The 2015 outburst of the accretion-powered pulsar IGR J00291+5934: INTEGRAL and Swift observations

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 ∼25 d. 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 (0.1-250 keV) persistent spectrum in 2015 is well described by a thermal Comptonization model with a column density of N_H ≈ 4 × 10²¹ cm^-2, a plasma temperature of kT_e ≈ 50 keV, 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 ∼150 keV 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 ≈ 1.5 × 10⁹ g cm^-2 following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first ∼20 s 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 ± 0.5 kpc.