Discovery of a broad O viii Lyα line in the ultracompact X-ray binary 4U 1543−624

Authors

  • O. K. Madej,

    Corresponding author
    1. Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, the Netherlands
    2. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
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  • P. G. Jonker

    1. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
    2. Harvard–Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
    3. Radboud University Nijmegen, PO Box 9102, 6500 HC Nijmegen, the Netherlands
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E-mail: O.Madej@sron.nl

ABSTRACT

We report the discovery of a broad emission feature at ∼0.7 keV in the spectra of the ultracompact X-ray binary 4U 1543−624, obtained with the high-resolution spectrographs of the XMM–Newton and Chandra satellites. We confirm the presence of the feature in the broad-band MOS2 spectrum of the source. As suggested before in the literature, the donor star in this source is a CO or ONe white dwarf, which transfers oxygen-rich material to the accretor, conceivably a neutron star. The X-rays reprocessed in this oxygen-rich accretion disc could give a reflection spectrum with O viii Lyα as the most prominent emission line. Apart from the feature at ∼0.7 keV, we confirm the possible presence of a weak emission feature at ∼6.6 keV, which was reported in the literature for this data set. We interpret the feature at ∼0.7 and ∼6.6 keV as O viii Lyα and Fe Kα emission, respectively, caused by X-rays reflected off the accretion disc in the strong gravitational field close to the accretor.

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