2006/12/31 by Philip J. Humphrey, David A. Buote
Physics and Astronomy · #Adaptive optics and wavefront sensing #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Galaxies: Formation, Evolution, Phenomena #Galaxy #Globular cluster #Luminosity #Luminosity function #Neutron star #Physics #Power law #Statistics #astro-ph
paper · pdf · doi:10.1086/592590
published as Astrophys.J.689:983-1004,2008 · 24 pages, 17 figures and 6 tables. Accepted for publication in the Astrophysical Journal. Expanded discussion and various minor revisions to improve robustness of results. Conclusions unchanged
arxiv created 2008/07/28 · openalex publication_date 2008/11/26 · arxiv updated 2011/02/11 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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- L X . The composite XLF is well fitted by a power law with a break at (2.21 + 0.65 −0.56 ) × 10 38 ergs s −1 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- L X XLF slope does not correlate with age, but may correlate with [α/Fe]. Considering only LMXBs with L X > 10 37 ergs s −1 , 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 L α GC Z γ Fe 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 L X > 10 37 ergs s −1 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 τ L and the duty cycle is F d , our results imply ~1.5(τ L /10 8 yr ) −1 F −1 d LMXBs are formed per gigayear per GC, and we place an upper limit of one active LMXB in the field per 3.4 × 10 9 L ☉ of V -band luminosity.