Magnetized Iron Atmospheres for Neutron Stars

Abstract

Pulsar surface temperatures deduced from soft X-rayobservations have important consequences for neutron star physics and theequation of state at supernuclear densities. It has been recognized,however, that inferred temperatures depend sensitively on the unknowncomposition of the neutron star surface. Motivated by recent lowresolution spectra from ROSAT and by future missions such as XMM andAXAF, we have estimated the atomic properties of iron in magneticfields B ~ 10(13) G. and used these data to compute model atmospheresand emergent spectra. Magnetic Fe energy levels and photon crosssections are estimated using a Hartree-Fock formalism. Computingionization equilibrium and normal mode opacities with these data, weconstruct our LTE atmospheres at 5.5 < Log(T_eff)< 6.5 and computeemergent spectra. We examine the dependence of the emergent spectraon T_eff and B. We also show the spectral variation with the anglebetween the magnetic field and the atmosphere normal and describe thesignificant limb darkening in the X-ray band. These results complement theauthors' low field model atmospheres for light and heavy elements usingdetailed opacities. We also compare with other authors' recent detailedcomputations of neutron star H model atmospheres in high fields. Thebroad spectral differences for different surface compositions may bediscernible with present X-ray data; we also note improvements needed toallow comparison of Fe models with future high quality spectra.