X-ray diagnostics of chemical composition of the accretion disc and donor star in ultra-compact X-ray binaries

Non-solar composition of the donor star in ultra-compact X-ray binaries (UCXBs) may have a pronounced effect on the fluorescent lines appearing in their spectra due to reprocessing of primary radiation by the accretion disc and the white dwarf surface. We show that the most dramatic and easily observable consequence of the anomalous C/O abundance is the significant, by more than an order of magnitude, attenuation of the Kα line of iron. It is caused by screening of the presence of iron by oxygen – in the C/O-dominated material the main interaction process for an E ≈ 7 keV photon is absorption by oxygen rather than by iron, contrary to the solar composition case. Ionization of oxygen at high mass accretion rates adds a luminosity dependence to this behaviour – the iron line is significantly suppressed only at low luminosity, log (LX) ≲ 37–37.5, and should recover its nominal strength at higher luminosity. The increase of the equivalent width of the Kα lines of carbon and oxygen, on the other hand, saturates at rather moderate values. Screening by He is less important, due to its low ionization threshold and because in the accretion disc it is mostly ionized. Consequently, in the case of the He-rich donor, the iron line strength remains close to its nominal value, determined by the iron abundance in the accretion disc. This opens the possibility of constraining the nature of donor stars in UCXBs by means of X-ray spectroscopy with moderate energy resolution.