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High frequency soft reverberation lags have now been detected from stellarmass and super massive black holes. Their interpretation involves reflection ofa hard source of photons onto an accretion disk, producing a delayed reflectedemission, with a time lag consistent with the light travel time between theirradiating source and the disk. Independently of the location of the clock,the kHz Quasi-Periodic Oscillation (QPO) emission is thought to arise from theneutron star boundary layer. Here, we search for the signature of reverberationof the kHz QPO emission, by measuring the soft lags and the lag energy spectrumof the lower kHz QPOs from 4U1608-52. Soft lags, ranging from ~15 to ~40microseconds, between the 3-8 keV and 8-30 keV modulated emissions are detectedbetween 565 and 890 Hz. The soft lags are not constant with frequency and showa smooth decrease between 680 Hz and 890 Hz. The broad band X-ray spectrum ismodeled as the sum of a disk and a thermal comptonized component, plus a broadiron line, expected from reflection. The spectral parameters follow a smoothrelationship with the QPO frequency, in particular the fitted inner disk radiusdecreases steadily with frequency. Both the bump around the iron line in thelag energy spectrum, and the consistency between the lag changes and theinferred changes of the inner disk radius, from either spectral fitting or theQPO frequency, suggest that the soft lags may indeed involve reverberation ofthe hard pulsating QPO source on the disk.

Soft lags in neutron star kHz Quasi Periodic Oscillations: evidence for reverberation?