2005A&A...444...15F

Simultaneous observations of the accretion-powered millisecond pulsar IGR J00291+5934 by the International Gamma-Ray Astrophysics Laboratory and Rossi X-ray Timing Explorer during the 2004 December outburst are analysed. The average spectrum is well described by thermal Comptonization with an electron temperature of 50 keV and Thomson optical depth τ_T∼1 in a slab geometry. The spectral shape is almost constant during the outburst. For the first time we detect a spin-up, {dot}(ν)=8.4x10^–13Hz/s, of an accreting millisecond pulsar. The ISGRI data reveal the pulsation of X-rays at a period of 1.67 milliseconds up to ∼150keV. The pulsed fraction is shown to increase from 6 per cent at 6keV to 12-20 per cent at 100keV. This is naturally explained by the action of the Doppler effect on the exponentially cutoff Comptonization spectrum from the hot spot. The nearly sinusoidal pulses show soft lags with a complex energy dependence, increasing up to 7keV, then decreasing to 15keV, and seemingly saturating at higher energies.