Introduction

The scattering of radiation by free electrons is one of the dominant sources of continuous opacity in the atmospheres of hot white dwarfs. Most model atmosphere calculations adopt the classical Thomson isotropic scattering approach, whereby only the direction of photon propagation changes as the result of a scattering event. More rigorously, finite electron mass instead implies that both momentum and energy exchange should actually occur.

The effects of Compton scattering on white dwarf model atmospheres was first investigated in detail by Madej (1994), who found that pure hydrogen models with temperatures of $10^5$ K show a significant depression of the X-ray continuum for wavelengths $< 50$ Å. Effects for models containing significant amounts of helium, or helium and heavier elements, were found to be much smaller or negligible, in keeping with expectations based on the relative importance of electron scattering as an opacity source as opposed to photoelectric absorption.

In a later paper, Madej (1998) computed the effects of Compton scattering for a model corresponding to the parameters of the DA white dwarf HZ 43. Differences between Compton and Thomson scattering model spectra are apparent for $\lambda < 50$ Å, and grow to orders of magnitude by 40 Å. While current X-ray instrumentation is not sufficiently sensitive to study the spectra of even the brightest DA white dwarfs in any detail at wavelengths $\lambda < 50$ Å, the spectral differences implied by the more rigorous Compton redistribution formalism will be of interest to more sensitive future missions. Moreover, the Chandra Low Energy Transmission Grating Spectrometer (LETG+HRC-S) effective area calibration is based on observed spectra of HZ 43 and Sirius B at wavelengths $\lambda > 60$ Å (Pease et al., 2003). It is therefore of current topical interest to re-examine the influence of Compton scattering for these stars and determine whether any significant differences might be discernible between Thomson and Compton scattering in the LETGS bandpass.

In this paper, we perform two independent and rigorous tests of the influence of Compton Scattering on the emergent spectra of hot DA white dwarfs. Our methods of calculation are outlined in §2, while results and conclusions are briefly discussed in §§3 and 5.