Understanding corona and disc evolution in black hole X-ray binaries through a comprehensive study of their broad-band variability and QPO characteristics

ABSTRACT The shape of the power spectrum of the black hole low-mass X-ray binary evolves systematically over different spectral states during an outburst. Therefore, the power colours (ratio of the variability amplitude at different frequency ranges) and the ‘hue’ parameter, quantifying the power spectral shape, can be utilized to identify the spectral states of the system. We present the comprehensive power colour analysis and subsequent identification of spectral states using the entire archival data (2016–2024) from AstroSat. We detected 29 QPOs (quasi-periodic oscillations), along with several associated harmonics and shoulders, and investigated their properties as a function of hue. We examined the evolution of the QPO RMS variability and time lag, along with hue and QPO frequency. We report a sign change in the average QPO time lag around the QPO frequency of $\sim$2 Hz for high-inclination sources, during the hard-to-hard intermediate state transition. At lower frequency, the hard lags showed an increasing trend reaching up to $\sim$36 ms, but the soft lags above 2 Hz remained confined within $\sim$10 ms, suggesting an evolution to a compact corona. Conversely, for low-inclination sources, no such transition was found. Furthermore, for high-inclination sources, the harmonic lag remains unaffected during state transition, in contrast to the QPO lag behaviour. Our results are consistent with a transition from an elongated jet-like corona to a compact corona and reveal vital clues about the dynamical evolution of the corona and disc.