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

¹ Institut für theoretische Physik, Universität Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
e-mail: vittorio-df@issibern.ch
² International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
³ SRON–Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁴ ISDC, Data centre for astrophysics, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁵ School of Physics and Astronomy, Monash University, VIC 3800, Australia
⁶ Monash Centre for Astrophysics, Monash University, VIC 3800, Australia
⁷ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁸ INAF–Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio Catone, 00078 Rome, Italy

Received: 24 August 2016
Accepted: 5 December 2016

Abstract

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.