Ultracompact X‐ray binaries consist of a neutron star or black hole that accretes material from a white dwarf donor star. The ultracompact nature is expressed in very short orbital periods of less than 1 h. In the case of 4U 0614+091 oxygen‐rich material from a CO or ONe white dwarf is flowing to the neutron star. This oxygen‐rich disc can reflect X‐rays emitted by the neutron star giving a characteristic emission spectrum. We have analysed high‐resolution Reflection Grating Spectrometer and broad‐band European Photon Imaging Camera (EPIC) spectra of 4U 0614+091 obtained by the XMM–Newton satellite. We detect a broad emission feature at ∼0.7 keV in both instruments, which cannot be explained by unusual abundances of oxygen and neon in the line of sight, as proposed before in the literature. We interpret this feature as O viii Lyα emission caused by reflection of X‐rays off highly ionized oxygen, in the strong gravitational field close to the neutron star.

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 Å) 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 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 ≈ 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 Xray 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. Besides a strong O VIII Lyα line the new reflection model also shows a strong O VIII K-edge (at 14.23 Å). We find that the absorption edge at ≈ 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 (if not entirely) in the interstellar medium, mitigating the 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.

Accretion onto the black hole in the system HDE 226868/Cygnus X-1 is powered by the strong line-driven stellar wind of the O-type donor star. We study the X-ray properties of the stellar wind in the hard state of Cyg X-1, as determined using data from the Chandra High Energy Transmission Gratings. Large density and temperature inhomogeneities are present in the wind, with a fraction of the wind consisting of clumps of matter with higher density and lower temperature embedded in a photoionized gas. Absorption dips observed in the light curve are believed to be caused by these clumps. This work concentrates on the non-dip spectra as a function of orbital phase. The spectra show lines of H-like and He-like ions of S, Si, Na, Mg, Al, and highly ionized Fe (Fe xvii-Fe xxiv). We measure velocity shifts, column densities, and thermal broadening of the line series. The excellent quality of these five observations allows us to investigate the orbital phase-dependence of these parameters. We show that the absorber is located close to the black hole. Doppler shifted lines point at a complex wind structure in this region, while emission lines seen in some observations are from a denser medium than the absorber. The observed line profiles are phase-dependent. Their shapes vary from pure, symmetric absorption at the superior conjunction to P Cygni profiles at the inferior conjunction of the black hole.