Speaker
Description
Black hole X-ray binaries are key laboratories for studying the behavior of matter in strong gravitational fields. During outbursts, these systems evolve through different spectral-timing states, whose distinct properties suggest changes in the geometry of the accretion flow. Previous studies have shown that the transition from the hard-intermediate state to the soft-intermediate state is typically associated with three major observational changes: the switch of Type-C quasi-periodic oscillations by Type-B QPOs, a sharp decrease in the fractional rms amplitude of the broadband noise, and a transition from a steady radio jet to discrete jet ejections.
Taking advantage of a joint-fitting approach to the power and cross spectra, weak variability components that are hidden or difficult to identify in the power spectrum alone can be detected. Using this method, we detect Type-B QPOs in the hard-intermediate state, suggesting that Type-B QPOs are not restricted to the soft-intermediate state. In this talk, I will present the detection of Type-B QPOs in the hard-intermediate states of two black hole X-ray binary systems, Swift J1727.8−1613 and MAXI J1820+070, and discuss their implications for the physical origin of Type-B QPOs and the geometry of the accretion flow.