The origin of the highly ionized iron emission (FeXXV at 6.7keV) characterizing the Galactic ridge X-ray emission (GRXE) remains a fundamental puzzle in high-energy astrophysics. Although the GRXE continuum is largely resolved into discrete populations of cataclysmic variables and coronally active stars, these sources exhibit Fe XXV equivalent widths significantly lower than that of the total GRXE, leaving the intense iron line emission unexplained. In this work, we cross-correlated the XMM-Newton survey of the inner Galactic disk with Gaia DR3 astrometry to identify and characterize hard X-ray sources (>2keV) with reliable stellar counterparts. We selected 107 X-ray sources located within the red giant branch of the color-magnitude diagram, many of which are verified long-period variables. These sources exhibit high X-ray luminosities (Lx~10^31^-10^33^erg/s), significantly exceeding the typical coronal saturation levels of single giants. Their X-ray spectra are notably harder than those of quiescent stellar coronae, with plasma temperatures reaching up to kT ~6keV and a prominent emission feature at ~6.7keV. The combination of high Lx, hard spectra, and intense FeXXV emission identifies this population as accretion-powered binaries associated with late-type giants. Our analysis demonstrates that this population contributes ~20% of the total GRXE continuum and ~40% of its iron line emission, providing a key component to resolving the Galactic X-ray background puzzle.

Cone search capability for table J/A+A/710/A243/tablea1 (Source properties for 107 SySt candidates)