On the torque exerted by a warped, magnetically threaded accretion disk

TL;DR

This paper develops a model to analyze the magnetic torque exerted by warped accretion disks on neutron stars, revealing that warping can enhance the spin-up effect compared to coplanar disks.

Contribution

It introduces a simplified two-component model for warped accretion disks and derives the magnetic torque, highlighting the impact of disk warping on neutron star spin evolution.

Findings

Warped disks are more likely to spin up neutron stars than coplanar disks.

The model shows the magnetic torque depends on initial disk parameters.

Warping influences the accretion torque magnitude and direction.

Abstract

Most astrophysical accretion disks are likely to be warped. In X-ray binaries the spin evolution of an accreting neutron star is critically dependent on the interaction between the neutron star magnetic field and the accretion disk. There have been extensive investigations on the accretion torque exerted by a coplanar disk that is magnetically threaded by the magnetic field lines from the neutron stars, but relevant works on warped/tilted accretion disks are still lacking. In this paper we develop a simplified two-component model, in which the disk is comprised of an inner coplanar part and an outer, tilted part. Based on standard assumption on the formation and evolution of the toroidal magnetic field component, we derive the dimensionless torque and show that a warped/titled disk is more likely to spin up the neutron star compared with a coplanar disk.We also discuss the possible influence of various initial parameters on the torque.