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

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.

arXiv comment5 pages, 4 figures, Accepted for publication in ApJL

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• Long-term coherent timing of the accreting millisecond pulsar IGR J17062-6143
• The highest-frequency kHz QPOs in neutron star low mass X-ray binaries
• Broadband mHz QPOs and spectral study of LMC X$-$4 with AstroSat
• Phase lags of quasi-periodic oscillations across source states in the low-mass X-ray binary 4U 1636-53
• On the frequency correlations of low-frequency QPOs with kilohertz QPOs in accreting millisecond X-ray pulsars
• Correlations between the frequencies of twin kHz QPOs and spins of neutron stars in LMXBs
• Probing the accretion disc structure by the twin kHz QPOs and spins of neutron stars in LMXBs
• The stochastic X-ray variability of the accreting millisecond pulsar IGR J17062-6143