Discovery of 6.4 keV line and soft X-ray emissions from G323.7$-$1.0 with Suzaku
Shigetaka Saji Hironori Matsumoto Masayoshi Nobukawa Kumiko K. Nobukaw Hideki Uchiyama Shigeo Yamauchi Katsuji Koyama
astro-ph.HE






1Division of Particle and Astrophysical Science,
Graduate School of Science,
 Nagoya University,
Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan
2Department of Earth and Space Science, Graduate School of Science, Osaka University,

 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan
3Faculty of Education, Nara University of Education, Takabatake-cho, Nara, Nara 630-8528, Japan
4Department of Physics, Faculty of Science, Nara Women's University, Kitauoyanishi-machi, Nara, Nara 630-8506, Japan
5Faculty of Education, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka, Shizuoka 422-8529, Japan
6Department of Physics, Graduate School of Science, Kyoto University, Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan

s saji@u.phys.nagoya-u.ac.jp

X-rays: ISM — ISM: supernova remnants —  ISM: individual (G323.7-1.0) — cosmic rays

Discovery of 6.4 keV line and soft X-ray emissions from G323.7-1.0 with Suzaku 
    Shigetaka Saji,1, 2*
Hironori Matsumoto,2
Masayoshi Nobukawa,3
Kumiko K. Nobukawa,4
Hideki Uchiyama,5
Shigeo Yamauchi,4

Katsuji Koyama,6

    December 30, 2023
==============================================================================================================================================





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 4.1σ.
The lower limit of the equivalent width (EW) is 1.2 keV.
The energy centroid of the 6.4 keV line is 6.40 ± 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 ∼ 10 MeV, which might be generated by the shock acceleration in G323.7-1.0.





§ INTRODUCTION

Many Supernova Remnants (SNRs) show iron Kα emission lines.
Since the energy centroids of the lines are lower than 6.7 keV in general, the iron is less ionized than the helium-like state <cit.>. 
These line emissions are thought to originate from thermal processes in the ionizing plasma.
<cit.> reported a peculiar 6.4 keV line from Kes 79. 
The spatial distribution of the line emission is correlated with a molecular cloud rather than the thermal X-ray radiation. 
Thus, the 6.4 keV line emission from Kes 79 is unlikely to originate from thermal processes. 
<cit.> also discovered a hint of the 6.4 keV emission line from 3C 391, which can be associated with molecular clouds.
Such a 6.4 keV line was also reported from five Galactic SNRs even though the electron temperature of their thermal plasma is lower than ∼ 1 keV <cit.>. Since almost no iron K-shell lines are emitted from such low-temperature plasma, the 6.4 keV line emission is thought to be generated by non-thermal processes such as interactions between low-energy cosmic-ray particles and adjacent cold gases, or photoionization.
Because of the large equivalent width (EW) of ≳ 400 eV and the absence of X-ray irradiating sources, the authors claimed that the low-energy cosmic-ray proton with the ∼MeV energy is the most plausible origin of the 6.4 keV line.

G323.7-1.0 is an SNR candidate discovered in the radio band by Molonglo Galactic Plane Survey <cit.>. The radio image shows an extremely faint oval shell with a size of ∼51'×38'.
Because of its large and faint radio shell, G323.7-1.0 would be an old SNR.
GeV and TeV gamma-ray emissions coincident with G323.7-1.0 have been discovered by Fermi and High Energy Stereoscopic System (H.E.S.S.) (<cit.>; <cit.>), respectively.
The distance to the gamma-ray source was estimated to be ∼ 5 kpc <cit.>.
No significant X-rays have been detected from G323.7-1.0 so far. We, therefore, searched for X-ray emission, thermal
and/or non-thermal, from G323.7-1.0 with Suzaku <cit.>. Then, we discovered spatially-extended 6.4 keV line emission. Although there is soft thermal emission which is not associated with the 6.4 keV line, 
no plasma hot enough to emit the Fe K lines is found from any region. Thus, the origin of the 6.4 keV line is due to non-thermal processes.
This paper discusses details of the 6.4 keV line emission and its origin. 
In this paper, the uncertainties are shown at the 90% confidence level, while the errors on the data points of spectra are given at the 1σ level.



§ OBSERVATION AND DATA REDUCTION



Suzaku observed four regions of G323.7-1.0. Figure <ref> shows the fields of view of the X-ray Imaging
Spectrometer (XIS) on the 843 MHz radio image of G323.7-1.0.
The Suzaku observations caught the eastern part of the radio shell. 
Suzaku did not observe the brightness peaks of the GeV and TeV gamma-ray emissions as shown with the white cross point
and the contours in figure <ref>, respectively.
The details of the observations are summarized in table <ref>.

The XIS consists of four sets of CCD cameras
(XIS0-3) <cit.>.  Each CCD chip is
placed on the focal plane of the X-Ray Telescope (XRT;
<cit.>). XIS0, 2 and 3 employ
front-illuminated (FI) CCDs, while XIS1 does a
back-illuminated (BI) CCD. The CCD has 1024 by 1024 pixels, corresponding to 17.8'×17.8'. Since XIS2 suffered serious
damage, it has not been usable since 2006 November 9. 
In the observations of table <ref>, the XIS was operated in the normal
clocking mode with the Spaced-row Charge Injection 
<cit.>. 





We analyzed the archival data with the , using  and the calibration database () released on 2016 Jun 7. 
The XIS pulse-height data for each X-ray event were converted to Pulse Invariant (PI) channel using the software  and the . 
The data taken at the South Atlantic Anomaly and low elevation
angles of < 5D from the night-earth rim and of < 20D
from the day-earth rim were excluded.
The ancillary response files and the redistribution matrix files were produced using   and  packages of , respectively.



§ ANALYSIS AND RESULTS

 




 §.§ Image



In order to search for X-ray sources in the G323.7-1.0 field,
XIS images in the soft (0.5–3 keV), hard (5–8 keV) and 6.3–6.5 keV bands were made. 
XIS1 (BI-CCD) data were not used to make the images above 5 keV, since the signal-to-noise ratio in the hard band is worse than that of the FI-CCDs.
The corners of the CCD chips illuminated by the ^55Fe calibration sources were excluded from the images above 5 keV.
The Non-X-ray Backgrounds (NXBs) were estimated from the data within ± 300 days of the observation using  <cit.>,
and were subtracted from the images. 
After the NXB subtraction, each image was divided by an exposure map simulated using the XRT+XIS simulator  <cit.> for vignetting corrections. 

Three panels in figure <ref> show the images thus obtained in the soft, hard and 6.3–6.5 keV bands.
The soft and hard-band images are binned with 8 × 8 pixels and then smoothed with a Gaussian function of σ = 5 bins. The 6.3–6.5 keV band image is binned with 32 × 32 pixels and smoothed with a Gaussian function of σ = 10 bins.

In the soft-band image, there are several point-like sources marked with solid small circles, while
a diffuse structure, indicated by the largest solid circle with a diameter of 8.4', is found in the southern field. 
This diffuse structure is called “the soft source" hereafter.
On the other hand, there is no prominent structure in the hard band except for one point source that is also seen in the soft band.
In the 6.3–6.5 keV band, some structures are seen as marked with dashed curves.
Because of their angular sizes of ≳ 5', which is larger than the half-power diameter of XRT (∼ 2'), they should not be point sources, but diffuse sources.
These sources are hereafter called “the 6.4 keV clumps”.






 §.§ Spectrum





  §.§.§ The 6.4 keV clumps

In order to investigate the 6.4 keV clumps, the “enhance region" was defined as illustrated with dashed curves in figure <ref>c. 
The whole field excluding the soft source, the point sources and the enhance region was defined as the reference region.
XIS0 and 3 spectra were extracted from the enhance and reference regions. 
 Although the enhance region consists of four parts, the data extracted from all areas were added to improve the statistics.
The NXBs were estimated using the data within ±300 days of the observation, and were subtracted from the enhance and reference spectra.

Since G323.7-1.0 is located on the Galactic plane (b = -1.D0), there should be emission lines of the Galactic Ridge X-ray Emission (GRXE). The GRXE exhibits lines of neutral iron (Fe I) Kα, helium-like iron (Fe XXV) Kα, hydrogen-like iron (Fe XXVI) Kα and Fe I Kβ at 6.40 keV, 6.68 keV, 6.97 keV and 7.06 keV, respectively (e.g., <cit.>). Thus we fitted the 5–8 keV band spectra with a model consisting of a power-law and four Gaussian functions. The energy centroids of the Gaussian functions were fixed to be 6.40 keV, 6.68 keV, 6.97 keV and 7.06 keV. The intensity of the Fe I Kβ line was set to 0.125 times that of the Fe I Kα line <cit.>, while those of the other lines were free parameters. The width of the Gaussian functions were fixed to be zero, except for the Fe XXV Kα line. Since the Fe XXV Kα line is a blend of the resonance, inter-combination and forbidden lines, the line width was fixed to be 23 eV based on <cit.>. 
Both of the photon index and the normalization of the power-law function were allowed to vary.

Figures <ref> show the spectra extracted from the enhance region and the reference region with the best-fit models.
Both of the model fittings are statistically acceptable with χ^2_ red = 0.84 (38 d.o.f.) for the enhance region and χ^2_ red = 0.88 (46 d.o.f.) for the reference region.
The best-fit parameters are summarized in table <ref>.
The 6.4 keV line intensity of the enhance region is significantly higher than that of the reference region. The significance level of the excess is 4.1σ.
On the other hand, the other parameters are consistent between the two regions.




Next we estimated the line intensities of the GRXE at the position of G323.7-1.0, (l, b) = (323.D7, -1.D0).
Using the longitude profile of <cit.> and the latitude profile of <cit.>, the GRXE line intensities are calculated to be 3.1 × 10^-9 (Fe I Kα), 1.5 × 10^-8 (Fe XXV Kα) and 1.6 × 10^-9 (Fe XXVI Kα) photons cm^-2 s^-1 arcmin^-2.
Figure <ref> shows the comparison of the line intensities between the enhance region, the reference region and the GRXE model. The intensity of the 6.4 keV line of the enhance region is significantly higher than the Fe I Kα line of the GRXE, while that of the reference region is consistent with the GRXE. The other two lines are almost consistent with the GRXE model, although the Fe XXVI Kα intensity of the reference region is slightly higher than the model.
Thus the 6.4 keV line enhancement cannot be explained by the GRXE.






Then, we calculated the EW of the 6.4 keV line enhancement. 
To estimate the enhanced component, the spectrum of the reference region was subtracted from that of the enhance region after correcting the difference of the area taking account of the vignetting.
Figure <ref> shows the spectrum of the enhance region after the subtraction. The spectrum was fitted with a model consisting of a power-law and two Gaussian functions. The energy centroids of the Gaussian functions were fixed to be 6.40 keV and 7.06 keV assuming the Fe I Kα and the Fe I Kβ lines, respectively. The intensity of the Fe I Kβ line was set to 0.125 times that of the Fe I Kα line. The width of each Gaussian function was fixed to be zero.  Both of the photon index and the normalization of the power-law component were free parameters.
The flux of the power-law component is consistent with zero.
The lower limit of the EW of the 6.4 keV line is 2.3 keV with a confidence level of 90 %. 
The continuum level and hence the EW are sensitive to the vignetting correction.
The lower limit of the EW of the 6.4 keV line is 1.2 keV when the uncertainty of the vignetting correction is assumed to be 5% according to Suzaku technical description[https://heasarc.gsfc.nasa.gov/docs/suzaku/prop_tools/suzaku_td].
When the energy centroid of the Gaussian function for the 6.4 keV line is allowed to vary, it becomes 6.40 ± 0.04 keV. The energy centroid is consistent with that of the Fe I Kα line.





  §.§.§ The soft source
 
In order to investigate the soft source, we extracted an XIS0 + 3 spectrum from the largest solid circle in figure <ref>a. 
The spectrum of the reference region was used as the background.
Figure <ref> shows the background-subtracted spectrum. 

Although the spectrum has poor statistics, a hint of an emission line can be seen at ∼ 1.4 keV. We, then, tried fitting the spectrum with an optically-thin thermal plasma model (APEC) suffering from an interstellar absorption (phabs) fixing the metal abundance of a solar value <cit.>. 
The model represents the spectrum well with the χ^2_ red = 0.44 (8 d.o.f.).
We obtained the plasma temperature kT=1.1^+0.5_-0.3 keV and the absorption column density N_ H=(1.8±0.6)× 10^22 cm^-2 as well as the normalization 2.8^+1.8_-1.1× 10^-4.
The normalization is the volume emission measure represented by 10^-14 n_ en_ H V/4π d^2, where n_ e, n_ H, V, and d are the electron density, the hydrogen density, the plasma volume, and the distance from the solar system, respectively.
The observed 1–8 keV flux is estimated to be 8.4× 10^-14 erg s^-1 cm^-2.






§ DISCUSSION
 


 §.§ The soft source

We constrained the interstellar absorption toward the soft source of N_ H=(1.8 ± 0.6)× 10^22 cm^-2. 
Assuming an averaged number density of 1H cm^-3, the absorption column density corresponds to a distance of 6 ± 2 kpc,
which is consistent with the distance d=5 kpc of the gamma-ray source coincident with G323.7-1.0 <cit.>.
Therefore, the soft source possibly belongs to G323.7-1.0, and is a thermal plasma generated by the shock heating.
The source radius is calculated to be 6 (d/5 kpc) pc based on the angular radius of ∼ 4.'2.
Using the plasma temperature and the volume emission measure (see section <ref>), 
the density and the total mass are calculated to be 0.06 cm^-3 and 1.9 M_⊙, respectively.
Here, the plasma volume is estimated assuming a sphere with the radius of 6 pc.

The soft source is located near the southeastern shell of G323.7-1.0 (<cit.>, figure <ref>a).
The size of the soft source is ∼8.'4, which is far smaller than the entire size of the radio shell of G323.7-1.0 (∼51'×38'; <cit.>).
If G323.7-1.0 is an old SNR and the age t is larger than 10000 yr, the plasma temperature is roughly estimated to be 0.3 (t/10^4 yr)^-6/5 keV using the Sedov solution assuming the explosion energy of 1 × 10^51 erg and the number density of the interstellar matter of 1H cm^-3. Soft X-rays from such low temperature plasma are difficult to be detected because of the interstellar absorption (N_ H∼ 2× 10^22 cm^-2). 
Shock heated plasma would make a shell-like soft X-ray emission along the radio shell. The soft source would be the hottest part of the shell emission, and the other regions would be unseen due to the large interstellar absorption.




 §.§ The 6.4 keV clumps

Thermal plasma in a non-equilibrium ionizing (NEI) state can emit the iron K-shell line at ∼ 6.4 keV. 
When we assume the solar abundance of <cit.> and the electron temperature of < 5 keV, the EW of the iron Kα line from the NEI plasma is calculated to be smaller than ∼ 300 eV using the NEI code in XSPEC <cit.>.
The iron abundance higher than ∼ 4 solar is required to explain the observed EW. Such high abundance can only be realized by iron-rich ejecta.
The upper limit of the observed line centroid is 6.44 keV, corresponding to an ionization degree of the Ne-like state (<cit.>; <cit.>; <cit.>).
If the 6.4 keV line comes from the NEI plasma, the ionization timescale is estimated to be t ≲ 300 (n_ e/1 cm^-3)^-1 yr using the NEI code. Here, n_ e is the electron number density in the NEI plasma.
The ejecta in G323.7-1.0 cannot be in such a low-ionization state since G323.7-1.0 has a large and extremely faint radio shell and hence would not be a young SNR. Indeed, <cit.> reported that only young SNRs exhibit such a low-ionization state using the Suzaku archive data of many SNRs. Also, the oldest SNR which shows the energy centroid of < 6.44 keV is RCW 86, the remnant of SN 185 <cit.>. 
Thus, the thermal plasma is unlikely as the origin of the observed 6.4 keV line.

Then, the 6.4 keV line emission is explained by non-thermal processes such as K-shell ionization by photons, non-thermal electrons or protons with the energy above the Fe K-edge (7.1 keV). In the following, we discuss which particle is the dominant source for the observed 6.4 keV line.

In the case of the photoionization, a bright X-ray source is required in the vicinity of G323.7-1.0.
Cir X-1, which is an X-ray binary hosting a neutron star, is located ∼ 2 degrees away from G323.7-1.0.
The distance from the solar system is 4.1 kpc <cit.>.
Thus, the minimum distance between G323.7-1.0 and Cir X-1 is 4.1  sin (2D) kpc = 140 pc.
Since the angular size of the largest 6.4 keV clump is ∼ 8', the actual size of the clump is ∼ 10 pc assuming that the distance from the solar system to G323.7-1.0 is the same as that to Cir X-1, 4.1 kpc.
Then, the hydrogen number density of the clump is estimated to be ∼ 600 cm^-3 based on an empirical relation between the density and size of n_ H = 180(D/40  pc)^-0.9 cm^-3 <cit.>, where D is the size of the clump. Note that there are no available data of ^12CO or HI with enough angular resolution to estimate the density of the clump.
Using the photon index of Cir X-1 (Γ = 2.6; <cit.>) and the cross-sections for the photoionization <cit.>, a luminosity of ∼ 2.2 × 10^39 erg s^-1 in the 0.1–100 keV band is required for Cir X-1 to generate the observed 6.4 keV line intensity. Since the luminosity exceeds the Eddington luminosity for a neutron star (∼ 2 × 10^38 erg s^-1) by an order of magnitude, the photoionization is unlikely to be the significant origin of the 6.4 keV line. 

A scenario that the iron in the cold and dense cloud is ionized by electrons with an energy of 10–100 keV might also be possible (e.g., <cit.>; <cit.>).
In this process, strong continuum emission must be generated by bremsstrahlung.
The EW of the iron Kα line for the bremsstrahlung is ∼ 400 eV at most assuming a solar abundance <cit.>. Since the lower limit of the EW of the observed 6.4 keV line is 1.2 keV, an anomalous large iron abundance of 3 solar or more is required. 

The 6.4 keV line can also be produced via ionization by protons with an energy of ∼ 10 MeV. 
In this process, the EW of the Fe I Kα emission is ∼ 1 keV or higher for gas with a solar abundance <cit.>. 
Thus the observed EW (>1.2 keV) can be well explained with the proton origin.
The Fe I Kα intensity is calculated by

    I_6.4 keV = 1/4π∫σ_6.4 keV  v  n_ H  l  Z_ Fe dN/dE  dE,

where σ_6.4 keV is the proton's cross section for the Fe I Kα emission <cit.>, v is the velocity of the proton, n_ H is the hydrogen number density of the clump, l is the thickness of the clump along the line-of-sight, Z_ Fe is the iron abundance of the clump, and dN/dE is the spectral number distribution of the proton.
Since the angular size of the largest 6.4 keV clump is ∼ 8', the thickness of the clump along the line-of-sight is assumed to be l = d  sin(8'). Here d is the distance from the solar system to the clump. 
When the spectrum of the proton is set to be dN/dE ∝ E^-2.7 assuming the Galactic cosmic-rays <cit.>, a proton energy density of ∼ 300 (100  cm^-3/n_ H)(5  kpc/d) eV cm^-3 in the 0.1–1000 MeV band is required to generate the observed 6.4 keV line intensity. 
Here the abundance of the iron Z_ Fe is assumed to be that of the Sun <cit.>.
This energy density is much higher than the canonical value for the Galactic cosmic-ray (∼ 1 eV cm^-3; <cit.>). Those protons are thought to be accelerated in G323.7-1.0.

The 6.4 keV clumps are located around or inside the faint radio shell (figure <ref>c).
The shell would be the diffusive shock acceleration cite of protons.
Higher-energy protons easily escape from the acceleration site (the SNR shell region), 
since their mean free paths are longer than those of lower-energy ones <cit.>.
Then the escaped high-energy protons would hit a high-density cloud located far from the shell and produces high-energy gamma-rays.
In fact, the brightest peak of the gamma-ray source is about 20' away from the 6.4 keV 
clumps (see figures <ref> and <ref>c).
The intensities of the 6.4 keV line and the high-energy gamma-rays depend on the density of low-energy protons and high-energy protons, respectively, as well as neutral gas density. More qualitative discussions on possible correlations between the 6.4 keV line and the gamma-ray intensities require detailed studies on the 6.4 keV line, gamma-rays and ^12CO or HI with an angular resolution better than a few arcmin. These would be future projects.





§ ACKNOWLEDGMENTS

SS is supported by Grant-in-Aid for Japan Society for the Promotion of Science (JSPS) Fellows number JP17J11471.
This work is also supported by MEXT KAKENHI Grant Number JP15H02070 (HM), JP25887028 (HU), JP15H02090, JP17K14289 (MN), and JSPS Number JP16J00548 (KKN).
KKN is supported by Research Fellowships of JSPS for Young Scientists. 



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