X-ray irradiation of white dwarf atmospehres : the soft X-ray spectrum of AM Herculis.

Abstract

We present model atmosphere calculations of white dwarfs irradiated at the surface by hard x-rays. We use a self-consistent treatment of Compton scattering as a noncoherent scattering process. The opacities include the photoionization edges of all individual subshells of the most abundant ions of 15 elements. Because of an 'inverse' blanketing effect we find a strong optically-thin temperature inversion above the photosphere. The emerging spectra are characterized by strong absorption edges which decrease in size with increasing irradiation. The numerical results are used to fit EXOSAT 1000 lines/mm-transmission-grating observations of the soft (0.1-0.3 keV) x-ray spectrum of AM Herculis in the reversed soft x-ray mode. Atmospheres with all the soft X-ray luminosity originating from large optical depths cannot explain the observed spectrum. Instead, a substantial fraction of the soft x-ray luminosity might be due to irradiation.