We present high-resolution spectroscopy of the neutron-star/low-mass X-ray binaries 2S 0918 - 549 and 4U 1543 - 624 with the High Energy Transmission Grating Spectrometer onboard the Chandra X-ray Observatory and the Reflection Grating Spectrometer onboard XMM-Newton .
Previous low-resolution spectra of both sources showed a broad line-like feature at 0.7 keV that was originally attributed to unresolved line emission .
We recently showed that this feature could also be due to excess neutral Ne absorption , and this is confirmed by the new high-resolution Chandra spectra .
The Chandra spectra are each well fit by an absorbed power-law + blackbody model with a modified Ne/O number ratio of 0.52 \pm 0.12 for 2S 0918 - 549 and 1.5 \pm 0.3 for 4U 1543 - 624 , compared to the interstellar-medium value of 0.18 .
The XMM spectrum of 2S 0918 - 549 is best fit by an absorbed power-law model with a Ne/O number ratio of 0.46 \pm 0.03 , consistent with the Chandra result .
On the other hand , the XMM spectrum of 4U 1543 - 624 is softer and less luminous than the Chandra spectrum and has a best-fit Ne/O number ratio of 0.54 \pm 0.03 .
The difference between the measured abundances and the expected interstellar ratio , as well as the variation of the column densities of O and Ne in 4U 1543 - 624 , supports the suggestion that there is absorption local to these binaries .
We propose that the variations in the O and Ne column densities of 4U 1543 - 624 are caused by changes in the ionization structure of the local absorbing material .
It is important to understand the effect of ionization on the measured absorption columns before the abundance of the local material can be determined .
This work supports our earlier suggestion that 2S 0918 - 549 and 4U 1543 - 624 are ultracompact binaries with Ne-rich companions .