New Constraints on Cosmic Particle Populations at the Galactic Center
  using X-ray Observations of the Molecular Cloud Sagittarius B2
  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 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 K$\alpha$ 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 time-varying 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 K$\alpha$ fluxes from cloud regions of
different densities, facilitating comparison with models of ambient low-energy
cosmic particle interactions throughout the cloud.
