Stellar black holes in globular clusters

Following the discovery of X-ray sources in globular clusters, the accretion of matter onto a central massive black hole was suggested as a possible explanation. Subsequently, it was found that these sources could be readily explained by thermonuclear instabilities on neutron-star surfaces and the black-hole models were abandoned. We show here, however, that the recent discovery of large populations of millisecond pulsars—and hence neutron stars—in globular clusters implies that several hundred stellar black holes (of about ten solar masses) should form within a typical cluster. In clusters of high central density, we find that the rapid dynamical evolution of the black-hole population will cause ejection of nearly all of the holes on a relatively short time-scale. But in systems of intermediate density, some of the surviving holes may capture a normal star to form a low-mass X-ray binary. We suggest that there may be one or more such binaries in the globular clusters surrounding our Galaxy. These systems will be quiescent most of the time—with only occasional X-ray outbursts—but future observations of the hard X-ray spectrum may indirectly establish their existence.