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

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 broadband 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 OVIII 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 OVIII Lyα and Fe Kα emission, respectively, caused by X-rays reflected off the accretion disc in the strong gravitational field close to the accretor. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.