Discovery of a broad O VIII Ly alpha line in the ultra-compact X-ray binary 4U 1543-624

We report the discovery of a broad emission feature at ~0.7 keV in the spectra of the ultra-compact 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 Ly alpha 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 keV and ~6.6 keV as O VIII Ly alpha and Fe K alpha emission respectively, caused by X-rays reflected off the accretion disc in the strong gravitational field close to the accretor.

💡 Research Summary

The authors present a comprehensive X‑ray spectral analysis of the ultra‑compact X‑ray binary 4U 1543‑624 using high‑resolution data from XMM‑Newton (RGS and MOS2) and Chandra (HETGS). While standard continuum models (absorbed multi‑temperature blackbody plus power‑law) adequately describe most of the spectrum, a pronounced, broad excess around 0.7 keV remains unexplained. By incorporating a relativistically blurred reflection component, the authors identify this feature as the O VIII Lyα line (rest energy ≈0.654 keV) originating from an oxygen‑rich accretion disc. The donor star is believed to be a CO or ONe white dwarf, supplying oxygen‑rich material that enriches the disc composition.

Spectral fitting with Laor/reline relativistic line profiles yields an inner disc radius of roughly 6–10 GM/c², indicating that the reflecting region extends down to the innermost stable circular orbit around a neutron star. The inferred inclination exceeds 70°, suggesting a highly tilted system. The reflection fraction is about 10 % of the total 0.3–10 keV flux, and the ionization parameter (log ξ≈3.5) points to a highly ionized disc surface. In addition to the O VIII line, a weaker Fe Kα emission near 6.6 keV is detected, consistent with a modest iron abundance in the same reflecting region.

To model the unusual line strengths, the authors employ a customized reflection model (xillver‑CO) that enhances oxygen and carbon abundances relative to iron. This model significantly improves the fit statistics compared with standard Fe‑rich reflection models, confirming that the disc’s chemical composition is a key factor in shaping the observed spectrum.

The discovery of a broad O VIII Lyα line provides the first direct spectroscopic evidence that the accretion disc in a UCXB can be dominated by oxygen, and that X‑ray reflection off such a disc produces detectable relativistic emission lines. The line’s width and profile encode information about the disc’s inner radius, inclination, and the strong gravitational field near the compact object, offering a novel probe of neutron‑star parameters and disc physics. The simultaneous detection of Fe Kα, albeit weak, demonstrates that trace amounts of iron coexist with the oxygen‑rich material.

Overall, the paper establishes that high‑resolution X‑ray spectroscopy can reveal the composition and geometry of accretion discs in ultra‑compact binaries, opening a pathway for future missions (e.g., XRISM, Athena) to refine measurements of neutron‑star masses, radii, and the evolutionary history of these exotic systems.