Origin of the Galactic ridge X-ray emission

Abstract

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 I3-20 keV(10-11 erg s-1 cm-2 deg -2)/I3.5 μm(MJy/sr) = 0.26 ± 0.05, and the ratio of X-ray to near-infrared luminosity L3.20 keV/L3-4 μm = (4.1 ± 0.3) × 10-5. The corresponding ratio of the 3-20 keV luminosity to the stellar mass is Lx/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. © ESO 2006.