X-ray Spectroscopy of the Isolated Neutron Star 1e1207.4-5209: Atmospheric Composition and Equation of State

Two broad absorption features were discovered in the Chandra/ACIS spectrum of the isolated neutron star 1E1207.4-5209 (Sanwal et al. 2002). Expanding on our previous paper (Hailey & Mori 2002), we describe the atomic physics methodology which leads to the identification of this neutron star’s atmospheric composition. The solution of Schrödinger’s equation in an intense magnetic field combined with simple atomic physics arguments leads to simultaneous determination of magnetic field strength, atmospheric elements and gravitational redshift. This determination does not require a solution to the equations of radiative transfer. Atomic physics arguments alone establish that the absorption features must be due to atomic transitions from He-like Oxygen or Neon at B ∼ 10 G, comparable to the magnetic field strength independently derived from the neutron star spin parameters. This result is robust to a surprising number of poorly understood atomic physics effects which might have served to compromise it. Further constraints on the atmosphere, as well as the neutron star equation of state, can be inferred if the mass of neutron stars is <∼ 1.7M⊙ as recent theoretical studies suggested. In that case Neon is ruled out, and the neutron star atmosphere is unambiguously Oxygen. Furthermore, under the assumption that the neutron star mass is near the canonical value of 1.4M⊙, the corresponding gravitational redshift for the Oxygen atmosphere constrains the equation of state of the interior of 1E1207.4-5209 to be quite stiff. Subject headings: atomic processes – magnetic fields – stars: neutron – equation of state – individual: 1E1207.4-5209