We 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 ( \sim 0.9 keV ) by an electron population with kT _ { e } \sim 30 keV , and at lower energies by a blackbody component with kT \sim 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 3 \sigma confidence level interval -6.6 \times 10 ^ { -13 } s/s < \dot { P } _ { orb } < 6.5 \times 10 ^ { -13 } s/s 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 ( hot-spot ) .