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

TL;DR

This study reveals a direct correlation between kHz QPO frequency and pulse amplitude in SAX J1808.4-3658, suggesting the QPO's origin is linked to orbital motion at the inner accretion disk edge.

Contribution

It provides the first evidence of a direct effect of kHz QPOs on millisecond pulsations, offering new insights into the QPO's origin and accretion dynamics.

Findings

Pulse amplitude changes by a factor of ~2 across QPO frequency transition.

QPO likely produced by azimuthal orbital motion at the inner disk edge.

Magnetospheric centrifugal inhibition may explain the observed phenomena.

Abstract

We study the relation between the 300-700 Hz upper kHz quasi-periodic oscillation (QPO) and the 401 Hz coherent pulsations across all outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the Rossi X-ray Timing Explorer. We find that the pulse amplitude systematically changes by a factor of ~2 when the upper kHz QPO frequency passes through 401 Hz: it halves when the QPO moves to above the spin frequency and doubles again on the way back. This establishes for the first time the existence of a direct effect of kHz QPOs on the millisecond pulsations and provides a new clue to the origin of the upper kHz QPO. We discuss several scenarios and conclude that while more complex explanations can not formally be excluded, our result strongly suggests that the QPO is produced by azimuthal motion at the inner edge of the accretion disk, most likely orbital motion. Depending on whether this azimuthal motion is faster or slower than the spin, the plasma then interacts differently with the neutron-star magnetic field. The most straightforward interpretation involves magnetospheric centrifugal inhibition of the accretion flow that sets in when the upper kHz QPO becomes slower than the spin.