Quasi-periodic pulse amplitude modulation in the accreting millisecond pulsar IGR J00291+5934

TL;DR

This paper presents a novel analysis method for studying the coupling between pulsation amplitude modulation and aperiodic variability in accreting millisecond X-ray pulsars, revealing new insights into accretion processes.

Contribution

The authors introduce a new frequency correction technique to analyze aperiodic pulsation variability, enabling detection of coupling with multiple QPOs in IGR J00291+5934.

Findings

Confirmed coupling between pulsations and 8 mHz QPO.

First detection of coupling with 20 mHz QPO and its harmonic.

Demonstrated the method's effectiveness with RXTE and XMM-Newton data.

Abstract

We introduce a new method for analysing the aperiodic variability of coherent pulsations in accreting millisecond X-ray pulsars. Our method involves applying a complex frequency correction to the time-domain light curve, allowing for the aperiodic modulation of the pulse amplitude to be robustly extracted in the frequency domain. We discuss the statistical properties of the resulting modulation spectrum and show how it can be correlated with the non-pulsed emission to determine if the periodic and aperiodic variability are coupled processes. Using this method, we study the 598.88 Hz coherent pulsations of the accreting millisecond X-ray pulsar IGR J00291+5934 as observed with the Rossi X-ray Timing Explorer and XMM-Newton. We demonstrate that our method easily confirms the known coupling between the pulsations and a strong 8 mHz QPO in XMM-Newton observations. Applying our method to the RXTE observations, we further show, for the first time, that the much weaker 20 mHz QPO and its harmonic are also coupled the pulsations. We discuss the implications of this coupling and indicate how it may be used to extract new information on the underlying accretion process.