The Torques Acting on Accreting Millisecond X-Ray Pulsars in the Outburst and Quiescent States, and During the Long-Term Evolution

TL;DR

This study models the torque and luminosity variations of five accreting millisecond X-ray pulsars across different states, revealing insights into their spin evolution and the forces influencing them.

Contribution

It applies a previously used model to analyze torque and luminosity changes in AMXPs during outburst and quiescence, enhancing understanding of their long-term evolution.

Findings

Most systems spin up during outbursts and spin down during quiescence.

Torques from disc-magnetosphere interaction, magnetic dipole radiation, and mass accretion align with observations.

Gravitational radiation torques cannot be ruled out due to uncertainties and noise.

Abstract

Accreting millisecond X-ray pulsars (AMXPs) are transient X-ray sources likely to be in the final evolutionary phases of low-mass X-ray binaries (LMXBs). In this work, we have investigated the torque and X-ray luminosity variations of five AMXPs during outburst and quiescence, using a model previously employed to explain some typical behaviours of LMXBs. Most of these systems spin up in the outburst state and spin down in quiescence, while they slow down on the long term. We have obtained reasonable results with the model in agreement with these observations. We have found that the torques produced by the disc-magnetosphere interaction, the magnetic dipole radiation, and the mass accretion are compatible with the X-ray luminosity and rotational properties of the sources in their outburst and quiescent states, and during the resultant long-term evolution. Nevertheless, our results are not sufficient to rule out the spin-down contribution of the gravitational radiation torques due to significant timing noise and uncertainties about the bolometric corrections for X-ray luminosity during the outbursts of some sources.