Impact of inclination on quasi-periodic oscillations from spiral structures

TL;DR

This paper models how inclination affects the observable properties of flux modulation caused by spiral structures in accretion disks, providing predictions for pulse profiles and power spectra relevant to understanding low-frequency QPOs.

Contribution

It introduces an analytical model linking inclination to observable QPO features, highlighting the impact of disk geometry on flux modulation signatures.

Findings

Amplitude of flux modulation varies strongly with inclination and frequency.

Higher inclination leads to stronger harmonic signatures in power spectra.

Pulse profiles become more complex at higher inclinations.

Abstract

Context. Quasi-periodic oscillations (QPOs) are a common feature of the power spectrums of X-ray binaries. Currently it is not possible to unambiguously differentiate the large number of proposed models to explain these phenomena through existing observations. Aims. We investigate the observable predictions of a simple model that generates flux modulation: a spiral instability rotating in a thin accretion disk. This model is motivated by the accretion ejection instability (AEI) model for low- frequency QPOs (LFQPOs). We are particularly interested in the inclination dependence of the observables that are associated with this model. Methods. We develop a simple analytical model of an accretion disk, which features a spiral instability. The disk is assumed to emit blackbody radiation, which is ray-traced to a distant observer. We compute pulse profiles and power spectra as observed from infinity. Results. We show that the amplitude of the modulation associated with the spiral rotation is a strong function of inclination and frequency. The pulse profile is quasi-sinusoidal only at low inclination (face-on source). As a consequence, a higher-inclination geometry leads to a stronger and more diverse harmonic signature in the power spectrum. Conclusions. We present how the amplitude depends on the inclination when the flux modulation comes from a spiral in the disk. We also include new observables that could potentially differentiate between models, such as the pulse profile and the harmonic content of the power spectra of high-inclination sources that exhibit LFQPOs. These might be important observables to explore with existing and new instruments.