The bicoherence analysis of type C quasi-periodic oscillations in Swift J1727.8-1613

Abstract

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 $f_1=f_2=f_{\mathrm{QPO}}$ with count rates. The diagonal structure at $f_1+f_2=f_{\mathrm{QPO}}$ 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 $f_1+f_2=f_{\mathrm{har}}$ 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.

14 pages, 6 figures, ApJ in press