Enhancement of the 6.4 keV line in the inner Galactic ridge: Proton-induced fluorescence?

TL;DR

The paper investigates the excess 6.4 keV iron line emission in the inner Galactic ridge, suggesting low-energy proton bombardment as a plausible origin over other scenarios, based on spatial and spectral analysis.

Contribution

It provides evidence for proton-induced fluorescence as the source of the 6.4 keV line excess in the Galactic center region.

Findings

Excess neutral iron line correlates with molecular gas density.

Spectrum shows large equivalent width inconsistent with electron bombardment.

Distribution favors proton bombardment over X-ray irradiation.

Abstract

A common idea for the origin of the Galactic diffuse X-ray emission, particularly that of the iron lines from neutral and highly ionized atoms, is a superposition of many cataclysmic variables and coronally active binaries. In this scenario, the flux should symmetrically distribute between the east and west on the plane with respect to Sagittarius A$^*$ because the stellar mass distribution determined by infrared observations is nearly symmetric. This symmetry is confirmed for the highly ionized iron line as well as the continuum emission. However, a clear excess of the neutral iron line in the near east of the Galactic center compared to the near-west side is found. The flux distribution of the excess emission well correlates with molecular column density. The X-ray spectrum of the excess emission is described by a power-law continuum plus a 6.4 keV line with the large equivalent width of $\sim1.3$ keV, which is hardly explained by the low-energy electron bombardment scenario. The longitudinal and latitudinal distribution of the excess emission disfavors the X-ray irradiation, neither by Sagittarius A$^*$ nor by nearby X-ray binaries. Then the low-energy proton bombardment is the most probable origin although the high energy density $\sim 80$ eV cm$^{-3}$ in 0.1-1000 MeV is required and there is no conventional proton source in the vicinity.