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

Impact of inclination on quasi-periodic oscillations from spiral structures