Atomic calculation for the atmospheres of strongly-magnetized neutron stars

Abstract

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 producing an extensive atomic data set including energy values and oscillator strengths in the so-called Landau regime ($B > 4.7\times10^9Z^2$ G). Conventional atmosphere models for B=0 are not applicable to typical field strengths of cooling neutron stars ($B \sim10^{12}-10^{13}$ G), since an atom no longer keeps its spherical shape. The elemental 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 ions as a function of magnetic field. To accomplish this efficiently, we minimized the iterations in the Hartree equation and treated exchange terms and higher Landau states by perturbation methods. This method has the effect of reducing the computation time significantly. Inclusion of higher Landau states gives us much more accurate data for inner orbitals 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 used in elements up to Fe with sufficient accuracy to be of use for spectroscopic missions such as {\it Chandra}, {\it XMM-Newton} and next-generation X-ray telescopes.