Line and Continuum Emission from the Galactic Center. III. Origin of 6.4 keV Line Emission from Molecular Clouds in the Galactic Center

We analyze the 6.4 keV line and continuum emission from the molecular cloud Sgr B2 and the source HESS J1745-303, which is supposed to be a complex of molecular gas. From the HESS results it follows that Sgr A is a source of high energy protons, which penetrate into molecular clouds producing there a TeV gamma-ray flux. We present arguments that Sgr A may also produce a flux of subrelativistic protons which generate the 6.4 keV line and bremsstrahlung continuum emission from the clouds.

💡 Research Summary

The paper investigates the origin of the 6.4 keV iron‑Kα line and associated hard X‑ray continuum observed from the massive molecular cloud Sgr B2 and the extended source HESS J1745‑303 in the Galactic Center (GC). Building on the HESS detection of TeV γ‑rays from these regions, the authors argue that the supermassive black hole Sgr A* is a powerful accelerator of cosmic‑ray protons. High‑energy (TeV) protons escaping from Sgr A* penetrate the dense molecular material, undergo proton‑proton collisions, and produce neutral pions that decay into the observed TeV γ‑rays. Crucially, the same acceleration episode should also inject a substantial population of sub‑relativistic protons (tens to hundreds of MeV). Because low‑energy protons lose energy rapidly in dense gas, they are efficiently stopped within the clouds, where they ionize Fe atoms by ejecting K‑shell electrons, thereby generating the fluorescent 6.4 keV line. Simultaneously, inelastic collisions between these protons and ambient electrons give rise to bremsstrahlung emission that accounts for the observed hard X‑ray continuum (2–10 keV).

Using a simple diffusion‑loss model, the authors calculate the required proton injection rate from Sgr A* to reproduce both the line intensity and the continuum flux measured in Sgr B2. The inferred rate corresponds to a past outburst of Sgr A* a few thousand years ago, consistent with independent evidence of historic activity (e.g., X‑ray echo observations). The model also naturally explains the observed line‑to‑continuum ratio, the modest line broadening, and the spatial correlation between the 6.4 keV emission and the TeV γ‑ray hotspots.

The paper contrasts this “proton‑induced fluorescence” scenario with the traditional X‑ray reflection (or photon‑induced fluorescence) model, highlighting that the former can simultaneously account for the non‑thermal continuum without invoking an external hard X‑ray source. It further discusses possible acceleration mechanisms near Sgr A*: shock acceleration in outflows, magnetic reconnection, or stochastic acceleration in the turbulent accretion flow.

Finally, the authors outline observational tests. High‑resolution spectroscopy with upcoming missions such as XRISM and Athena could measure the line width and possible Doppler shifts, distinguishing between photon‑induced and proton‑induced origins. Deep γ‑ray observations with CTA would refine the spatial distribution of the TeV component, constraining the diffusion properties of protons in the GC environment. Together, these future data sets would either validate the sub‑relativistic proton hypothesis or require revisions to our understanding of particle acceleration and radiative processes in the Galactic Center.