X-ray luminosity function of faint point sources in the Milky Way

S. Sazonov; M. Revnivtsev; M. Gilfanov; E. Churazov; R. Sunyaev

doi:doi:10.1051/0004-6361:20054297

Free access

Issue		A&A Volume 450, Number 1, April IV 2006


Page(s)		117 - 128
Section		Galactic structure, stellar clusters, and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20054297

A&A 450, 117-128 (2006)
DOI: 10.1051/0004-6361:20054297

X-ray luminosity function of faint point sources in the Milky Way

S. Sazonov^{1, 2}, M. Revnivtsev^{1, 2}, M. Gilfanov^{1, 2}, E. Churazov ^{1, 2} and R. Sunyaev^{1, 2}

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany
e-mail: sazonov@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia

(Received 3 October 2005 / Accepted 15 December 2005 )

Abstract
We assessed the contribution to the X-ray (above 2 keV) luminosity of the Milky Way by different classes of low-mass binary systems and single stars. We began by using the RXTE Slew Survey of the sky at $\vert b\vert>10^\circ$ to construct an X-ray luminosity function (XLF) of nearby X-ray sources in the range $10^{30}~{\rm erg}~{\rm s}^{-1}<L_{\rm x}<10^{34}~{\rm erg}~{\rm s}^{-1}$ (where $L_{\rm x}$ is the luminosity over 2-10 keV), occupied by coronally active binaries (ABs) and cataclysmic variables (CVs). We then extended this XLF down to $L_{\rm x}\sim 10^{27.5}~{\rm erg}~{\rm s}^{-1}$ using the Rosat All-Sky Survey in soft X-rays and available information on the 0.1-10 keV spectra of typical sources. We found that the local cumulative X-ray (2-10 keV) emissivities (per unit stellar mass) of ABs and CVs are $(2.0\pm 0.8)$ $\times$ 10²⁷ and $(1.1\pm 0.3)$ $\times$ 10²⁷ erg s^-1 $M_\odot^{-1}$ , respectively. In addition to ABs and CVs representing old stellar populations, young stars locally emit $(1.5\pm 0.4)$ $\times$ 10²⁷ erg s $^{-1}~M_\odot^{-1}$ . Finally to the XLF of ABs and CVs we attached a high luminosity branch (up to ~10³⁹ erg s^-1) composed of neutron-star and black-hole low-mass X-ray binaries (LMXBs), derived in previous work. The combined XLF covers ~12 orders of magnitude in luminosity. The estimated combined contribution of ABs and CVs to the 2-10 keV luminosity of the Milky Way is ~2 $\times$ 10³⁸ erg s^-1, or ~3% of the integral luminosity of LMXBs averaged over nearby galaxies. The XLF obtained in this work is used elsewhere to assess the contribution of point sources to the Galactic ridge X-ray emission.

Key words: stars: luminosity function, mass function -- Galaxy: structure -- X-rays: binaries -- X-rays: galaxies -- X-rays: stars

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