Where Is the Supervirial Gas? III. Insights from X-Ray Shadow Observations and a Revised Model for the Soft Diffuse X-Ray Background

Abstract Shadow observations provide a powerful tool to separate foreground components of the soft diffuse X-ray background (SDXB) from the background components. Such observations have now established that the “local” foreground is made of the solar wind charge exchange and the Local Bubble, and the background emission is from the extended circumgalactic medium (CGM) of the Milky Way and from the unresolved extragalactic sources. New data and careful analyses of the SDXB led to two new discoveries in recent years: (1) excess emission near 0.5 keV that is identified as the N vii emission line, and (2) excess emission near 0.8−1.0 keV that is identified with an additional, supervirial temperature hot thermal component of the CGM. The goal of this paper is to use Suzaku shadow observations along six sightlines to determine whether either of these components is from the “local” sources. We eliminate the ambiguity regarding the origin of N vii emission, ruling out the local origin. We confirm that the Milky Way CGM contains nitrogen-rich plasma, with a supersolar average (N/O) of 2.6 ± 0.5, and suggest that nitrogen-enhanced plasma is widespread throughout the CGM. We find supersolar Ne abundance in two sightlines, also from the CGM. Similarly, we rule out the local origin of the hot thermal component and confirm that it is present beyond the shadowing clouds. Furthermore, we provide a revised model of the SDXB, which is crucial for extragalactic astronomy.