The Spin Distribution of Fast-spinning Neutron Stars in Low-mass X-Ray Binaries: Evidence for Two Subpopulations

Abstract

Abstract We study the current sample of rapidly rotating neutron stars in both accreting and non-accreting binaries in order to determine whether the spin distribution of accreting neutron stars in low-mass X-ray binaries (LMXBs) can be reconciled with current accretion torque models. We perform a statistical analysis of the spin distributions and show that there is evidence for two subpopulations among LMXBs, one at a relatively low spin frequency, with an average of and a broad spread, and a peaked population at higher frequency with an average spin frequency of . We show that the two subpopulations are separated by a cut-point at a frequency of . We also show that the spin frequency of radio millisecond pulsars (RMSPs) does not follow a log-normal distribution and shows no evidence for the existence of distinct subpopulations. We discuss the uncertainties of different accretion models and speculate that either the accreting neutron star cut-point marks the onset of gravitational waves as an efficient mechanism to remove angular momentum or some of the neutron stars in the fast subpopulation do not evolve into RMSPs.