Time lags in the kilohertz quasi-periodic oscillations of the low-mass X-ray binary 4U 1608-52

Since (part of) the high-energy radiation emitted in X-rays from the surface of the neutron star (NS) is reprocessed an re-emitted at lower energies in the accretion disk, one can constrain the size of the (re-)emitting region measuring the time lags between low- and high-energy photons of the kilohertz quasi-periodic oscillations (kHz QPOs). We studied the system 4U 1608-52, in which the frequency of the kHz QPOs ranges from 540 to 1060 Hz. If these QPOs come from the inner edge of the accretion disk, the time lags should change with frequency, as the inner disc radius changes. We find a significant dependence of the time lags with energy, but a weak (if any) dependence with frequency and use these results to constrain the location at which these QPOs are produced.