On the lack of X-ray pulsation in most neutron star low-mass X-ray binaries

TL;DR

This study explores why most neutron star low-mass X-ray binaries lack pulsations, suggesting that high accretion rates keep the inner disc from reaching the surface, thus suppressing pulsations, and explains the rarity of accreting millisecond X-ray pulsars.

Contribution

The paper introduces a model linking inner disc radius to neutron star parameters, explaining pulsation absence and the transient nature of AMXPs.

Findings

Most LMXBs have accretion rates above the critical threshold for pulsation suppression.

X-ray pulsations are possible when the neutron star's spin period is below a critical value.

The model aligns with observed luminosity ranges of AMXPs and explains their scarcity.

Abstract

We have investigated whether the lack of X-ray pulsations from most neutron star (NS) low-mass X-ray binaries (LMXBs) could be due to the extension of their inner disc to the NS surface. To estimate the inner disc radii, we have employed the model, recently proposed to account for the torque reversals of LMXBs. In this model, the inner disc radius depends on the spin period as well as the dipole moment and the mass inflow rate of the disc. Our model results indicate that most LMXBs have mass accretion rates above the minimum critical rates required for the inner disc to reach down to the NS surface and thereby quench the pulsed X-ray emission. For most sources X-ray pulsations are allowed when the period decreases below a certain critical value. For the same parameters, the model is also consistent with the observed X-ray luminosity ranges of the individual accreting millisecond X-ray pulsars (AMXPs). The paucity of AMXPs compared to the majority population of non-pulsing LMXBs is explained, as well as the fact that AMXPs are transient sources.