observations of Sgr B2, from 2018. These reveal small-scale variations within lower density portions of the Sgr B2 complex, including brightening features, yet still enable the best upper limits on X-rays from low-energy cosmic particles in Sgr B2. We also present Fe Kα fluxes from cloud regions of different densities, facilitating comparison with models of ambient low-energy cosmic particle interactions throughout the cloud.