[Paper Review] Accreting strongly magnetised neutron stars: X-ray Pulsars

Alexander A. Mushtukov, Sergey S. Tsygankov|arXiv (Cornell University)|Apr 29, 2022

High-pressure geophysics and materials24 citations

TL;DR

This review synthesizes observational and theoretical progress on X-ray pulsars, accreting strongly magnetised neutron stars, covering pulsations, luminosity variability, spectra, magnetic field structure, and ULX pulsars.

ABSTRACT

X-ray pulsars (XRPs) are accreting strongly magnetised neutron stars (NSs) in binary systems with, as a rule, massive optical companions. Very reach phenomenology and high observed flux put them into the focus of observational and theoretical studies since the first X-ray instruments were launched into space. The main attracting characteristic of NSs in this kind of system is the magnetic field strength at their surface, about or even higher than $10^{12}\,{ m G}$, that is about six orders of magnitude stronger than what is attainable in terrestrial laboratories. Although accreting XRPs were discovered about 50 years ago, the details of the physical mechanisms responsible for their properties are still under debate. Here we review recent progress in observational and theoretical investigations of XRPs as a unique laboratory for studies of fundamental physics (plasma physics, QED and radiative processes) under extreme conditions of ultra-strong magnetic field, high temperature, and enormous mass density.

Motivation & Objective

• Explain how strong magnetic fields shape accretion geometry and X-ray emission in XRPs.
• Summarize timing and spectral features across broad luminosity ranges.
• Discuss the diversity of XRP sub-classes and their transients (BeXRBs, SFXTs, ULX pulsars).
• Highlight observational diagnostics for magnetic field strength, accretion regimes, and beam patterns.

Proposed method

• Survey and synthesize multi-wavelength observational results (timing, spectral, variability).
• Relate observed pulsation properties to beam patterns and accretion geometry.
• Explain cyclotron resonance features as tracers of surface magnetic fields.
• Discuss theoretical models of accretion flow, magnetosphere interaction, and voltage-dominated regimes.
• Compare sub-classes (persistent, BeXRBs, SFXTs, ULX pulsars) and their luminosity evolution.

Experimental results

Research questions

• RQ1 What are the dominant observational signatures (timing, spectral, variability) that reveal the physics of accretion onto strongly magnetised neutron stars?
• RQ2 How does the magnetic field strength and geometry influence the inner accretion flow, beam patterns, and observed pulsations across luminosity states?
• RQ3 What mechanisms drive the wide range in XRP luminosities, including ULX pulsars, and how do they relate to accretion regimes?
• RQ4 Can cyclotron resonance features serve as robust probes of magnetic field structure and its evolution in XRPs?
• RQ5 How do BeXRBs, SFXTs, and other XRP sub-classes differ in accretion physics and observational phenomenology?

Key findings

• XRPs exhibit coherent pulsations due to misalignment of magnetic and rotation axes, with pulse profiles varying with energy and luminosity.
• Luminosities span from ~10^32 to ~10^41 erg/s, with ULX pulsars representing the high end and possible beaming effects influencing apparent luminosity.
• PDS features show breaks and QPOs whose frequencies correlate with luminosity, reflecting inner-disc dynamics and propagating fluctuations.
• Spectra are typically cutoff power-laws with occasional cyclotron lines; at low luminosities spectra can become two-component, impacting cyclotron line interpretation.
• BeXRBs, SFXTs, and ULX pulsars illustrate diverse accretion regimes and instabilities, including radiation-pressure dominated discs and rapid variability on short timescales.
• Cyclotron line energies show correlations with luminosity, indicating changes in emission region geometry across critical luminosities.