The Milky Way in X-rays for an outside observer

Abstract

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 - 1035 erg s-1, 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 ~ 3-4 kpc of the Galaxy. In addition HMXBs are more concentrated towards the Galactic Plane (scale heights of ~ 150 and ~ 410 pc 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 2-10 keV luminosities of all X-ray binaries in the Galaxy, averaged over 1996-2000, are ~ 2-3 x 1039 (LMXB) and ~ 2-3 x 1038 (HMXB) erg s-1. Normalised to the stellar mass and the star formation rate, respectively, these correspond to ~ 5 x 1028 erg s-1 M-1sun for LMXBs and ~ 5x 1037 erg s-1/(Msun yr-1) 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 1.4 Msun 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.