| Abstract: The QPOs (quasi-periodic oscillations) observed in the power spectrum of black hole low-mass X-ray binary (BH-LMXB) systems are exceptional features for studying the effects of strong gravity and the associated accretion physics. We searched the entire archival data (2016-2024) of BH-LMXBs from AstroSat and found 29 QPOs, along with several associated harmonics and shoulders. We then performed a timing and spectral analysis of these observations to investigate the origin and modulation of the QPOs. Notably, we detect a sign reversal in the average QPO time lag between hard and soft photons around 2 Hz for the high inclination sources during the hard-to-hard intermediate state transition. At lower frequency, the hard lags showed an increasing trend reaching up to ∼36 ms, but the soft lags above 2 Hz remained confined within ∼10 ms, suggesting the presence of a compact corona. The transition frequency indicates the radius of Lense-Thirring (LT) precession beyond which the corona transits from elongated (jet-like) to a compact corona, leading to soft lags. Furthermore, for high inclination sources, the harmonic lag remains unaffected during the state transition, in contrast to the QPO lag behavior. Conversely, for low inclination sources, the time lag does not exhibit any transition, but the RMS spectral slope changes sign at a similar QPO frequency value. The difference in lag behavior between high and low-inclination sources above 2 Hz arises because relativistic effects become stronger at smaller radii leading to variation of QPO flux, which is not the case at larger radii. The spectral properties are closely correlated with the timing features, exhibiting a positive correlation of the QPO frequency with the photon index and the thermal flux fraction. This study offers critical insights into the transformation of the coronal structure in association with the state evolution of BH-LMXBs. |
