Lense-Thirring precession of neutron-star accretion flows: Relativistic versus classical precession

TL;DR

This paper investigates the Lense-Thirring precession of neutron-star accretion flows using relativistic models, revealing complex dependencies on neutron star spin and quadrupole moments that explain observed oscillation frequencies.

Contribution

It provides a comprehensive relativistic analysis of precession frequencies considering neutron star quadrupole effects, surpassing previous linear approximations.

Findings

Precession frequencies depend non-linearly on neutron star spin.

Maxima in precession frequency occur at low spin values.

Different neutron star spins can produce similar precession frequencies.

Abstract

The vertical (Lense-Thirring) precession of the innermost accretion flows has been discussed as a sensitive indicator of the rotational properties of neutron stars (NSs) and their equation of state because it vanishes for a non-rotating star. In this work, we apply the Hartle-Thorne spacetimes to study the frequencies of the precession for both geodesic and non-geodesic (fluid) flows. We build on previous findings on the effect of the NS quadrupole moment, which revealed the importance of the interplay between the relativistic and classical precession. Because of this interplay, the widely used Lense-Thirring metric, linear in the NS angular momentum, is insufficient to calculate the behaviour of the precession frequency across an astrophysically relevant range of NS angular momentum values. We find that even for a moderately oblate NSs, the dependencies of the precession frequency on the NS angular momentum at radii within the innermost accretion region have maxima that occur at relatively low values of the NS angular momentum. We conclude that very different groups of accreting NSs -- slow and fast rotators -- can display the same precession frequencies. This may explain the lack of evidence for a correlation between the frequencies of the observed low-frequency quasiperiodic oscillations and the NS spin. In our work, we provide a full, general description of precession behaviour, and also examples that assume specific NS and quark star (MIT bag) equation of state. Our calculations are reproducible using the associated Wolfram Mathematica notebook.