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

TY - JOUR

T1 - Discovery of a soft X-ray 8 mHz QPO from the accreting millisecond pulsar IGR J00291+5934

AU - Iaria, Rosario

AU - Di Salvo, Tiziana

AU - D'Aì, null

AU - Riggio, null

AU - Bozzo, null

AU - Pintore, null

AU - Sanna, null

AU - Burderi, null

AU - Papitto, null

AU - Ferrigno, null

PY - 2017

Y1 - 2017

N2 - 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 is characterized by a flaring-like behaviour, 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 behaviour. 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 behaviour. We suggest that the unusual variability of IGR J00291+5934 observed by XMM-Newton and NuSTAR could be produced by a heartbeat-like mechanism, similar to that observed 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 sustained in the literature for a number of other neutron star binaries displaying a similar behaviour, cannot be solidly tested in the case of IGR J00291+5934 due to the paucity of type I X-ray bursts detected from this source.

AB - 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 is characterized by a flaring-like behaviour, 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 behaviour. 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 behaviour. We suggest that the unusual variability of IGR J00291+5934 observed by XMM-Newton and NuSTAR could be produced by a heartbeat-like mechanism, similar to that observed 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 sustained in the literature for a number of other neutron star binaries displaying a similar behaviour, cannot be solidly tested in the case of IGR J00291+5934 due to the paucity of type I X-ray bursts detected from this source.

KW - Astronomy and Astrophysics

KW - Pulsars: individual: IGR J00291+5934

KW - Space and Planetary Science

KW - Stars: neutron

KW - X-rays: binaries

UR - http://hdl.handle.net/10447/228373

UR - http://mnras.oxfordjournals.org/

M3 - Article

VL - 466

SP - 3450

EP - 3459

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

ER -