Nonlinear Variability Observed with Insight-HXMT in MAXI J1820+070 and MAXI J1535-571

We present the timing analysis of the nonlinear variability in two black hole low mass X-ray binaries MAXI J1820+070 and MAXI J1535-571 by using the bicoherence, a measure of phase coupling at different Fourier frequencies. We found different patterns, e.g., "cross" and "hypotenuse," for LFQPOs in different outburst states. When they can be clearly distinguished, bicoherence patterns are similar over a wide energy range of 1-100 keV. It is intriguing that in some type-C QPOs we found the patterns that are normally observed in type-B QPOs. On the contrary, the "hypotenuse" pattern, a characteristic of type-C QPOs, was detected in a type-B QPO. This suggests that different types of QPOs may originate from similar underlying mechanisms. In addition, we speculate that the nonlinear variability may be a promising approach to disentangle distinct QPO models which assume different interactions between the broadband noise and QPO components.