Neutron stars in globular clusters: formation and observational manifestations

Abstract

Population synthesis is used to model the number of neutron stars in globular clusters that are observed as LMXBs and millisecond PSRs. The dynamical interaction between binary and single stars in a GC are assumed to take place with a permanently replenished "background" of single stars whose density distribution keeps track with the cluster evolution as a whole and evolution of single stars. We use the hypothesis (Podsiadlowski et al) that NS forming in binary systems from components with initial masses \sim 8-12 M_\odot during the electron-capture collapse of the degenerate O-Ne-Mg core do not acquire a high space velocities (kicks). The remaining NSs (i.e. from single stars with M>8 M_\odot or binary comonents with M>12 M_\odot) are assumed to be born with high kicks, as found from obsrevations of single pulsars (Hobbs et al. 2005). Under this assumption, a sizeable fraction of NSs remain in GCs (about 1000 NSs in a GC with a mass of 5\times 10^5 M_\odot). The number of ms PSRs formed in the cluster via accretion spin-up in binaries is then about 10, which is consistent with observations. Our modelling reproduces the observed shape of the X-ray luminosity function for accreting NSs in binaries with normal and degenerate components and the distribution of spin periods of ms PSRs in GCs under the assumption of accretion-driven magnetic field decay of NSs up to a bottom value of 10^8 G. The number of LMXBs and ms PSRs dynamically expelling from GCs is also calculated.