The double-peaked 2008 outburst of the accreting milli-second X-ray pulsar, IGR J00291+5934

Abstract

Context. In August 2008, the accreting milli-second X-ray pulsar (AMXP), 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 J00291+5934 from Faulkes Telescope North, the Isaac Newton Telescope, the Keck Telescope, PAIRITEL, the Westerbork Synthesis Radio Telescope and the Swift, XMM-Newton and RXTE X-ray missions. We study the outburst's evolution at various wavelengths, allowing us to probe accretion physics in this AMXP. Methods. We study the light curve morphology, presenting the first radio-X-ray spectral energy Distributions (SEDs) for this source and the most detailed UV-IR SEDs for any outbursting AMXP. We show simple models that attempt to identify the emission mechanisms responsible for the SEDs. We analyse short-timescale optical variability, and compare a medium resolution optical spectrum with those from 2004. Results. The outburst morphology is unusual for an AMXP, comprising two 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. We detect optical variability of ∼0.05 mag, on timescales of minutes, but find no periodic modulation. In the optical, the SEDs contain a blue component, indicative of an irradiated disc, and a transient near-infrared (NIR) excess. This excess is consistent with a simple model of an optically thick synchrotron jet (as seen in other outbursting AMXPs), however we discuss other potential origins. The optical spectrum shows a double-peaked Hα profile, a diagnostic of an accretion disc, but we do not clearly see other 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 constraints to model disc-jet coupling in accreting neutron stars.

A late jet rebrightening revealed from multiwavelength monitoring of the black hole candidate XTE J1752−223★

“…However, similar SEDs have been reported (e.g. Lewis et al 2010) and the spectral shape of the NIR excess could be explained by the jet base (Markoff, Nowak & Wilms 2005) or a cooling feature in the jet (Pe’er & Casella 2009). SED [1] also shows a similar H − K S colour, but that SED was acquired from a different telescope so it is unlikely to be a common calibration issue (and the effect of an incorrect extinction cannot account for the spectral shape here in the IR).…”

The influence of spin on jet power in neutron star X-ray binaries

“…Like BHs (e.g. Corbel & Fender 2002; Coriat et al 2009), an optical/IR excess above the accretion disc flux has been reported for some NS XRBs, and identified as likely synchrotron emission from the jet (Wang et al 2001; Krauss et al 2005; Giles et al 2005; Greenhill et al 2006; Migliari et al 2006, 2010; Russell et al 2007; Torres et al 2008; Lewis et al 2010). These are generally systems with short orbital periods and hence smaller and dimmer accretion discs, comprising the AMXPs and the ultracompacts.…”

“…We use the jet flux density measured in the same waveband for each source; the NIR K ‐band (2 μm) in which the jet is typically much brighter than the disc component. Some observations contained K ‐band fluxes, which we take directly as the jet flux density (IGR J00291+5934 from Torres et al 2008 and Lewis et al 2010 and SAX J1808.4−3658 from Wang et al 2001). For sources in which no K ‐band observations were made, we infer the K ‐band flux density of the jet from the SED.…”

“…All the flux densities are de‐reddened to account for interstellar dust extinction. For some observations of XTE J0929−314 (Giles et al 2005), SAX J1808.4−3658 (Greenhill, Giles & Coutures 2006), IGR J00291+5934 (Lewis et al 2010) and XTE J1814−338 (Krauss et al 2005), optical SEDs were obtained only, for which we take the flux density from the reddest (lowest frequency) filter, i.e. the filter with the most prominent excess ( I band or y band at 1 μm).…”

A tool to separate optical/infrared disc and jet emission in X-ray transient outbursts: the colour-magnitude diagrams of XTE J1550-564

“…Monitoring of the outburst in two wavebands is required, which emphasizes the importance of such campaigns (see e.g. Jain et al 2001; Lewis et al 2008; 2010; Coriat et al 2009). The colour–magnitude diagrams show:…”