2017A&A...599A..88D

The pulsar IGR J00291+5934 is the fastest-known accretion-powered X-ray pulsar, discovered during a transient outburst in 2004. In this paper, we report on INTEGRAL and Swift observations during the 2015 outburst, which lasts for ∼25d. The source has not been observed in outburst since 2008, suggesting that the long-term accretion rate has decreased by a factor of two since discovery. The averaged broad-band (0.1-250keV) persistent spectrum in 2015 is well described by a thermal Comptonization model with a column density of N_H~=4x10²¹cm^–2, a plasma temperature of kT_e~=50keV, and a Thomson optical depth of τ_T~=1. Pulsations at the known spin period of the source are detected in the INTEGRAL data up to the ∼150keV energy band. We also report on the discovery of the first thermonuclear burst observed from IGR J00291+5934, which lasts around 7 min and occurs at a persistent emission level corresponding to roughly 1.6% of the Eddington accretion rate. The properties of the burst suggest it is powered primarily by helium ignited at a depth of y_ign~=1.5x10⁹g/cm² following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first ∼20s of the burst provide indications of a photospheric radius expansion phase. Assuming this is the case, we infer a source distance of d=4.2±0.5kpc.