Low-mass X-ray binaries and globular clusters in early-type galaxies. I. Chandra observations.
HUMPHREY P.J. and BUOTE D.A.
Abstract (from CDS):
We present a Chandra survey of LMXBs in 24 early-type galaxies. Correcting for detection incompleteness, the X-ray luminosity function (XLF) of each galaxy is consistent with a power law with negative logarithmic differential slope, β∼2.0. However, β strongly correlates with incompleteness, indicating the XLF flattens at low-LX. The composite XLF is well fitted by a power law with a break at 2.21+0.65–0.56x1038 ergs/s and β=1.40+0.10–0.13 and 2.84+0.39–0.30 below and above it, respectively. The break is close to the Eddington limit for a 1.4 M☉ neutron star, but the XLF shape rules out its representing the division between neutron star and black hole systems. Although the XLFs are similar, we find evidence of some variation between galaxies. The high-LX XLF slope does not correlate with age, but may correlate with [α/Fe]. Considering only LMXBs with LX>1037 ergs/s, matching the LMXBs with globular clusters (GCs) identified in HST observations of 19 of the galaxies, we find the probability a GC hosts an LMXB is proportional to LαGCZγFe where α=1.01±0.19 and γ=0.33±0.11. Correcting for GC luminosity and color effects, and detection incompleteness, we find no evidence that the fraction of LMXBs with LX>1037 ergs/s in GCs (40%), or the fraction of GCs hosting LMXBs (∼6.5%) varies between galaxies. The spatial distribution of LMXBs resembles that of GCs, and the specific frequency of LMXBs is proportional to the GC specific luminosity, consistent with the hypothesis that all LMXBs form in GCs. If the LMXB lifetime is τL and the duty cycle is Fd, our results imply ∼1.5τL/108/yr/Fd LMXBs are formed per gigayear per GC, and we place an upper limit of one active LMXB in the field per 3.4x109 L☉ of V-band luminosity.
Galaxies: Elliptical and Lenticular, cD - Galaxies: Star Clusters - X-Rays: Binaries - X-Rays: Galaxies
VizieR on-line data:
<Available at CDS (J/ApJ/689/983): table1.dat table6.dat>