Paper detail

Detailed atmosphere modelling for the neutron star 1E1207.4-5209: Evidence of Oxygen/Neon atmosphere

We present a comprehensive investigation of the two broad absorption features observed in the X-ray spectrum of the neutron star 1E1207.4-5209 based on a recent analysis of the 260 ksec XMM-Newton data by Mori et al. 2005. Expanding on our earlier work (Hailey & Mori 2002) we have examined all previously proposed atmospheric models for 1E1207.4-5209. Using our atomic code, which rapidly solves Schrodinger's equation for arbitrary ion in strong magnetic field (Mori & Hailey 2002), we have systematically constructed atmospheric models by calculating polarization-dependent LTE opacities and addressed all the physics relevant to strongly-magnetized plasmas. We have been able to rule out virtually all atmospheric models because they either do not sustain an ionization balance consistent with the claimed atmosphere composition or because they predict line strengths and line widths which are inconsistent with the data. Only Oxygen or Neon atmospheres at B~10^{12} G provide self-consistent atmospheric solutions of appropriate ionization balance and with line widths, strengths and energies consistent with the observations. The observed features are likely composed of several bound-bound transition lines from highly-ionized oxygen/neon and they are broadened primarily by motional Stark effects and magnetic field variation over the line-emitting region. Further considerations of plausible mechanisms for the formation of a mid-Z atmosphere likely rule out Neon atmospheres, and have important implications for the fallback mechanism in supernova ejecta. Future high resolution spectroscopy missions such as Constellation-X will be able to resolve predicted substructure in the absorption features and will measure magnetic field strength and gravitational redshift independently to better than 10% accuracy.