INTEGRAL and RXTE observations of accreting millisecond pulsar IGR J00291+5934 in outburst

¹ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique (CNRS FRE 2591), 91191 Gif-sur-Yvette, France e-mail: mfalanga@cea.fr
² Unité mixte de recherche Astroparticule et Cosmologie, 11 place Berthelot, 75005 Paris, France
³ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ Astronomy Division, PO Box 3000, 90014 University of Oulu, Finland
⁵ Laboratoire de l'Univers et de ses Théories, Observatoire de Paris, 92195 Meudon Cedex, France
⁶ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁷ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
⁸ School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK
⁹ INTEGRAL Science Data Centre, 1290 Versoix, Switzerland
¹⁰ Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy

Received: 19 May 2005
Accepted: 18 August 2005

Abstract

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 in a slab geometry. The spectral shape is almost constant during the outburst. For the first time we detect a spin-up, , of an accreting millisecond pulsar. The ISGRI data reveal the pulsation of X-rays at a period of 1.67 milliseconds up to  keV. The pulsed fraction is shown to increase from 6 per cent at 6 keV to 12–20 per cent at 100 keV. 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 7 keV, then decreasing to 15 keV, and seemingly saturating at higher energies.