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

TL;DR

This study analyzes 14 years of X-ray timing data from the pulsar SAX J1808.4-3658, revealing unique aperiodic variability, a novel ~410 Hz QPO behavior, and insights into the dynamics of twin kHz QPOs and their relation to neutron star spin.

Contribution

It provides the first detailed analysis of the ~410 Hz QPO, links low-frequency phenomena to precession effects, and measures the evolution of twin kHz QPOs in SAX J1808.4-3658.

Findings

The ~410 Hz QPO appears only when the upper kHz QPO is below the spin frequency.

The ~410 Hz QPO is likely a retrograde beat against the spin frequency.

Twin kHz QPOs maintain a frequency separation proportional to the spin frequency.

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 $(0.446 \pm 0.009) \nu_{spin}$.