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

TL;DR

This study uses bicoherence analysis on Swift J1727.8-1613's 2023 outburst data to characterize type C QPOs, revealing energy-dependent patterns and correlations with spectral hardness, and suggesting a low-inclination system.

Contribution

It introduces a detailed bicoherence analysis of type C QPOs in Swift J1727.8-1613, identifying new patterns and energy-dependent features not previously documented.

Findings

Type C QPOs are confirmed in the source.

A new 'parallel' bicoherence pattern is identified in 10-20 keV band.

Energy-dependent transition from 'web' to 'hypotenuse' pattern observed.

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