The Spin Distribution of Millisecond X-ray Pulsars

TL;DR

This paper reviews the observed sharp cutoff in the spin frequencies of accreting millisecond X-ray pulsars and explores the hypothesis that gravitational radiation limits their spin-up, with potential for gravitational wave detection.

Contribution

It discusses various explanations for the spin frequency cutoff and highlights how long-term X-ray timing can test the gravitational wave emission hypothesis.

Findings

The spin frequency cutoff occurs sharply above 730 Hz.

Gravitational radiation may limit neutron star spin-up.

Long-term X-ray timing can test gravitational wave models.

Abstract

The spin frequency distribution of accreting millisecond X-ray pulsars cuts off sharply above 730 Hz, well below the breakup spin rate for most neutron star equations of state. I review several different ideas for explaining this cutoff. There is currently considerable interest in the idea that gravitational radiation from rapidly rotating pulsars might act to limit spin up by accretion, possibly allowing eventual direct detection with gravitational wave interferometers. I describe how long-term X-ray timing of fast accreting millisecond pulsars like the 599 Hz source IGR J00291+5934 can test the gravitational wave model for the spin frequency limit.