Can disk-magnetosphere interaction models and beat frequency models for quasi-periodic oscillation in accreting X-ray pulsars be reconciled?

TL;DR

This paper reviews magnetic threaded disk models and tests their predictions against observations of X-ray pulsars with quasi-periodic oscillations and torque reversals, finding current models insufficient to explain all observed phenomena.

Contribution

It critically compares different disk-magnetosphere interaction prescriptions and highlights the need for improved models to reconcile QPOs with torque behavior in X-ray pulsars.

Findings

None of the existing prescriptions reproduces observed QPO and torque behavior.

Current models cannot fully explain the combination of QPOs and torque reversals.

The problem of disk threading by magnetic fields remains unsolved.

Abstract

In this paper we review some aspects of the theory of magnetic threaded disks. We discuss in particular the equations that determine the position of the inner disk boundary by using different prescriptions for the neutron star-accretion disk interaction. We apply the results to several accretion powered X-ray pulsars that showed both quasi-periodic oscillations in their X-ray flux and spin-up/spin-down torque reversals. Under the hypothesis that the beat-frequency model is applicable to the quasi-periodic oscillations, we show that these sources provide an excellent opportunity to test models of the disk-magnetosphere interaction. A comparison is carried out between the magnetospheric radius obtained with all the prescriptions used in threaded disk models; this shows that none of those prescriptions is able to reproduce the combination of quasi-periodic oscillations and torque behaviour observed for different X-ray luminosity levels in the X-ray pulsars in the present sample. This suggests that the problem of accretion disk threading by stellar magnetic field is still lacking a comprehensive solution. We discuss briefly an outline of possible future developments in this field.