Magnetospheric Flows in X-ray Pulsars I: Instability at super-Eddington regime of accretion

TL;DR

This paper models the dynamics of accretion flows in X-ray pulsars near and above the Eddington luminosity, revealing flow instability and quasi-periodic fluctuations influenced by magnetic inclination and radiation anisotropy.

Contribution

It introduces a model of magnetospheric accretion flow that accounts for gravity, radiative, and centrifugal forces, highlighting flow instability at high accretion rates.

Findings

Accretion flow becomes unstable at high mass accretion rates.

Flow tends to fluctuate quasi-periodically with a period similar to free-fall time.

Magnetic inclination and radiation anisotropy influence stability and fluctuations.

Abstract

Within the magnetospheric radius, the geometry of accretion flow in X-ray pulsars is shaped by a strong magnetic field of a neutron star. Starting at the magnetospheric radius, accretion flow follows field lines and reaches the stellar surface in small regions located close to the magnetic poles of a star. At low mass accretion rates, the dynamic of the flow is determined by gravitational attraction and rotation of the magnetosphere due to the centrifugal force. At the luminosity range close to the Eddington limit and above it, the flow is additionally affected by the radiative force. We construct a model simulating accretion flow dynamics over the magnetosphere, assuming that the flow strictly follows field lines and is affected by gravity, radiative and centrifugal forces only. The magnetic field of a NS is taken to be dominated by the dipole component of arbitrary inclination with respect to the accretion disc plane. We show that accretion flow becomes unstable at high mass accretion rates and tends to fluctuate quasi-periodically with a typical period comparable to the free-fall time from the inner disc radius. The inclination of a magnetic dipole with respect to the disc plane and strong anisotropy of X-ray radiation stabilise the mass accretion rate at the poles of a star, but the surface density of material covering the magnetosphere fluctuates even in this case.