EDP Sciences
Free Access
Volume 391, Number 3, September I 2002
Page(s) 923 - 944
Section Galactic structure and dynamics
DOI https://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)

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, $L_{\rm X}
\ge 10^{34} {-} 10^{35}$  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 $\approx$ 150 and $\approx$ 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 \times 10^{39}$ (LMXB) and ~ $ 2{-}3 \times
10^{38}$ (HMXB) erg s -1. Normalised to the stellar mass and the star formation rate, respectively, these correspond to ~ $ 5 \times
10^{28}$  erg s $^{-1}~M^{-1}_{\odot}$ for LMXBs and ~ $ 5\times
10^{37}$  erg s -1/( $M_{\odot}$ 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  $M_{\odot}$ 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

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