The spectral and timing properties of the kHz QPOs in 4U 1636-53

Combined spectral-timing techniques are the key to understand the variability, environment and internal composition of neutron-stars in X-ray binary systems. We analyzed all the available RXTE observations of the neutron-star low-mass X-ray binary 4U 1636-53 and studied the rms fractional amplitude of its pair of kilohertz quasi-periodic oscillations (QPOs) as a function of energy and frequency. By combining the observations with the QPOs within chosen frequency ranges we produced frequency-resolved fractional rms amplitude spectra. By selecting light-curves in different energy bands we created energy-resolved relations of the fractional rms amplitude vs. frequency of the kHz QPOs. We also studied the relation between the frequency of the kHz QPOs and the physical parameters that describe the energy spectrum of 4U 1636-53. We obtained results that, for the first time, show the connection between the spectral parameters and the dynamical and radiative properties of the kHz QPOs. We discuss the implications of our findings on our understanding of the mechanism that produces the kHz QPOs. Our results demonstrate that a simultaneous description of the time and energy domains is needed to reveal the origin of the kHz QPOs.