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

Abstract

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 $\sim 25$ 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 $N_{\mathrm{H}}\approx 4\times 10^{21}$ cm${}^{-2}$, a plasma temperature of $k{T}_{\mathrm{e}}\approx 50$ keV, and a Thomson optical depth of ${\tau }_{\mathrm{T}}\approx 1$. Pulsations at the known spin period of the source are detected in the Integral data up to the $\sim 150$ 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 $y_{\mathrm{ign}}\approx 1.5\times 10^9$ g cm${}^{-2}$ following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first $\sim 20$ s of the burst provide indications of a photospheric radius expansion phase. Assuming this is the case, we infer a source distance of $d=4.2\pm 0.3$ kpc.

8 Pages, 6 figures, 5 tables. Accepted for publication on 5 December 2016 by the Journal A&A

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