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 quasiperiodic 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 low-energy (LE) and high-energy (HE) bands. In the bicoherence patterns, there are correlations between horizontal and vertical bicoherence at f1 = f2 = fQPO with count rates. The diagonal structure at f1 + f2 = fQPO becomes more prominent with increasing energy. Additionally, we discovered a new bicoherence pattern in the medium energy band from 10 to 20 keV; the diagonal structure at f1 + f2 = fhar 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.