The double peaked 2008 outburst of the acccreting millisecond X-ray pulsar IGR J 00291 + 5934 Journal Item

Context. In August 2008, the accreting milli-second X-ray pulsar (AM XP), IGR J00291+5934, underwent an outburst lasting ∼ 100 days, the first since its discovery in 2004. Aims. We present data from the 2008 double-peaked outburst of IGR J 00291+5934 from Faulkes Telescope North, the Isaac Newton Telescope, the Keck Telescope, PAIRITEL, the Westerbork Sy nthesis Radio Telescope and the Swift, XMM-Newton and RXTE X ray missions. We study the outburst’s evolution at various w avelengths, allowing us to probe accretion physics in this A MXP. Methods. We study the light curve morphology, presenting the first rad io–X–ray Spectral Energy Distributions (SEDs) for this sou rce and the most detailed UV–IR SEDs for any outbursting AMXP. We show simple models that attempt to identify the emission mec hanisms responsible for the SEDs. We analyse short-timescal e optical variability, and compare a medium resolution opti cal spectrum with those from 2004. Results. The outburst morphology is unusual for an AMXP, comprising t wo peaks, the second containing a ‘plateau’ of ∼ 10 days at maximum brightness within 30 days of the initial activity . This has implications on duty cycles of short-period X-ray transients. The X-ray spectrum can be fitted by a single, hard power-law. W e detect optical variability of ∼ 0.05 magnitudes, on timescales of minutes, but find no periodic modulation. In the optical, the SEDs contain a blue component, indicative of an irradiated d isc, and a transient near-infrared (NIR) excess. This excess is consi stent with a simple model of an optically thick synchrotron j et (as seen in other outbursting AMXPs), however we discuss other potenti al origins. The optical spectrum shows a double-peaked H α profile, a diagnostic of an accretion disc, but we do not clearly see oth r lines (e.g. He I, II) that were reported in 2004. Conclusions. Optical/IR observations of AMXPs appear to be excellent for studying the evolution of both the outer accretion disc and the inner jet, and may eventually provide us with tight co nstraints to model disc-jet coupling in accreting neutron s tars.