An Insight-HXMT View of the Evolution of the Type-C Quasiperiodic Oscillation during the Flaring State of Swift J1727.8-1613

TL;DR

This study analyzes the evolution of type-C QPOs in Swift J1727.8-1613 during flaring, revealing energy-dependent behaviors and a new 4 Hz spectral break indicating accretion flow geometry changes.

Contribution

It provides the first detection of a 4 Hz break in QPO rms-frequency relation across multiple energy bands and links spectral parameter changes to accretion geometry.

Findings

QPO rms decreases with frequency below 10 keV

QPO rms remains stable above 10 keV

A common 4 Hz break in rms-frequency relation was detected

Abstract

We present a detailed analysis of the evolution of type-C quasiperiodic oscillations (QPOs) observed during the flaring state of the recently discovered black hole X-ray binary Swift J1727.8-1613, utilizing data from the Insight Hard X-ray Modulation Telescope. By examining the relation between the QPO fractional rms amplitude and QPO frequency across various energy bands, we discover that the behavior significantly differs between these energy bands. Below 10 keV, the QPO fractional rms generally decreases with increasing QPO frequency, whereas above 10 keV, the QPO fractional rms remains relatively stable with frequency. Additionally, we report, for the first time, the detection of a common break at around 4 Hz in the relation between QPO fractional rms and frequency in both the 2-4 and 50-100 keV energy bands. We also find that the evolution of all the spectral parameters alters its behavior at around 4 Hz, with the changes in all parameters becoming flatter. This suggests a significant change in the geometry of the accretion flow. We attribute the observed break to the overall changes in the spectrum.