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

¹ Faulkes Telescope Project, School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, CF24 3AA, Wales, UK e-mail: fraser.lewis@faulkes-telescope.com
² Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
³ Division of Earth, Space and Environment, University of Glamorgan, Pontypridd, CF37 1DL, Wales, UK
⁴ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands
⁵ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Department of Astrophysics, IMAPP, Radboud University Nijmegen, Toernooiveld 1, 6525 ED, Nijmegen, The Netherlands
⁸ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
⁹ Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹⁰ Astronomical Institute of the Romanian Academy, Cutitul de Argint 5, RO-040557 Bucharest, Romania
¹¹ Research Center for Atomic Physics and Astrophysics, Atomistilor 405, RO-077125 Bucharest, Romania
¹² Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA
¹³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
¹⁴ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
¹⁵ ESAC, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
¹⁶ University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹⁷ Department of Astronomy, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA 94720, USA
¹⁸ Miller Fellow
¹⁹ Las Cumbres Observatory Global Telescope Project, 6740 Cortona Dr., Santa Barbara, CA 93117, USA

Received: 8 March 2010
Accepted: 3 May 2010

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.