Detailed Atmosphere Modeling for the Neutron Star 1E1207.4−5209: Evidence of Oxygen/Neon Atmosphere

We present a comprehensive investigation of the two broad absorption featuresobserved in the X-ray spectrum of the neutron star 1E1207.4-5209 based on arecent analysis of the 260 ksec XMM-Newton data by Mori et al. 2005. Expandingon our earlier work (Hailey & Mori 2002) we have examined all previouslyproposed atmospheric models for 1E1207.4-5209. Using our atomic code, whichrapidly solves Schrodinger's equation for arbitrary ion in strong magneticfield (Mori & Hailey 2002), we have systematically constructed atmosphericmodels by calculating polarization-dependent LTE opacities and addressed allthe physics relevant to strongly-magnetized plasmas. We have been able to ruleout virtually all atmospheric models because they either do not sustain anionization balance consistent with the claimed atmosphere composition orbecause they predict line strengths and line widths which are inconsistent withthe data. Only Oxygen or Neon atmospheres at B~10^{12} G provideself-consistent atmospheric solutions of appropriate ionization balance andwith line widths, strengths and energies consistent with the observations. Theobserved features are likely composed of several bound-bound transition linesfrom highly-ionized oxygen/neon and they are broadened primarily by motionalStark effects and magnetic field variation over the line-emitting region.Further considerations of plausible mechanisms for the formation of a mid-Zatmosphere likely rule out Neon atmospheres, and have important implicationsfor the fallback mechanism in supernova ejecta. Future high resolutionspectroscopy missions such as Constellation-X will be able to resolve predictedsubstructure in the absorption features and will measure magnetic fieldstrength and gravitational redshift independently to better than 10% accuracy.Comment: 21 pages, 12 figures, resubmitted to Ap