Discovery of 6.4 keV line and soft X-ray emissions from G323.7$-$1.0 with Suzaku

TL;DR

This study analyzes Suzaku X-ray data of the SNR candidate G323.7-1.0, discovering a 6.4 keV iron line and soft X-ray enhancements, suggesting cosmic-ray interactions rather than photoionization as the origin.

Contribution

First detection of a 6.4 keV line and soft X-ray enhancements in G323.7-1.0, indicating cosmic-ray induced K-shell ionization in a supernova remnant candidate.

Findings

6.4 keV line detected at 4.1 sigma significance

Large equivalent width suggests cosmic-ray proton ionization

Soft X-ray emission associated with the shell structure

Abstract

In this paper, the Suzaku X-ray data of the Galactic Supernova Remnant (SNR) candidate G323.7$-$1.0 are analyzed to search for X-ray emission. Spatially-extended enhancements in the 6.4 keV line and in soft X-rays are found inside or on the radio shell. The soft X-ray enhancement would be the hottest part of the shell-like X-ray emission along the radio shell. The 6.4 keV line enhancement is detected at a significance level of $4.1 \sigma$. The lower limit of the equivalent width (EW) is 1.2 keV. The energy centroid of the 6.4 keV line is $6.40 \pm 0.04$ keV, indicating that the iron is less ionized than the Ne-like state. If the 6.4 keV line originates from ionizing non-equilibrium thermal plasma, presence of iron-rich ejecta in a low-ionization state is required, which is disfavored by the relatively old age of the SNR. The 6.4 keV line enhancement would be due to K-shell ionization of iron atoms in a dense interstellar medium by high-energy particles. Since there is no irradiating X-ray source, the origin of the 6.4 keV line enhancement is not likely the photoionization. The large EW can only be explained by K-shell ionization due to cosmic-ray protons with an energy of $\sim 10$ MeV, which might be generated by the shock acceleration in G323.7$-$1.0.