K-shell Emission of Neutral Iron Line from Sgr B2 Excited by Subrelativistic Protons

TL;DR

This study analyzes the origin of the 6.4 keV iron line emission in the Galactic center, distinguishing between past X-ray illumination and proton impact, and predicts future variations based on cosmic ray diffusion.

Contribution

It introduces a method to differentiate X-ray and proton excitation of iron lines using equivalent width and time variation analysis, with predictions for future emission changes.

Findings

Proton impact can produce distinguishable iron line features.

The line intensity evolution depends on cosmic ray diffusion coefficients.

Predicted significant decrease in line emission from Sgr B2 within a few years.

Abstract

We investigated the emission of K$\alpha$ iron line from the massive molecular clouds in the Galactic center (GC). We assume that at present the total flux of this emission consists of time variable component generated by primary X-ray photons ejected by Sagittarius A$^\ast$ (Sgr A$^\ast$) in the past and a relatively weak quasi-stationary component excited by impact of protons which were generated by star accretion onto the central black hole. The level of background emission was estimated from a rise of the 6.4 keV line intensity in the direction of several molecular clouds, that we interpreted as a stage when the X-ray front ejected by Sgr A$^\ast$ entered into these clouds. The 6.4 keV emission before this intensity jump we interpreted as emission generated by subrelativistic cosmic rays there. The cross-section of K$\alpha$ vacancies produced by protons differs from that of electrons or X-rays. Therefore, we expect that this processes can be distinguished from the analysis of the equivalent width of the iron line and time variations of the width can be predicted. The line intensity from the clouds depends on their distance from Sgr A$^\ast$ and the coefficient of spacial diffusion near the Galactic center. We expect that in a few years the line intensity for the cloud G\,0.11$-$0.11 which is relatively close to Sgr A$^\ast$ will decreases to the level $\lesssim$ 10% from its present value. For the cloud Sagittarius B2 (Sgr B2) the situation is more intricate. If the diffusion coefficient $D\gtrsim 10^{27}$ cm$^2$ s$^{-1}$ then the expected stationary flux should be about 10% of its level in 2000. In the opposite case the line intensity from Sgr B2 should drop down to zero because the protons do not reach the cloud.