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

TL;DR

This paper reports the discovery of a broad O VIII Ly alpha emission line in the X-ray spectrum of the ultra-compact binary 4U 1543-624, indicating reflection off an oxygen-rich accretion disk near a neutron star.

Contribution

It provides the first detection of a broad O VIII Ly alpha line in this system, supporting the presence of oxygen-rich material and reflection processes in the accretion disk.

Findings

Detection of a broad ~0.7 keV emission feature attributed to O VIII Ly alpha.

Confirmation of a weak ~6.6 keV emission feature likely due to Fe K alpha.

Evidence of X-ray reflection from an oxygen-rich accretion disk near a neutron star.

Abstract

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.