Disappearance of the Fe K$\alpha$ emission line in Ultra Compact X-ray Binaries 4U 1543-624 and Swift J1756.9-2508

We investigate the long-term variability of the K$\alpha$ line of iron in the spectra of two Ultra Compact X-ray Sources (UCXBs) with C/O-rich donors. We revisit archival observations by five different X-ray telescopes, over a ~twenty year period. Adopting physically motivated models for the spectral continuum, we probe the long-term evolution of the source emission in a self-consistent manner enabling physical interpretation of potential variability in the primary X-ray emission continuum and/or any emission lines from reflection off the accretion disk. We find that the spectral shape and flux of the source emission (for both objects) has remained almost constant throughout all the observations, displaying only minor variability in some spectral parameters and the source flux (largest variation is a ~25% drop in the flux of Swift J1756.9-2508). We note a striking variability of the Fe K$\alpha$ line which fluctuates from a notable equivalent width of ~66-100 eV in 4U 1543-624 and ~170 eV in Swift J1756.9-2508, to non-detections with upper limits of 2-8 eV. We argue that the disappearance of the iron line is due to the screening of the Fe K$\alpha$ line by the overabundant oxygen in the C/O-rich UCXBs. This effect is cancelled when oxygen becomes fully ionized in the inner disk region, resulting in the variability of the Fe K$\alpha$ line in an otherwise unaltered spectral shape. This finding supports earlier predictions on the consequences of H-poor, C/O-rich accretion disk on reflection induced fluorescent lines in the spectra of UCXBs.