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.

Discovery of millihertz quasi-periodic oscillations in the X-ray binary EXO 0748−676

“…For example, different composition of the burning material, different accretion geometry (i.e., accretion mainly onto the NS equator vs. spherical accretion onto most of the NS surface), different NS spin frequency and/or magnetic field, and a combination of all these, can affect the burning process and how we detect it. Ferrigno et al (2017) detected a QPO at an approximately constant frequency of ∼ 8 mHz in the accreting millisecond X-ray pulsar (AMXP) IGR J00291+5934. The QPO was persistently present during an 80-ksec XMM-Newton observation.…”

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

“…Additionally, two harmonically related QPOs are observed at ∼ 20 mHz and ∼ 40 mHz that appear more like the low-frequency QPOs observed in black holes (Linares et al 2007) than any feature normally seen in accreting neutron stars. The 2015 outburst showed a similar band-limited noise structure, but different QPOs (Ferrigno et al 2017). In that outburst a prominent 8 mHz QPO was observed at low (< 2 keV) energies.…”

“…We can distinguish two instances of mHz QPOs, namely the 8 mHz QPO observed with XMM-Newton in the 2015 outburst (Ferrigno et al 2017) and the ∼ 20 mHz and its harmonics as seen in RXTE observations of earlier outbursts (Linares et al 2007;Hartman et al 2011). Because the two versions of millihertz variability are not observed simultaneously, nor in the same energy band, it is unclear if they share the same underlying physical mechanism (Ferrigno et al 2017). In the following we therefore first discuss the XMM-Newton and RXTE results separately, before briefly considering them together.…”

“…In a similar example, Ferrigno et al (2017) measured pulse properties over short time segments. Rather than using the flux distribution, they computed the power spectrum for a series of consecutive pulse amplitude measurements.…”

The Stochastic X-Ray Variability of the Accreting Millisecond Pulsar MAXI J0911–655

“…This feature is not seen MAXI J0911, but if it is due to 'heartbeat' variability (Belloni et al 2000;Altamirano et al 2011), as suggested by Ferrigno et al (2017), then such flaring may be a transient phenomenon and MAXI J0911 may in fact be a good candidate to monitor for such variability.…”