The bicoherence study of quasi-periodic oscillations in MAXI J1535-571

Bicoherence is a way to measure the phase coupling of triplets of Fourier frequencies. We use this method to analyze quasi-periodic oscillations (QPOs) in the black hole X-ray binary MAXI J1535-571 during its 2017 September-October outburst. The bicoherence provides an interesting new diagnostic to uncover QPO behaviour and the relationships between QPO harmonics and broadband noise. The bicoherence pattern of type-C QPOs starts as a 'web' pattern and changes to a 'hypotenuse' pattern after the presence of type-B QPOs, indicating that MAXI J1535-571 is a low-inclination source. The intensity of bicoherence also exhibits variations across different energy bands. We try to explain the bicoherence results in the scenario of a dual-corona geometry.