Ferrigno, C.C.FerrignoBozzo, E.E.BozzoSanna, A.A.SannaPintore, FabioFabioPintorePAPITTO, ALESSANDROALESSANDROPAPITTORiggio, A.A.RiggioBurderi, L.L.BurderiDi Salvo, T.T.Di SalvoIaria, R.R.IariaD'AI', ANTONINOANTONINOD'AI'2020-08-312020-08-3120170035-8711http://hdl.handle.net/20.500.12386/27005We 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.STAMPAenArticle10.1093/mnras/stw33442-s2.0-85014919092000398418600067https://academic.oup.com/mnras/article/466/3/3450/27363002017MNRAS.466.3450FFIS/05 - ASTRONOMIA E ASTROFISICA