Formation of Ultracompact X-Ray Binaries in Dense Star Clusters

Abstract

Bright, ultracompact X-ray binaries observed in dense star clusters, such as\nGalactic globular clusters, must have formed relatively recently, since their\nlifetimes as persistent bright sources are short (e.g., ~10^8 yr above 10^36\nerg/s for a 1.4 Msun neutron star accreting from a degenerate helium companion\nwith an initial mass of ~0.2 Msun). Therefore, we can use the present\nconditions in a cluster core to study possible dynamical formation processes\nfor these sources. Here we show that direct physical collisions between neutron\nstars and red giants can provide a sufficient formation rate to explain the\nobserved numbers of bright sources. These collisions produce tight, eccentric\nneutron star -- white dwarf binaries that decay to contact by gravitational\nradiation on timescales ~10^6-10^10 yr, usually shorter and often much shorter\nthan the cluster age.\n