The origin of the 6.4 keV line emission and H$_2$ ionization in the diffuse molecular gas of the Galactic center region

TL;DR

This study explores the origins of the 6.4 keV line emission and H₂ ionization in the Galactic Center, attributing the line to past X-ray activity of Sgr A* and H₂ ionization to cosmic rays.

Contribution

It distinguishes the ionization sources for Fe atoms and H₂ molecules, linking Fe ionization to Sgr A*'s past activity and H₂ ionization to cosmic rays, providing new insights into GC gas processes.

Findings

Fe atoms ionized by Sgr A*'s X-ray flares

H₂ ionized mainly by cosmic rays, not photons

Sgr A*'s past activity lasted hundreds of years

Abstract

We investigate the origin of the diffuse 6.4 keV line emission recently detected by Suzaku and the source of H_2ionization in the diffuse molecular gas of the Galactic Center (GC) region. We show that Fe atoms and H_2 molecules in the diffuse interstellar medium of the GC are not ionized by the same particles. The Fe atoms are most likely ionized by X-ray photons emitted by Sgr A* during a previous period of flaring activity of the supermassive black hole. The measured longitudinal intensity distribution of the diffuse 6.4 keV line emission is best explained if the past activity of Sgr A$* lasted at least several hundred years and released a mean 2-100 keV luminosity > 10^38} erg s^{-1}. The H_2 molecules of the diffuse gas can not be ionized by photons from Sgr A*, because soft photons are strongly absorbed in the interstellar gas around the central black hole. The molecular hydrogen in the GC region is most likely ionized by low-energy cosmic rays, probably protons rather than electrons, whose contribution into the diffuse 6.4 keV line emission is negligible.