Temporal evolution of quasi-periodic oscillations in an accreting black hole Swift J1727.8-1613: coevolution of the disk-corona during the state transition

Abstract: Low-frequency quasi-periodic oscillations (QPOs) are commonly observed in black hole X-ray binaries, and their frequency has been found to correlate with various energy spectral properties. In this work, we present a detailed timing analysis of Swift J1727.8-1613, revealing a novel two-branch correlation between the QPO frequency and the observed disk emission, which differs from previous findings of a single correlation. Specifically, at QPO frequencies below 3 Hz, the QPO frequency is negatively correlated with the observed disk emission. Such a negative relation transitions to a positive one, as the QPO frequency exceeds approximately 3 Hz. The correlation between QPO frequency and Compton flux exhibits an opposite trend, with a positive correlation at lower frequencies and a negative correlation at higher ones. We interpret these behaviors as signatures of an evolving disk-corona geometry, within the framework of a Lense-Thirring precessing hot flow. Additionally, we find that during the flare state, the QPO fractional root-mean-square (rms) remains nearly constant above 15 keV, but increases with energy below this threshold. The slope of the rms-energy relation becomes steeper as the energy spectrum softens.