Origin of the Galactic ridge X-ray emission

¹ Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: mikej@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

Received: 28 September 2005
Accepted: 31 January 2006

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 (10^-11 erg s^-1 cm^-2 deg^-2)/(MJy/sr) = , and the ratio of X-ray to near-infrared luminosity . The corresponding ratio of the 3–20 keV luminosity to the stellar mass is  erg s^-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  erg s^-1 cm^-2 (2–10 keV) will need to be reached in future observations.