Discovery of a soft X-ray 8 mHz QPO from the accreting millisecond pulsar IGR J00291+5934

Abstract

In this paper, we report on the analysis of the peculiar X-ray variability displayed by the accreting millisecond X-ray pulsar IGR J00291+5934 in a 80 ks-long joint NuSTAR and XMM-Newton observation performed during the source outburst in 2015. The light curve of the source was characterized by a flaring-like behavior, with typical rise and decay time scales of ~120 s. The flares are accompanied by a remarkable spectral variability, with the X-ray emission being generally softer at the peak of the flares. A strong quasi periodic oscillation (QPO) is detected at ~8 mHz in the power spectrum of the source and clearly associated with the flaring-like behavior. This feature has the strongest power at soft X-rays (<3 keV). We carried out a dedicated hardness-ratio resolved spectral analysis and a QPO phase-resolved spectral analysis, together with an in-depth study of the source timing properties, to investigate the origin of this behavior. We suggest that the unusual variability of IGR J00291+5934 observed by XMM-Newton and NuSTAR could be produced by an heartbeat-like mechanism, similar to that operating in black-hole X-ray binaries. The possibility that this variability, and the associated QPO, are triggered by phases of quasi-stable nuclear burning, as suggested in the literature for a number of other neutron star binaries displaying a similar behavior, cannot be solidly tested in the case of IGR J00291+5934 due to the paucity of type-I X-ray bursts observed from this source.

Submitted to MNRAS on 23 Sept 2016. Modified according to the referee's suggestions. Comments are welcomed. One reference updated in this version

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• The stochastic X-ray variability of the accreting millisecond pulsar MAXI J0911-655
• Detection of millihertz quasi-periodic oscillations in the low-mass X-ray binary 4U 1730--22 with NICER
• XMM-Newton and NICER measurement of the rms spectrum of the millihertz quasi-periodic oscillations in the neutron-star low-mass X-ray binary 4U 1636-53
• Drifts of the marginally stable burning frequency in the X-ray binaries 4U 1608--52 and Aql X--1
• Detection of a strong ~2.5 Hz modulation in the Newly Discovered Millisecond Pulsar MAXI J1816-195
• Quasi-periodic pulse amplitude modulation in the accreting millisecond pulsar IGR J00291+5934