Spectral and timing properties of IGR J00291+5934 during its 2015 outburst

Abstract

We report on the spectral and timing properties of the accreting millisecond\nX-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015\noutburst. The source is in a hard state dominated at high energies by a\ncomptonization of soft photons ($\\sim0.9$ keV) by an electron population with\nkT$_e\\sim30$ keV, and at lower energies by a blackbody component with\nkT$\\sim0.5$ keV. A moderately broad, neutral Fe emission line and four narrow\nabsorption lines are also found. By investigating the pulse phase evolution, we\nderived the best-fitting orbital solution for the 2015 outburst. Comparing the\nupdated ephemeris with those of the previous outbursts, we set a $3\\sigma$\nconfidence level interval $-6.6\\times 10^{-13}$ s/s $< \\dot{P}_{orb} < 6.5\n\\times 10^{-13}$ s/s on the orbital period derivative. Moreover, we\ninvestigated the pulse profile dependence on energy finding a peculiar\nbehaviour of the pulse fractional amplitude and lags as a function of energy.\nWe performed a phase-resolved spectroscopy showing that the blackbody component\ntracks remarkably well the pulse-profile, indicating that this component\nresides at the neutron star surface (hot-spot).\n