The Milky Way in X-rays for an outside observer

Log(N)-Log(S) and luminosity function of X-ray binaries from RXTE/ASM data

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
² Space Research Institute, Moscow, Russia

Corresponding author: H.-J. Grimm, grimm@mpa-garching.mpg.de

Received: 18 September 2001
Accepted: 3 June 2002

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,  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 ~ kpc of the Galaxy. In addition HMXBs are more concentrated towards the Galactic Plane (scale heights of ≈ and ≈  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 ~ (LMXB) and ~ (HMXB) erg s^-1. Normalised to the stellar mass and the star formation rate, respectively, these correspond to ~ erg s for LMXBs and ~ erg s^-1/( 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 ~. Although the average LMXB luminosity function shows a break near the Eddington luminosity for a 1.4  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.