A likely inclination dependence in the non-linear variability of quasi-periodic oscillations from black hole binaries

We present a systematic analysis of the effects of orbital inclination angle on the non-linear variability properties of type-B and type-C quasi-periodic oscillations (QPOs) from black hole binaries. We use the bicoherence, a measure of phase coupling at different Fourier frequencies for our analysis. We find that there is a likely inclination-dependent change in the non-linear properties of type-C QPOs as the source transitions from a hard intermediate state to a soft intermediate state. High inclination (edge-on) sources show a change from a 'web' to a 'cross' pattern, while the low inclination (face-on) sources show a change from a 'web' to a 'hypotenuse' pattern. We present a scenario of a moderate increase in the optical depth of the Comptonizing region as a possible explanation of these effects. The bicoherence of type-B QPOs do not exhibit any measurable inclination dependence.