Origin of the 6.4-keV line of the Galactic Ridge X-ray Emission

TL;DR

This study reveals that the 6.4-keV line in the Galactic Ridge X-ray Emission is asymmetrically distributed and correlates with cold interstellar medium, indicating diffuse fluorescence caused by ionizing particles across the Galactic plane.

Contribution

First high-statistics observation showing the asymmetric distribution of the 6.4-keV line and its correlation with cold ISM, supporting a diffuse fluorescence origin for the Galactic ridge X-ray emission.

Findings

6.4-keV line is asymmetrically distributed with respect to the Galactic center.

The 6.4-keV line flux correlates with molecular gas column density.

Diffuse fluorescence from cold ISM likely causes the 6.4-keV emission across the Galactic plane.

Abstract

We report the first results from high-statistics observation of the 6.4-keV line in the region of $l= +1.5^\circ$ to $+3.5^\circ$ (hereafter referred to as GC East), with the goal to uncover the origin of the Galactic ridge X-ray emission (GRXE). By comparing this data with that from the previous observations in the region $l=-1.5^\circ$ to $-3.5^\circ$ (hereafter referred to as GC West), we discovered that the 6.4-keV line is asymmetrically distributed with respect to the Galactic center, whereas the 6.7-keV line is symmetrically distributed. The distribution of the 6.4-keV line follows that of $^{13}$CO and its flux is proportional to the column density of the molecular gas. This correlation agrees with that seen between the 6.4-keV line and the cold interstellar medium (ISM) (H$_{\rm I}$ $+$ H$_2$) in the region $|l|>4^\circ$. This result suggests that the 6.4-keV emission is diffuse fluorescence from the cold ISM not only in GC East and West but also in the entire Galactic plane. This observational result suggests that the surface brightness of the 6.4-keV line is proportional to the column density of the cold ISM in the entire Galactic plane. For the ionizing particles, we consider X-rays and low energy cosmic-ray protons and electrons .