Proposed SMEX to spectrally analyze the diffuse x-ray background: The Baryonic Extragalactic Structure Tracer (BEST)

The Baryonic Extragalactic Structure Tracer (BEST) is a SMEX-class mission that is designed to map the hot million-degree diffuse intergalactic and interstellar gas with high spectral resolution. It consists of an imaging X-ray spectrometer that can, over a 1-2 year mission, map the entire sky and conduct deep pointed observations of selected regions to profoundly extend our understanding of hot matter in the Universe. BEST will be able to detect and characterize the missing baryons in the current epoch, which are primarily in moderately overdense intergalactic regions and are predicted to account for 10 - 20% of the soft X-ray background, and also determine the properties of the hot Galactic halo and the hot Galactic gas, crucial to understanding the evolution and dynamics of our Galaxy and its interstellar medium.