While there are many dynamical mechanisms and models that try to explain the origin and phenomenology of quasi-periodic oscillations (QPOs) in low-mass X-ray binaries, few of them address how the radiative processes occurring in these extreme environments give rise to the rich set of variability features actually seen in the X-ray light curves of these systems.
A step towards that end is the study of the energy and frequency dependence of the phase lags of the QPOs in the light curves.
Here we studied the phase lags of all QPOs in the range of 1 Hz to 1300 Hz detected in the low-mass X-ray binary 4U 1636-53 using a methodology that allowed us to study, for the first time, the dependence of the phase lags upon energy and frequency as the source changes its states as it moves through the colour-colour diagram.
Our results suggest that within the context of models of up-scattering Comptonization, the phase lags dependencies upon frequency and energy can be used to extract size scales and physical conditions of the medium that produces the lags.