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

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 |b| > 10° to construct an X-ray luminosity function (XLF) of nearby X-ray sources in the range 10 30 erg s-1 < Lx < 1034 erg s-1 (where Lx is the luminosity over 2-10 keV), occupied by coronally active binaries (ABs) and cataclysmic variables (CVs). We then extended this XLF down to Lx ∼ 1027.5 erg 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 ± 0.8) × 1027 and (1.1 ± 0.3) × 1027 erg s-1 M·-1, respectively. In addition to ABs and CVs representing old stellar populations, young stars locally emit (1.5 ± 0.4) × 1027 erg s -1 M·-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 × 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. © ESO 2006.