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

Abstract

Located $\sim100$ 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 K$\alpha$ line at 6.4 keV and continuum emission beyond 10 keV, indicating high-energy, non-thermal processes in the cloud. The Sgr B2 complex is an X-ray reflection nebula whose total emissions have decreased 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 K$\alpha$ and hard X-ray emissions, and it provides a window into the luminous 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 X-ray reflection, in this case we can assume that the X-ray flux contribution from ionization by 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 B complex, including brightening features, and enable the best upper limits on ionization of molecular clouds by low-energy cosmic particles in the inner $\sim100$ pc of the Galaxy.