The aperiodic X-ray variability of the accreting millisecond pulsar SAX J1808.4-3658

Abstract

We have studied the aperiodic variability of the 401 Hz accreting millisecond X-ray pulsar SAX J1808.4-3658 using the complete data set collected with the Rossi X-ray Timing Explorer over 14 years of observation. The source shows a number of exceptional aperiodic timing phenomena that are observed against a backdrop of timing properties that show consistent trends in all five observed outbursts and closely resemble those of other atoll sources. We performed a detailed study of the enigmatic ~410 Hz QPO, which has only been observed in SAX J1808.4-3658. We find that it appears only when the upper kHz QPO frequency is less than the 401 Hz spin frequency. The difference between the ~410 Hz QPO frequency and the spin frequency follows a similar frequency correlation as the low frequency power spectral components, suggesting that the ~410 Hz QPO is a retrograde beat against the spin frequency of a rotational phenomenon in the 9 Hz range. Comparing this 9 Hz beat feature with the Low-Frequency QPO in SAX J1808.4-3658 and other neutron star sources, we conclude that these two might be part of the same basic phenomenon. We suggest that they might be caused by retrograde precession due to a combination of relativistic, classical and magnetic torques. Additionally we present two new measurements of the lower kHz QPO, allowing us, for the first time, to measure the frequency evolution of the twin kHz QPOs in this source. The twin kHz QPOs are seen to move together over 150 Hz, maintaining a centroid frequency separation of $(0.446\pm 0.009){\nu }_{spin}$.

18 pages, 9 figures, 7 tables. Accepted for publication in ApJ

References in corpus (8)

• Global General Relativistic MHD Simulation of a Tilted Black-Hole Accretion Disk
• X-ray time variability across the atoll source states of 4U 1636--53
• A decade of timing an accretion-powered millisecond pulsar: The continuing spin down and orbital evolution of SAX J1808.4-3658
• Order in the Chaos: Spin-up and Spin-down during the 2002 Outburst of SAX J1808.4-3658
• Swift observations of SAX J1808.4-3658: monitoring the return to quiescence
• Pulse amplitude depends on kHz QPO frequency in the accreting millisecond pulsar SAX J1808.4-3658
• Timing and spectral properties of the accreting millisecond pulsar SWIFT J1756.9-2508
• Discovery of 1-5 Hz flaring at high luminosity in SAX J1808.4-3658

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• Hydrogen-triggered X-ray Bursts from SAX J1808.4-3658? The Onset of Nuclear Burning
• Constraints on the magnetic field structure in accreting compact objects from aperiodic variability
• Spectral Analysis of the 2019 and 2022 Outbursts of SAX J1808.4-3658
• The stochastic X-ray variability of the accreting millisecond pulsar IGR J17062-6143