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Issue A&A
Volume 452, Number 1, June II 2006
Page(s) 169 - 178
Section Galactic structure, stellar clusters, and populations
DOI http://dx.doi.org/10.1051/0004-6361:20054268



A&A 452, 169-178 (2006)
DOI: 10.1051/0004-6361:20054268

Origin of the Galactic ridge X-ray emission

M. Revnivtsev1, 2, S. Sazonov1, 2, M. Gilfanov1, 2, E. Churazov1, 2 and R. Sunyaev1, 2

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 $I_{\rm 3{-}20~ keV}$(10-11 erg s-1 cm-2 deg-2)/ $I_{\rm 3.5~\mu m}$(MJy/sr) = $0.26\pm0.05$, and the ratio of X-ray to near-infrared luminosity $L_{\rm 3-20~ keV}/L_{\rm 3-4 ~\mu m}= (4.1\pm0.3)\times10^{-5}$. The corresponding ratio of the 3-20 keV luminosity to the stellar mass is $L_{\rm x}/M= (3.5\pm0.5) \times 10^{27}$ erg s-1 $M_{\odot}^{-1}$, 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 ${\sim}10^{-16}$ erg s-1 cm-2 (2-10 keV) will need to be reached in future observations.


Key words: stars: binaries: general -- Galaxy: bulge -- Galaxy: disk -- X-rays: general -- X-rays: stars

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© ESO 2006

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