Abstract
Exploiting the presence of kilohertz quasi-periodic oscillations (QPOs) in the timing power spectrum, we find that the soft X-ray emission of the neutron star X-ray binary and atoll source 4U 1636-536, modulated at the QPO frequency, lags behind that of the hard X-ray emission. Emission in the 3.8-6.4 keV band is delayed by 25.0±3.3 μs relative to the 9.3-69 keV band. The delay increases in magnitude with increasing energy. The soft lag could be produced by Comptonization of hard photons injected into a cooler electron cloud or by intrinsic spectral softening of the emission during each oscillation cycle.
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