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 .