X-ray diagnostics of 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.