Energy dependence of the low-frequency quasi-periodic oscillations in Swift J1727.8-1613

Based on observations from the Insight-Hard X-ray Modulation Telescope (Insight-HXMT), an analysis of Type-C quasi-periodic oscillations (QPOs) observed during the outburst of the new black hole candidate Swift J1727.8-1613 in 2023 was conducted. This analysis scrutinized the QPO's evolution throughout the outburst, particularly noting its rapid frequency escalation during two flare events. Utilizing the energy range covered by Insight-HXMT, a dependency of the QPO frequency on energy was observed. Below approximately 3 Hz, minimal variations in frequency with energy were noted, whereas clear variations with photon energy were observed when it exceeded approximately 3 Hz. Additionally, a sharp drop in the rate of change was observed when the frequency exceeded approximately 8 Hz. This behavior, similar to several previously reported sources, suggests the presence of a common underlying physical mechanism. Moreover, the QPO rms-frequency relationship can be explained by the Lense-Thirring precession model. The relationship between rms-energy and phase lag with frequency suggests the black hole system as a high-inclination source.