PhD Thesis (PDF) - Department of Astronomy - University of Virginia

7.1.2 The Connection Between Globular Clusters and Low-
Mass X-ray Binaries
Low-mass X-ray binaries form efficiently in GCs. Our study of the connection
between LMXBs and GCs in NGC 4697 was a trial study for our more developed sur-
vey. By combining Chandra X-ray Observatory and Hubble Space Telescope-Advanced
Camera for Surveys (HST-ACS) observations of early-type galaxies, the most accurate
identification of LMXBs and GCs to date allows us to shed critical light on the GC-
LMXB connection. We use a study of 270 GCs with LMXBs and 6,488 GCs without
detectable X-ray emission from a sample of eleven massive early-type galaxies in the
Virgo cluster. Globular clusters that are more massive, are redder, and have smaller
radii are more likely to contain LMXBs. Unlike Galactic GCs, a large number of GCs
with LMXBs have relaxation times > 2.5 Gyr; GCs do not need to survive for more
than five relaxation timescales to produce LMXBs. The expected number of LMXBs
per GC (λ) is found to vary with the GC mass, color, and half-mass radius as
λ ∝ M 1.24±0.08 10 0.9+0.2
−0.1 (G475−Z850) −2.2
r +0.3
−0.4
278
h,cor . (7.1)
Earlier studies had suggested that the number of LMXBs per GC might just be
linearly proportional to GC mass; we are able to rule out this simple dependence.
Based on the expected number of LMXBs per GC, we predict that most GCs with
high X-ray luminosities contain a single LMXB. The detailed dependence of λ on GC
properties appears mainly due to a dependence on a combination of mass and radius,
and a dependence on color. We show that these two dependencies are essentially