The Milky Way in X-rays for an outside observer

Abstract

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, L X ≥ 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 ≈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 ∼2−3 × 10 39 (LMXB) and ∼2−3 × 10 38 (HMXB) erg s −1 . Normalised to the stellar mass and the star formation rate, respectively, these correspond to ∼5 × 10 28 erg s −1 M −1 for LMXBs and ∼5 × 10 37 erg s −1 /(M 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 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.

HMXB, ULX and star formation

“…(ii) The galaxy has sufficiently high SFR/mass ratio, to ensure that the population of X-ray binaries is dominated by HMXBs and the LMXB contribution of can be safely ignored [5]. For the Milky Way we explicitly selected HMXBs, based on results of [7]. For each galaxy, the star formation rate was determined combining the results from conventional SFR indicators, [9] (FIR, UV, H α and radio).…”

Joint Discussion 6 Neutron stars and black holes in star clusters

“…The LMXB XLF of the Milky Way (Grimm et al 2002) becomes flatter at these lower luminosities. Gilfanov (2004) suggested a significant "universal" flattening below 5 × 10 37 erg/s in the LMXB XLF.…”

“…Each of these parameters is potentially related to the formation and evolution of XRBs in a given stellar population: the distribution of luminosities (slope) has been found to be related to the age of the population (see below); breaks in the XLF are a possible indication of multiple or evolving XRB populations in the same galaxy; the normalization is a measure of the total number of XRBs. Grimm et al (2002) first reported differences in the XLFs of different types of binaries, by deriving the "young-short-lived" high-mass X-ray binary (HMXB) and "old LMXB" XLFs for the Milky Way. They found that the HMXB XLF is well fitted by a single power-law, while the LMXB XLF may show both high-and low-luminosity breaks.…”

Cumulative luminosity functions of the X-ray point source population in M31

“…This is comparable with our α = 1.96 for 37.7 < log(L X erg s −1 ) < 38.2 in the Core luminosity function. Grimm et al (2002) studied the Milky Way and created luminosity functions for the high and low mass binary populations separately. Their cumulative luminosity functions had α = 0.64 for HMXBs and α = 0.26 for LMXBs.…”