Galactic Globular Clusters with Luminous X-Ray Binaries

Abstract

Luminous X-ray binaries (>1E34 erg/s, LMXBs) have a neutron star or black hole primary, and in globular clusters, most of these close binaries are expected to be have evolved from wider binaries through dynamical interactions with other stars. We sought to find a predictor of this formation rate that is representative of the initial properties of globular clusters rather than of the highly evolved core quantities. Models indicate the half-light quantities best reflect the initial conditions, so we examine whether the associated dynamical interaction rate, proportional to L^1.5 r^-2.5, is useful in understanding the presence of luminous LMXBs in the Galactic globular cluster system. We find that while LMXB clusters with large values of L^1.5 r^-2.5 preferentially host LMXBs, the systems must also have half-mass relaxation times below about 1E9 yr. This relaxation time effect probably occurs because several relaxation times are required to modify binary separations, a timescale that must be shorter than cluster ages. The frequency of finding an LMXB cluster is enhanced if the cluster is metal-rich and if it is close to the bulge region. The dependence upon metallicity is most likely due either to differing initial mass functions at the high mass end, or because bulge systems evolve more rapidly from tidal interactions with the bulge. This approach can be used to investigate globular cluster systems in external galaxies, where core properties are unresolved.