Hartree-Fock calculations for the photoionisation of helium and helium-like ions in neutron star magnetic fields

TL;DR

This paper presents detailed Hartree-Fock calculations of photoionisation cross sections for helium and helium-like ions in extremely strong neutron star magnetic fields, aiding interpretation of neutron star X-ray spectra.

Contribution

It introduces a comprehensive method for calculating photoionisation in high magnetic fields, including bound and continuum states, for two-electron systems relevant to neutron star atmospheres.

Findings

Calculated photoionisation cross sections for helium and helium-like oxygen.

Identified spectral features relevant for neutron star atmosphere modeling.

Provided data for interpreting X-ray observations of neutron stars.

Abstract

We derive the photoionisation cross section in dipole approximation for many-electron atoms and ions for neutron star magnetic field strengths in the range of 10^7 to 10^9 T. Both bound and continuum states are treated in adiabatic approximation in a self-consistent way. Bound states are calculated by solving the Hartree-Fock-Roothaan equations using finite-element and B-spline techniques while the continuum orbital is calculated by direct integration of the Hartree-Fock equations in the mean-field potential of the remaining bound orbitals. We take into account mass and photon density in the neutron star's atmosphere, finite nuclear mass as well as thermal occupation of the levels. The data may be of importance for the quantitative interpretation of observed x-ray spectra that originate from the thermal emission of isolated neutron stars. They can serve as input for modeling neutron star atmospheres as regards chemical composition, magnetic field strength, temperature, and redshift. Our main focus in this paper lies on helium and helium-like oxygen. These two-electron systems are simple enough to calculate all possible transitions when limiting the quantum numbers and should show all the basic structures and behaviour of other two-electron systems up to iron.