X-ray reflection in oxygen-rich accretion discs of ultracompact X-ray binaries

Abstract

We present spectroscopic X-ray data of two candidate ultracompact X-ray binaries (UCXBs): 4U 0614+091 and 4U 1543−624. We confirm the presence of a broad O viii Lyα reflection line (at ≈18 Å) using XMM–Newton and Chandra observations obtained in 2012 and 2013. The donor star in these sources is a carbon–oxygen or oxygen–neon–magnesium white dwarf. Hence, the accretion disc is enriched with oxygen which makes the O viii Lyα line particularly strong. Modelling the X-ray reflection off a carbon- and oxygen-enriched, hydrogen- and helium-poor disc with models assuming solar composition likely biases several of the best-fitting parameters. In order to describe the X-ray reflection spectra self-consistently, we modify the currently available xillver reflection model. We present initial grids that can be used to model X-ray reflection spectra in UCXBs with carbon–oxygen-rich (and hydrogen- and helium-poor) accretion disc. We find that the new reflection model provides a better overall description of the reflection spectra of 4U 0614+091 and 4U 1543−624 than the reflection models that assume solar abundances.