Diffuse X-ray sky in the Galactic center

Abstract

Abstract The Galactic diffuse X-ray emission (GDXE) in the Milky Way Galaxy is spatially and spectrally decomposed into the Galactic center X-ray emission (GCXE), the Galactic ridge X-ray emission (GRXE), and the Galactic bulge X-ray emission (GBXE). The X-ray spectra of the GDXE are characterized by the strong K-shell lines of the highly ionized atoms, and the brightest lines are the K-shell transition (principal quantum number transition of n = 2 → 1) of neutral iron (Fe i-Kα), He-like iron (Fe xxv-Heα), and He-like sulfur (S xv-Heα). Accordingly, the GDXE is composed of a high-temperature plasma of ∼7 keV (HTP) and a low-temperature plasma of ∼1 keV, which emit the Fe xxv-Heα and S xv-Heα lines, respectively. The Fe i-Kα line is emitted from nearly neutral irons, and hence the third component of the GDXE is a cool gas (CG). The Fe i-Kα distribution in the GCXE region is clumpy (Fe i-Kα clump), associated with giant molecular cloud (MC) complexes (Sagittarius A, B, C, D, and E) in the central molecular zone. The origin of the Fe i-Kα clumps is the fluorescence and Thomson scattering from the MCs irradiated by past big flares of the supermassive black hole Sagittarius A*. The scale heights and equivalent widths of the Fe i-Kα, Fe xxv-Heα, and Fe xxvi-Lyα (n = 2 → 1 transition of H-like iron) lines are different among the GCXE, GBXE, and GRXE. Therefore, their structures and origins are separately examined. This paper gives an overview of the research history and the present understandings of the GDXE, while in particular focusing on the origin of the HTP and CG in the GCXE.