# The Spin Distribution of Fast Spinning Neutron Stars in Low Mass X-Ray Binaries: Evidence for Two Sub-Populations

## TL;DR

This study analyzes the spin distribution of neutron stars in low-mass X-ray binaries, revealing two distinct sub-populations with different average spin frequencies and discussing implications for accretion models and gravitational wave emission.

## Contribution

It provides the first statistical evidence for two separate sub-populations of neutron stars based on spin frequency in low-mass X-ray binaries, with a proposed cut-point at 540 Hz.

## Key findings

- Evidence for two sub-populations at ~300 Hz and ~575 Hz
- A spin frequency cut-point at ~540 Hz separating the groups
- Radio millisecond pulsars do not show similar sub-population structure

## 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 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 sub-populations among low-mass X-ray binaries, one at relatively low spin frequency, with an average of ~300 Hz and a broad spread, and a peaked population at higher frequency with average spin frequency of ~575 Hz. We show that the two sub-populations are separated by a cut-point at a frequency of ~540 Hz. We also show that the spin frequency of radio millisecond pulsars does not follow a log-normal distribution and shows no evidence for the existence of distinct sub-populations. 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 sub-population do not evolve into radio millisecond pulsars.