X-ray Pulsars

TL;DR

X-ray pulsars emit X-rays through accretion of material, with variability driven by wind and disk instabilities, magnetic interactions, and thermonuclear processes, revealing complex accretion dynamics and state transitions.

Contribution

This paper synthesizes observational and theoretical insights into the mechanisms driving X-ray variability and state transitions in X-ray pulsars, highlighting the role of magnetic fields and accretion regimes.

Findings

Wind-driven flows cause large X-ray variability on hours scale.

Disk instabilities lead to quasi periodic oscillations.

Magnetic field strength influences accretion geometry and emission.

Abstract

X-ray pulsars shine thanks to the conversion of the gravitational energy of accreted material to X-ray radiation. The accretion rate is modulated by geometrical and hydrodynamical effects in the stellar wind of the pulsar companions and/or by instabilities in accretion discs. Wind driven flows are highly unstable close to neutron stars and responsible for X-ray variability by factors $10^3$ on time scale of hours. Disk driven flows feature slower state transitions and quasi periodic oscillations related to orbital motion and precession or resonance. On shorter time scales, and closer to the surface of the neutron star, X-ray variability is dominated by the interactions of the accreting flow with the spinning magnetosphere. When the pulsar magnetic field is large, the flow is confined in a relatively narrow accretion column, whose geometrical properties drive the observed X-ray emission. In low magnetized systems, an increasing accretion rate allows the ignition of powerful explosive thermonuclear burning at the neutron star surface. Transitions from rotation- to accretion-powered activity has been observed in rare cases and proved the link between these classes of pulsars.