Exploring Disk-Corona Dynamics in Black Hole X-ray Binaries through a comprehensive study of variability and QPO characteristics using AstroSat

Black hole low-mass X-ray binary (BH-LMXB) systems provide exceptional laboratories for exploring accretion physics under strong gravity. These systems undergo state transitions during outbursts, driven by variations in mass accretion rate. The conditions and triggering mechanisms behind state transitions are still challenging to understand and quantify, but investigation of variability properties provides insights into their underlying causes. State transitions involve changes in the accretion disk and the size of the corona, with significant variability in emission properties. Using AstroSat observations of 10 BH-LMXBs, we performed detailed timing analysis during outburst, employing an innovative power-color technique. This method uses ratio of variability amplitudes across distinct frequency ranges to estimate spectral states through hue. We used the variation of total RMS with hue enabling a robust characterization of spectral state evolution. We detected multiple quasi-periodic oscillations (QPOs) and associated harmonics across different states, providing valuable insights into the emission mechanisms of the inner accretion disk. Our analysis revealed the evolution of QPO variability amplitude spectra with hue and QPO frequency, indicating the change in the geometry of the corona in various states. A key finding was a reversal in the average QPO time lag between hard and soft photons at a QPO frequency of 2 Hz, coinciding with the hard-to-hard-intermediate state transition. This reversal, accompanied by a slope change in the QPO RMS spectra, supports a transition in the corona geometry from an elongated, jet-like structure producing hard lags to a compact corona where soft lags emerge via reverberation. The transition frequency near 2 Hz likely represents the critical radius for Lense-Thirring precession, beyond which the angular momentum misalignment might have altered the disk dynamics, triggering the state change. These findings provide a better understanding of the physical mechanisms driving state transitions and their impact on disk-corona interplay in BH-LMXBs.