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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 [738], 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 1- 1000Mo .. Although this is pure speculation, it does provide some indirect suggestion of possible black hole binaries in globular clusters.

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