

3.4 Black holes
There have been very few observations of black hole binaries in
globular clusters. Although there have been hints of possible black
hole binaries in extragalactic globular cluster systems [7, 38], there are
no known black hole binaries in the galactic globular cluster
system. All of the globular cluster LMXBs exhibit the X-ray
variability that is indicative of nuclear burning on the surface of
a neutron star. Possible evidence of black holes in globular
clusters can be found by extrapolating the velocity
dispersion-black hole mass relation for galaxies with bulges to
globular clusters. Using velocity dispersion data for 22 galactic
globular clusters, Zheng [169] finds possible
black hole masses in the range of
. Although
this is pure speculation, it does provide some indirect suggestion
of possible black hole binaries in globular clusters.

