Where is the Supervirial Gas? III. Insights from X-ray Shadow Observations and a revised Model for the Soft Diffuse X-ray Background

TL;DR

This study uses Suzaku shadow observations to clarify the origins of specific X-ray emissions, confirming they originate from the Milky Way's circumgalactic medium and providing a revised model of the soft diffuse X-ray background.

Contribution

The paper demonstrates that the NVII and super-virial temperature emissions are from the CGM, not local sources, and introduces a revised model for the SDXB.

Findings

NVII emission is from the CGM, not local sources.

Super-solar N/O ratio (~2.6) indicates nitrogen-rich plasma in the CGM.

Hot thermal component is present beyond shadowing clouds, from the CGM.

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 NVII emission line, and (2) excess emission near 0.8-1.0 keV that is identified with an additional, super-virial 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 NVII emission, ruling out the local origin. We confirm that the Milky Way CGM contains nitrogen-rich plasma, with a super-solar average (N/O) of 2.6+-0.5, and suggest that nitrogen-enhanced plasma is widespread throughout the CGM. We find super-solar Ne abundance in two sighlines, 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 soft diffuse X-ray background, which is crucial for extragalactic astronomy.