The nature of the X-ray sources constituting the 6.7 keV Galactic ridge emission

Abstract

We reanalyze the deep Chandra X-ray observations near the Galactic center and show that reliable identifications of X-ray sources can be obtained with the Gaia EDR3 data to investigate which types of stellar sources are responsible for the X-ray emission observed from the Galactic ridge (GRXE). In the central 3 arcmin region 318 X-ray sources are detected, about one-third of which can be identified with objects listed in Gaia EDR3; however, only 22 objects have parallaxes and colors and can be placed into a color-magnitude diagram and thus be identified as coronal X-ray emitters. A rather large fraction of the X-ray sources cannot be identified with Gaia EDR3 entries, and we discuss the optical brightnesses of these sources. We analyze the counting events obtained in the 6.7 keV iron line spectral region and show that they are mainly caused by background events; however, 237 events can be associated with the detected X-ray sources, and we carry out an intensity measurement of the whole iron line complex. Our analysis shows that the mean energy of this iron line complex is located at a wavelength of ≈1.87 Å, where a variety of emission lines of iron ions in ionization stages FeXXIII–FeXXV are located; another line at 7.0 keV is only marginally detected, while the fluorescent 6.4 keV neutral iron line is clearly not seen. We demonstrate that only a few of the detected X-ray sources are responsible for the bulk of the observed iron line emission. We discuss to what extent coronal emission can be held responsible and demonstrate that M dwarfs and active binary systems like RS CVn systems do not significantly contribute to the observed emission; instead, it appears that the Galactic ridge emission is produced by optically fainter sources. Among the known population of cataclysmic variables, polars and dwarf novae appear to be the most promising candidates as main contributors to the GRXE.