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

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.