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

摘要

We study the Log(N)-Log(S) and X-ray luminosity function in the 2-10 keVenergy band, and the spatial (3-D) distribution of bright, log(L_X) > 34-35erg/s, X-ray binaries in the Milky Way. In agreement with theoreticalexpectations and earlier results we found significant differences between thespatial distributions of low (LMXB) and high (HMXB) mass X-ray binaries. Thevolume density of LMXB sources peaks strongly at the Galactic Bulge. HMXBs tendto avoid the inner 3-4 kpc of the Galaxy, HMXBs are more concentrated towardsthe Galactic Plane and show clear signatures of the spiral structure in theirspatial distribution. LMXB sources have a flatter Log(N)-Log(S) distributionand luminosity function than HMXBs. The integrated 2-10 keV luminosities ofX-ray binaries, averaged over 1996--2000, are 2-3 * 10^39 (LMXB) and 2-3 *10^38 (HMXB) erg/s. Normalised to the stellar mass and the star formation rate,respectively, these correspond to 5 * 10^28 erg/s/M_sol for LMXBs and 5 * 10^37erg/s/(M_sol/yr) for HMXBs. Due to the shallow slopes of the luminosityfunctions the integrated emission of X-ray binaries is dominated by the 5-10most luminous sources which determine the appearance of the Milky Way in thestandard X-ray band for an outside observer. In particular variability ofindividual sources or an outburst of a bright transient source can increase theintegrated luminosity of the Milky Way by as much as a factor of ~2. Althoughthe average LMXB luminosity function shows a break near the Eddingtonluminosity for a 1.4 M_sol neutron star, at least 11 sources showed episodes ofsuper-Eddington luminosity during ASM observations. We provide the maps ofdistribution of X-ray binaries in the Milky Way in various projections, whichcan be compared to images of nearby galaxies taken by CHANDRA and XMM-Newton.