Origin of the Galactic ridge X-ray emission

We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that the GRXE intensity follows the Galactic near-infrared surface brightness closely and thus traces the Galactic stellar mass distribution. The GRXE consists of two spatial components that can be identified with the bulge/bar and the disk of the Galaxy. The parameters of these components determined from X-ray data are compatible with those derived from near-infrared data. The inferred ratio of X-ray to near-infrared surface brightness I_3{-20 keV}(10^-11 erg s^-1 cm^-2 deg^-2)/I_{3.5 μ m}(MJy/sr) = 0.26±0.05, and the ratio of X-ray to near-infrared luminosity L_{3-20 keV}/L_{3-4 μ m}= (4.1±0.3)×10^-5. The corresponding ratio of the 3-20 keV luminosity to the stellar mass is L_x/M= (3.5±0.5) × 10²⁷ erg s^-1 M_⊙^-1, which agrees within the uncertainties with the cumulative emissivity per unit stellar mass of point X-ray sources in the Solar neighborhood, determined in an accompanying paper (Sazonov et al.). This suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly cataclysmic variables and coronally active binaries. The fractional contributions of these classes of sources to the total X-ray emissivity determined from the Solar neighborhood data can also explain the GRXE energy spectrum. Based on the luminosity function of local X-ray sources we predict that, in order to resolve 90% of the GRXE into discrete sources, a sensitivity limit of ∼10^-16 erg s^-1 cm^-2 (2-10 keV) will need to be reached in future observations.