Galactic Globular Clusters with Luminous X‐Ray Binaries

Luminous X-ray binaries (>1E34 erg/s, LMXBs) have a neutron star or blackhole primary, and in globular clusters, most of these close binaries areexpected to be have evolved from wider binaries through dynamical interactionswith other stars. We sought to find a predictor of this formation rate that isrepresentative of the initial properties of globular clusters rather than ofthe highly evolved core quantities. Models indicate the half-light quantitiesbest reflect the initial conditions, so we examine whether the associateddynamical interaction rate, proportional to L^1.5 r^-2.5, is useful inunderstanding the presence of luminous LMXBs in the Galactic globular clustersystem. We find that while LMXB clusters with large values of L^1.5 r^-2.5preferentially host LMXBs, the systems must also have half-mass relaxationtimes below about 1E9 yr. This relaxation time effect probably occurs becauseseveral relaxation times are required to modify binary separations, a timescalethat must be shorter than cluster ages. The frequency of finding an LMXBcluster is enhanced if the cluster is metal-rich and if it is close to thebulge region. The dependence upon metallicity is most likely due either todiffering initial mass functions at the high mass end, or because bulge systemsevolve more rapidly from tidal interactions with the bulge. This approach canbe used to investigate globular cluster systems in external galaxies, wherecore properties are unresolved.Comment: 20 pages, 8 figures; accepted in The Astrophysical Journa