2023MNRAS.524.5869R

We investigate spectro-temporal properties for two black hole X-ray binary sources, MAXI J1535-571 and H 1743-322, during their hard and hard-intermediate states. For MAXI J1535-571, we analyse SWIFT/XRT, NUSTAR, and NICER observations, specifically focusing on the occurrence of type-C Quasi-periodic Oscillations (QPOs). Regarding H 1743-322, we analyse multi-epoch observations of NICER and AstroSat, identifying a type-C QPO with centroid frequency ranging from 0.1-0.6 Hz. In both sources, we fit the spectra with a relativistic truncated disc and a power-law component. In MAXI J1535-571, we also observe an additional relativistically smeared iron-line. Through temporal and spectral analysis, we estimate the QPO centroid frequency and spectral parameters, such as the accretion rate and inner disc radii. We test the origin of type-C QPOs as relativistic precession frequency, and dynamic frequency (i.e. the inverse of the sound crossing time $\frac{r}{c_\mathrm{ s}(r)}$). The dependence of QPO frequency on both the accretion rate and inner disc radii favours the QPO origin as dynamic frequency. We discuss the implications of these results.