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[33.16] Atomic calculation for the model atmospheres of cooling neutron stars

Kaya Mori, C.J. Hailey (Columbia Astrophysics Laboratory)

Complete modeling of radiative transfer in neutron star atmospheres is in progress, taking into account the anisotropy induced by magnetic fields, non-ideal effects and general relativity. As part of our modeling, we present a novel atomic calculation method (Multiconfigurational Effective Potential Method) producing an extensive atomic data set including energy values and oscillator strengths in the so-called Landau regime (B > 4.7\times10⁹Z² G). Conventional atmosphere models for B=0 are not applicable to typical field strengths of cooling neutron stars (B ~ 10¹²-10¹³ G), since an atom no longer keeps its spherical shape so the wave functions must be expanded in terms of Landau states and a component along the field. The composition and the configuration of the magnetic field in the atmosphere are presently unknown, so that atomic data must be produced for ground and excited states of several ion states in various magnetic fields. To accomplish this, we reduced computation time significantly by minimizing the iterations in the Hartree equation and treating exchange terms and higher Landau states by perturbation methods. Inclusion of higher Landau states gives us much more accurate data for inner orbits unlike other methods based on the adiabatic approximation. While existing atomic data in the Landau regime are available only for low Z atoms, our method can be readily extended to elements up to Fe with sufficient accuracy to be of relevance for spectroscopic missions such as XMM-Newton, Chandra and next-generation X-ray telescopes. Opacity tables based on our atomic data base will be used to construct model atmospheres by comparison with observation data from recent X-ray telescopes.