Strong signature of right-handed circularly polarized photoionization close to the cyclotron line in the atmosphere of magnetic white dwarfs

TL;DR

This paper predicts a strong right-handed circular polarization signature in the photoionization spectrum of magnetic white dwarf atmospheres near the cyclotron line, revealing new polarization-dependent absorption features.

Contribution

It introduces a novel calculation of polarization-dependent photoionization opacity in magnetic white dwarf atmospheres using combined atomic equilibrium models and approximate cross sections.

Findings

Strong right-handed circular polarization absorption predicted near cyclotron resonance.

Absorption shows a deep trough at energies below the cyclotron fundamental.

Polarization jump caused by multiple photoionization continua from atomic states.

Abstract

Magnetic fields break the symmetry of the interaction of atoms with photons with different polarizations, yielding chirality and anisotropy properties. The dependence of the absorption spectrum on the polarization, a phenomenon known as dichroism, is present in the atmosphere of magnetic white dwarfs. Its evaluation for processes in the continuum spectrum has been elusive so far due to the absence of appropriate ionization equilibrium models and incomplete data on photoionization cross sections. We combined rigorous solutions to the equilibrium of atomic populations with approximate cross sections to calculate the absolute opacity due to photoionization in a magnetized hydrogen gas. We predict a strong right-handed circularly polarized absorption ($\chi^+$) formed blueward of the cyclotron resonance for fields from about 14 to several hundred megagauss. In energies lower than the cyclotron fundamental, this absorption shows a deep trough with respect to linear and left-handed circular polarizations that steepens with the field strength. The jump in $\chi^+$ is due to the confluence of a large number of photoionization continua produced by right-handed circularly polarized transitions from atomic states with a nonnegative magnetic quantum number toward different Landau levels.