On the origin of the iron fluorescent line emission from the Galactic Ridge

Abstract

The Galactic Ridge X-ray Emission (GRXE) spectrum has strong iron emission lines at 6.4, 6.7, and 7.0keV, each corresponding to the neutral (or low-ionized), He-like, and H-like iron ions. The 6.4keV 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 the origin of the cold fluorescent matter, we examined the contribution of the 6.4keV 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 6.4keV lines of hSSs are systematically higher than those of mCVs, such that the EWs of the merged hSSs and mCVs are 179-11+46eV and 93-3+20eV, respectively. The EW of hSSs compares favorably with the typical EWs of the 6.4keV line in the GRXE of 90–300eV depending on Galactic positions. Average 6.4keV line luminosities of the hSSs and mCVs are 9.2×1039 and 1.6×1039photonss−1, respectively, indicating that hSSs are intrinsically more efficient 6.4keV line emitters than mCVs. We estimated required space densities of hSSs and mCVs to account for all the GRXE 6.4keV line emission flux to be 2×10-7pc−3 and 1×10-6pc−3, respectively. We also estimated the actual 6.4keV line contribution from the mCVs with a known space density, which is as much as 20% of the observed GRXE flux, and for the hSSs, for which only five hSSs are known, we noted that they could contribute a significant percentage to the observed GRXE flux since we believe there is still more hSSs yet to be discovered in the Galaxy. We therefore conclude that the GRXE 6.4keV line flux could be significantly explained by hSSs and mCVs 6.4keV line flux.