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 Issue A&A Volume 391, Number 3, September I 2002 923 - 944 Galactic structure and dynamics http://dx.doi.org/10.1051/0004-6361:20020826

A&A 391, 923-944 (2002)
DOI: 10.1051/0004-6361:20020826

## 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
H.-J. Grimm1, M. Gilfanov1, 2 and R. Sunyaev1, 2

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

(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 ~ 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 ~ (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 ~ 2. 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.

Key words: X-rays: binaries -- X-rays: galaxies -- Galaxy: general -- Galaxy: structure -- galaxies: spiral -- stars: luminosity function

Offprint request: H.-J. Grimm, grimm@mpa-garching.mpg.de