2002A&A...391..923G

We study the Log(N)-Log(S) and X-ray luminosity function in the 2-10 keV energy band, and the spatial (3-D) distribution of bright, L_X≥10³⁴-10³⁵erg/s, X-ray binaries in the Milky Way. In agreement with theoretical expectations and earlier results we found significant differences between the spatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. The volume density of LMXB sources peaks strongly at the Galactic Bulge whereas HMXBs tend to avoid the inner ∼3-4kpc of the Galaxy. In addition HMXBs are more concentrated towards the Galactic Plane (scale heights of ≃150 and ≃410pc for HMXB and LMXB correspondingly) and show clear signatures of the spiral structure in their spatial distribution. The Log(N)-Log(S) distributions and the X-ray luminosity functions are also noticeably different. LMXB sources have a flatter Log(N)-Log(S) distribution and luminosity function. The integrated 2-10keV luminosities of all X-ray binaries in the Galaxy, averaged over 1996-2000, are ∼2-3x10³⁹ (LMXB) and ∼2-3x10³⁸ (HMXB) erg/s. Normalised to the stellar mass and the star formation rate, respectively, these correspond to ∼5x10²⁸erg/s/M_☉ for LMXBs and ∼5x10³⁷erg/s/(M_☉/yr) for HMXBs. Due to the shallow slopes of the luminosity functions the integrated emission of X-ray binaries is dominated by the ∼5-10 most luminous sources which determine the appearance of the Milky Way in the standard X-ray band for an outside observer. In particular variability of individual sources or an outburst of a bright transient source can increase the integrated luminosity of the Milky Way by as much as a factor of ∼2. Although the average LMXB luminosity function shows a break near the Eddington luminosity for a 1.4M_☉ neutron star, at least 12 sources showed episodes of super-Eddington luminosity during ASM observations. We provide the maps of distribution of X-ray binaries in the Milky Way in various projections, which can be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.