Impact of inclination on quasi-periodic oscillations from spiral structures

¹ AstroParticule & Cosmologie (APC), UMR 7164, Université Paris Diderot, 10 rue Alice Domon et Leonie Duquet, 75205 Paris Cedex 13, France
e-mail: varniere@apc.univ-paris7.fr
² Observatoire de Paris/LESIA, 5 place Jules Janssen, 92195 Meudon Cedex, France
³ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warszawa, Poland

Received: 6 November 2015
Accepted: 18 March 2016

Abstract

Context. Quasi-periodic oscillations (QPOs) are a common feature of the power spectrum 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.