X‐Ray Spectroscopy of the Low‐Mass X‐Ray Binaries 2S 0918−549 and 4U 1543−624: Evidence for Neon‐rich Degenerate Donors

We present high-resolution spectroscopy of the neutron-star/low-mass X-raybinaries 2S 0918-549 and 4U 1543-624 with the High Energy Transmission GratingSpectrometer onboard the Chandra X-ray Observatory and the Reflection GratingSpectrometer onboard XMM-Newton. Previous low-resolution spectra of bothsources showed a broad line-like feature at 0.7 keV that was originallyattributed to unresolved line emission. We recently showed that this featurecould also be due to excess neutral Ne absorption, and this is confirmed by thenew high-resolution Chandra spectra. The Chandra spectra are each well fit byan absorbed power-law + blackbody model with a modified Ne/O number ratio of0.52+/-0.12 for 2S 0918-549 and 1.5+/-0.3 for 4U 1543-624, compared to theinterstellar-medium value of 0.18. The XMM spectrum of 2S 0918-549 is best fitby an absorbed power-law model with a Ne/O number ratio of 0.46+/-0.03,consistent with the Chandra result. On the other hand, the XMM spectrum of 4U1543-624 is softer and less luminous than the Chandra spectrum and has abest-fit Ne/O number ratio of 0.54+/-0.03. The difference between the measuredabundances and the expected interstellar ratio, as well as the variation of thecolumn densities of O and Ne in 4U 1543-624, supports the suggestion that thereis absorption local to these binaries. We propose that the variations in the Oand Ne column densities of 4U 1543-624 are caused by changes in the ionizationstructure of the local absorbing material. It is important to understand theeffect of ionization on the measured absorption columns before the abundance ofthe local material can be determined. This work supports our earlier suggestionthat 2S 0918-549 and 4U 1543-624 are ultracompact binaries with Ne-richcompanions.Comment: 11 pages, 5 figures, major revisions including addition of XMM spectral analysis, accepted for publication in the Astrophysical Journal, vol. 59