The Bicoherence Analysis of Type-C Quasiperiodic Oscillations in Swift J1727.8−1613

We present the results of bicoherence analysis for Swift J1727.8-1613 during its 2023 outburst, using data from Insight-HXMT. Our analysis focused on observations with quasi-periodic oscillations (QPOs) of frequencies greater than 1 Hz, revealing that all of them belong to type C QPOs. We found a strong correlation between the QPO frequency and the hardness ratio, as well as a linear relationship between the QPO RMS and the hardness ratio. The bicoherence analysis revealed a transition from a "web" pattern to a "hypotenuse" pattern in the LE and HE energy bands. In the bicoherence patterns, there are correlations between horizontal and vertical bicoherence at f_1=f_2=f_ QPO with count rates. The diagonal structure at f_1+f_2=f_ QPO becomes more prominent with increasing energy. Additionally, we discovered a new bicoherence pattern in the medium energy band from 10 – 20 keV, the diagonal structure at f_1+f_2=f_ har is prominent only in this energy band, which we refer to as the "parallel" pattern. The bicoherence analysis indicates that the source is likely a low-inclination source.