Low‐Mass X‐Ray Binaries and Globular Clusters in Early‐Type Galaxies. I.ChandraObservations

We present a Chandra survey of LMXBs in 24 early-type galaxies. Correctingfor detection incompleteness, the X-ray luminosity function (XLF) of eachgalaxy is consistent with a powerlaw with negative logarithmic differentialslope, beta~2.0. However, beta strongly correlates with incompleteness,indicating the XLF flattens at low-Lx. The composite XLF is well-fitted by apowerlaw with a break at 2.21(+0.65,-0.56)E38 erg/s and beta=1.40(+0.10,-0.13)and 2.84(+0.39,-0.30) below and above it, respectively. The break is close tothe Eddington limit for a 1.4Msun neutron-star, but the XLF shape rules out itsrepresenting the division between neutron-star and black-hole systems. Althoughthe XLFs are similar, we find evidence of some variation between galaxies. Thehigh-Lx XLF slope does not correlate with age, but may correlate with[alpha/Fe]. Considering only LMXBs with Lx>1E37 erg/s, matching the LMXBs withglobular clusters (GCs) identified in HST observations of 19 of the galaxies,we find the probability a GC hosts an LMXB is proportional to LGC^alphaZFe^gamma} where alpha=1.01+/-0.19 and gamma=0.33+/-0.11. Correcting for GCluminosity and colour effects, and detection incompleteness, we find noevidence that the fraction of LMXBs with Lx>1e37 erg/s in GCs (40%), or thefraction of GCs hosting LMXBs (~6.5%) varies between galaxies. The spatialdistribution of LMXBs resembles that of GCs, and the specific frequency ofLMXBs is proportional to the GC specific luminosity, consistent with thehypothesis that all LMXBs form in GCs. If the LMXB lifetime is tau and the dutycycle is Fd, our results imply ~1.5 (tau/1E8 yr)^-1 /Fd LMXBs are formed perGyr per GC and we place an upper limit of 1 active LMXB in the field per3.4E9Lsun of V-band luminosity.Comment: 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 unchange