A time-dependent Comptonisation model for the spectral-timing properties of QPOs in Low-Mass X-ray Binaries

Spectral-timing modeling of the X-ray variability in neutron-star (NS) and black-hole (BH) X-ray binaries provides a powerful tool to study the physical and geometrical properties of the accretion/ejection flow in these systems. Power density spectra (PDS) of these sources show a variety of quasi-periodic oscillations (QPOs) that span a wide range of frequencies. While most models address the dynamical properties of these QPOs, the radiative mechanism that produces the observed variability remains unexplained. We have recently developed (Karpouzas+2020) a variable-Comptonisation model that allows us to simultaneously fit the energy-dependent phase lags and fractional rms amplitudes of these QPOs. This model provides access to physical properties of the Comptonising regions that cannot be probed through fits to the time-averaged spectra. In this talk, we will describe the model and show results from fits to the lower kHz QPO in the NS XRB 4U 1636-56, the Type-B QPO in MAXI J1348-630, and the Type-C QPO in GRS 1915+105 using archival data from the RXTE mission and new data from the NICER timing instrument.