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We study the relation between the 300-700 Hz upper kHz quasi-periodicoscillation (QPO) and the 401 Hz coherent pulsations across all outbursts ofthe accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the RossiX-ray Timing Explorer. We find that the pulse amplitude systematically changesby a factor of ~2 when the upper kHz QPO frequency passes through 401 Hz: ithalves when the QPO moves to above the spin frequency and doubles again on theway back. This establishes for the first time the existence of a direct effectof kHz QPOs on the millisecond pulsations and provides a new clue to the originof the upper kHz QPO. We discuss several scenarios and conclude that while morecomplex explanations can not formally be excluded, our result strongly suggeststhat the QPO is produced by azimuthal motion at the inner edge of the accretiondisk, most likely orbital motion. Depending on whether this azimuthal motion isfaster or slower than the spin, the plasma then interacts differently with theneutron-star magnetic field. The most straightforward interpretation involvesmagnetospheric centrifugal inhibition of the accretion flow that sets in whenthe upper kHz QPO becomes slower than the spin.

Pulse amplitude depends on kHz QPO frequency in the accreting millisecond pulsar SAX J1808.4-3658