Quasi-periodic Pulse Amplitude Modulation in the Accreting Millisecond Pulsar IGR J00291+5934

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.

Long-term coherent timing of the accreting millisecond pulsar IGR J17062-6143

“…Indeed, the sample of known AMXPs shows ample evidence for such a transfer mechanism. On rapid timescales (seconds), the stochastic and quasi-periodic noise of the accretion disk has been shown to modulate the pulse amplitude (Menna et al 2003;Bult et al 2017). Pulse amplitudes also routinely show variations on slower daylong timescales, which are generally understood to be stochastically driven, and are obviously correlated with variations in the source luminosity (Chou et al 2008), spectroscopy (Kajava et al 2011), or temporal variability (Bult & van der Klis 2015).…”

The stochastic X-ray variability of the accreting millisecond pulsar IGR J17062-6143

“…While the pulsar properties of AMXPs alone make them interesting targets for study, these systems generally also exhibit the phenomenology seen in nonpulsating low-mass X-ray binaries (LMXBs), including accretion rate dependent state transitions (van Straaten et al 2005), quasi-periodic variability (QPOs) (Wijnands & van der Klis 1998;van Straaten et al 2005), and thermonuclear X-ray bursts (in 't Zand et al 1999;Chakrabarty et al 2003). Through this combination of pulsar and LMXB properties the different observ-ables of AMXPs offer complementary views of the same underlying accretion processes (Wijnands et al 2003;Chakrabarty et al 2003;Bult & van der Klis 2015a;Bult et al 2017).…”