On the geometric nature of low-frequency quasi-periodic oscillations in neutron-star low-mass X-ray binaries

TL;DR

This paper investigates the geometric origin of ~1 Hz QPOs in a neutron-star binary, showing their evolution with spectral states and linking them to accretion disk precession, with implications for inclination-dependent observational effects.

Contribution

It demonstrates that the ~1 Hz QPOs in high-inclination neutron-star binaries are due to disk precession, unifying their behavior with atoll source QPOs and highlighting geometric effects.

Findings

QPO frequency increases from 0.4 Hz to 25 Hz across spectral states.

QPOs follow similar relations with noise components as in atoll sources.

QPOs are likely caused by precession of a misaligned inner accretion disk.

Abstract

We report on a detailed analysis of the so-called ~1 Hz quasi-periodic oscillation (QPO) in the eclipsing and dipping neutron-star low-mass X-ray binary EXO 0748-676. This type of QPO has previously been shown to have a geometric origin. Our study focuses on the evolution of the QPO as the source moves through the color-color diagram, in which it traces out an atoll-source-like track. The QPO frequency increases from ~0.4 Hz in the hard state to ~25 Hz as the source approaches the soft state. Combining power spectra based on QPO frequency reveals additional features that strongly resemble those seen in non-dipping/eclipsing atoll sources. We show that the low-frequency QPOs in atoll sources and the ~1 Hz QPO in EXO 0748-676 follow similar relations with respect to the noise components in their power spectra. We conclude that the frequencies of both types of QPOs are likely set by (the same) precession of a misaligned inner accretion disk. For high-inclination systems, like EXO 0748-676, this results in modulations of the neutron-star emission due to obscuration or scattering, while for lower-inclination systems the modulations likely arise from relativistic Doppler boosting and light-bending effects.