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

TL;DR

This paper reports on the 2015 outburst of the fastest-known accretion-powered pulsar IGR J00291+5934, including spectral analysis, pulsation detection up to 150 keV, and the first observed thermonuclear burst, providing insights into its accretion and burst properties.

Contribution

It presents the first thermonuclear burst observation from IGR J00291+5934 and detailed spectral and timing analysis during its 2015 outburst, expanding understanding of this pulsar's behavior.

Findings

Detection of pulsations up to 150 keV.

Observation of the first thermonuclear burst from the source.

Estimated source distance of 4.2 ± 0.3 kpc.

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 $\sim25$ 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 $N_{\rm H} \approx4\times10^{21}$ cm$^{-2}$, a plasma temperature of $kT_{\rm e} \approx50$ keV, and a Thomson optical depth of $\tau_{\rm T}\approx1$. Pulsations at the known spin period of the source are detected in the Integral data up to the $\sim150$ 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 $y_{\rm ign}\approx1.5\times10^9$ g cm$^{-2}$ following the exhaustion by steady burning of the accreted hydrogen. The Swift/BAT data from the first $\sim20$ 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.