Super-Eddington Magnetized Neutron Star Accretion Flows: a Self-similar Analysis

TL;DR

This paper develops a self-similar model for super-Eddington accretion disks around magnetized neutron stars, revealing how magnetic interactions influence disk structure and neutron star spin-up in high-accretion regimes.

Contribution

It introduces a self-similar solution framework for super-Eddington magnetized neutron star accretion disks, detailing the magnetosphere-disk interaction and boundary layer properties.

Findings

Magnetosphere truncation radius is proportional to the Alfvén radius with a coefficient of 0.34-0.71.

Super-Eddington accretion can spin up neutron stars to rapid rotation.

The model applies to ultraluminous X-ray binaries and active galactic nucleus disks.

Abstract

The properties of super-Eddington accretion disks exhibit substantial distinctions from the sub- Eddington ones. In this paper, we investigate the accretion process of a magnetized neutron star (NS) surrounded by a super-Eddington disk. By constructing self-similar solutions for the disk structure, we study in detail an interaction between the NS magnetosphere and the inner region of the disk, revealing that this interaction takes place within a thin boundary layer. The magnetosphere truncation radius is found to be approximately proportional to the Alfv\'en radius, with a coefficient ranging between 0.34-0.71, influenced by the advection and twisting of a magnetic field, NS rotation, and radiation emitted from an NS accretion column. Under super-Eddington accretion, the NS can readily spin up to become a rapid rotator. The proposed model can be employed to explore the accretion and evolution of NSs in diverse astrophysical contexts, such as ultraluminous X-ray binaries or active galactic nucleus disks.