Sanna, A.A.SannaPintore, FabioFabioPintoreBozzo, E.E.BozzoFerrigno, C.C.FerrignoPAPITTO, ALESSANDROALESSANDROPAPITTORiggio, A.A.RiggioDi Salvo, T.T.Di SalvoIaria, R.R.IariaD'AI', ANTONINOANTONINOD'AI'EGRON, ELISE MARIE JEANNEELISE MARIE JEANNEEGRONBurderi, L.L.Burderi2020-08-312020-08-3120170035-8711http://hdl.handle.net/20.500.12386/26997We report on the spectral and timing properties of the accreting millisecond X-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015 outburst. The source is in a hard state dominated at high energies by a Comptonization of soft photons (∼0.9 keV) by an electron population with kT<SUB>e</SUB> ∼ 30 keV, and at lower energies by a blackbody component with kT ∼ 0.5 keV. A moderately broad, neutral Fe emission line and four narrow absorption lines are also found. By investigating the pulse phase evolution, we derived the best-fitting orbital solution for the 2015 outburst. Comparing the updated ephemeris with those of the previous outbursts, we set a 3σ confidence level interval -6.6 × 10<SUP>-13</SUP> s s<SUP>-1</SUP> < dot{P}_{orb} < 6.5 × 10^{-13} s s<SUP>-1</SUP> on the orbital period derivative. Moreover, we investigated the pulse profile dependence on energy finding a peculiar behaviour of the pulse fractional amplitude and lags as a function of energy. We performed a phase-resolved spectroscopy showing that the blackbody component tracks remarkably well the pulse profile, indicating that this component resides at the neutron star surface (hotspot).STAMPAenArticle10.1093/mnras/stw33322-s2.0-85017402487000398418600029https://academic.oup.com/mnras/article/466/3/2910/27338612017MNRAS.466.2910SFIS/05 - ASTRONOMIA E ASTROFISICA