On the Origin of the Absorption Features in SS433

We present high-resolution optical spectroscopy of the X-ray binary system SS433, obtained over a wide range of orbital phases. The spectra display numerous weak absorption features, and include the clearest example seen to date of those features, resembling a mid-A type supergiant spectrum, that have previously been associated with the mass donor star. However, the new data preclude the hypothesis that these features originate solely within the photosphere of the putative mass donor, indicating that there may be more than one region within the system producing an A supergiant-like spectrum, probably an accretion disc wind. Indeed, whilst we cannot confirm the possibility that the companion star is visible at certain phase combinations, it is possible that all supergiant-like features observed thus far are produced solely in a wind. We conclude that great care must be taken when interpreting the behaviour of these weak features.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure

X-Ray Spectroscopy of the Low-Mass X-ray Binaries 2S 0918-549 and 4U1543-624: Evidence for Neon-Rich Degenerate Donors

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+/-0.12 for 2S 0918-549 and 1.5+/-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+/-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+/-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.Comment: 11 pages, 5 figures, major revisions including addition of XMM spectral analysis, accepted for publication in the Astrophysical Journal, vol. 59

Phase separation in the crust of accreting neutron stars

Nucleosynthesis, on the surface of accreting neutron stars, produces a range of chemical elements. We perform molecular dynamics simulations of crystallization to see how this complex composition forms new neutron star crust. We find chemical separation, with the liquid ocean phase greatly enriched in low atomic number elements compared to the solid crust. This phase separation should change many crust properties such as the thermal conductivity and shear modulus. The concentration of carbon, if present, is enriched in the ocean. This may allow unstable thermonuclear burning of the carbon and help explain the ignition of the very energetic explosions known as superbursts.Comment: 8 pages, 6 figures, minor changes, Physical Review E in pres