Iron Fluorescent Line Emission from the mCvs and Hard X-ray Emitting Symbiotic Stars as a Source of the Iron Fluorescent Line Emission from the Galactic Ridge

TL;DR

This study investigates the origin of the 6.4 keV iron fluorescent line in the Galactic Ridge X-ray Emission, finding that magnetic cataclysmic variables likely account for most of this emission, with symbiotic stars contributing as well.

Contribution

It provides the first detailed spectral analysis of hSSs and mCVs, quantifying their contributions to the 6.4 keV line in the GRXE, and compares their efficiencies and luminosities.

Findings

hSSs have higher equivalent widths of 6.4 keV lines than mCVs

mCVs can largely explain the 6.4 keV line flux in GRXE

hSSs may have a non-negligible contribution to the 6.4 keV emission

Abstract

The Galactic Ridge X-ray Emission (GRXE) spectrum has strong iron emission lines at 6.4, 6.7, and 7.0~keV, each corresponding to the neutral (or low-ionized), He-like, and H-like iron ions. The 6.4~keV fluorescence line is due to irradiation of neutral (or low ionized) material (iron) by hard X-ray sources, indicating uniform presence of the cold matter in the Galactic plane. In order to resolve origin of the cold fluorescent matter, we examined the contribution of the 6.4~keV line emission from white dwarf surfaces in the hard X-ray emitting symbiotic stars (hSSs) and magnetic cataclysmic variables (mCVs) to the GRXE. In our spectral analysis of 4~hSSs and 19~mCVs observed with Suzaku, we were able to resolve the three iron emission lines. We found that the equivalent-widths (EWs) of the 6.4~keV lines of hSSs are systematically higher than those of mCVs, such that the average EWs of hSSs and mCVs are $179_{-11}^{+46}$~eV and $93_{-3}^{+20}$~eV, respectively. The EW of hSSs compares favorably with the typical EWs of the 6.4~keV line in the GRXE of 90--300~eV depending on Galactic positions. Average 6.4~keV line luminosities of the hSSs and mCVs are $9.2\times 10^{39}$ and $1.6\times 10^{39}$~photons~s$^{-1}$, respectively, indicating that hSSs are intrinsically more efficient 6.4~keV line emitters than mCVs. We compare expected contribution of the 6.4 keV lines from mCVs with the observed GRXE 6.4 keV line flux in the direction of $(l,b) \approx (28.5\arcdeg, 0\arcdeg$). We conclude that almost all the 6.4 keV line flux in GRXE may be explained by mCVs within current undertainties of the stellar number densities, while contribution from hSSs may not be negligible.