X-ray reflection in oxygen-rich accretion discs of ultra-compact X-ray binaries

Abstract

We present spectroscopic X-ray data of two candidate ultra-compact X-ray binaries: 4U~0614+091 and 4U~1543$-$624. We confirm the presence of a broad O VIII Ly$\alpha$ reflection line (at $\approx 18\mathring{A}$) using {\it XMM-Newton} and {\it 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 Ly$\alpha$ line particularly strong. We also confirm the presence of a strong absorption edge at $\approx 14$ Å so far interpreted in the literature as due to absorption by neutral neon in the circumstellar and interstellar medium. However, the abundance required to obtain a good fit to this edge is $\approx 3-4$ times solar, posing a problem for this interpretation. Furthermore, 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 parameters. In order to describe the X-ray reflection spectra self-consistently we modify the currently available {\sc xillver} reflection model. We present initial grids that can be used to model X-ray reflection spectra in UCXBs with carbon-oxygen-rich 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$-$624 than the reflection models that assume solar abundances. Besides a strong O VIII Ly$\alpha$ line the new reflection model also shows a strong O VIII K-edge (at $14.23$ Å). We find that the absorption edge at $\approx 14$ Å 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 in the interstellar medium, mitigating the problem of the apparent very high neon abundance.

10 pages, 7 figures, submitted to MNRAS

References in corpus (4)

• Relativistic X-ray Lines from the Inner Accretion Disks Around Black Holes
• X-ray reflection in accreting stellar-mass black hole systems
• High-Resolution X-ray Spectroscopy of the Interstellar Medium II: Neon and Iron Absorption Edges
• Time-resolved optical photometry of the ultra-compact binary 4U0614+091

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• Disk reflection and a possible disk wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9-342058
• Testing jet geometries and disk-jet coupling in the neutron star LMXB 4U 0614+091 with the internal shocks model
• High-Resolution X-Ray Spectroscopy of Interstellar Iron Toward Cygnus X-1 and GX 339-4
• On the impact of an intermediate duration X-ray burst on the accretion environment in IGR J17062-6143