Pulse Profiles of Accreting Neutron Stars from GRMHD Simulations

TL;DR

This paper uses 3D GRMHD simulations to study accretion hotspots and X-ray pulse properties of millisecond pulsars, revealing how magnetic inclination affects hotspot shape and pulse variability, consistent with observations.

Contribution

It introduces a comprehensive GRMHD simulation approach to model accretion hotspots and pulse variability, incorporating relativistic ray tracing and scattering effects, advancing understanding of pulsar emission mechanisms.

Findings

Hotspot shapes depend on magnetic inclination.

Pulse amplitudes range between 1-12% rms, matching observations.

Turbulent accretion flow causes broadband variability.

Abstract

The pulsed X-ray emission from the neutron star surface acts as a window to study the state of matter in the neutron star interior. For accreting millisecond pulsars, the surface X-ray emission is generated from the `hotspots' formed due to the magnetically channeled accretion flow hitting the stellar surface. The emission from these hotspots is modulated by stellar rotation giving rise to pulsations. Using global three-dimensional general relativistic magnetohydrodynamic (GRMHD) simulations of the star-disk system, we investigate the accretion hotspots and the corresponding X-ray pulse properties of accreting millisecond pulsars with dipolar magnetic fields. The accretion spot morphologies in our simulations are entirely determined by the accretion columns and vary as a function of the stellar magnetic inclination. For lower magnetic inclinations, the hotspots are shaped like crescents around the magnetic axis and are transformed into elongated bars for higher inclinations. We model the X-ray pulses resulting from the simulated hotspots using general-relativistic ray tracing calculations and quantify the variability of the pulsed signal. The pulse amplitudes in our simulations usually range between $1 - 12 \%$ rms and are consistent with the observed values. We find that the turbulent accretion flow in the GRMHD simulations introduces significant broadband variability on a timescale similar to the stellar rotational period. We also explore the impact of electron scattering absorption and show that along with being a key factor in determining the pulse characteristics, this also introduces significant additional variability and higher harmonics in the bolometric light curve of the accreting sources.