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

We present spectroscopic X-ray data of two candidate ultra-compact X-ray binaries: 4U 0614+091 and 4U 1543−624. We confirm the presence of a broad O VIII Lyα reflection line (at≈ 18 A) using XMM-Newton and Chandra observations obtained in 2012 and 2013. The donor star in these sources is 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. We also confirm the presence of a strong absorption edge at≈ 14 A so far interpreted in the literature as due to absorption by neutra l neon in the circumstellar and interstellar medium. However, the abundance required to obtain a good fit to this edge is ≈ 3− 4 times solar, posing a problem for this interpretation. Furt hermore, modeling 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-fit para meters. In order to describe the Xray reflection spectra self-consistently we modify the curr ently availablexillver 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 th e reflection spectra of 4U 0614 +091 and 4U 1543−624 than the reflection models that assume solar abundances. Besides a strong O VIII Lyα line the new reflection model also shows a strong O VIII K-edge (at 14.23 A). We find that the absorption edge at ≈ 14 A present in the data can be described by a O VIII K-edge formed due to reflection in the accretion disc and a Ne I K-edge originating mostly (if not entirely) in the interstellar medium, mitigating th e problem of the apparent very high neon abundance. Additionally, based on the spectral properties of 4U 1543−624 we consider a scenario in which this source is accreting near the Eddington limit.