Results of high resolution soft X-ray spectroscopy of hot white dwarfs with EXOSAT

Abstract

We present the final results of the high resolution X-ray spectroscopy of hot white dwarfs, obtained with the 500 lines mm^-1 Transmission Grating Spectrometer on EXOSAT. These data, which are unique in wavelength coverage, spectral resolution, and statistical quality, have offered us the first detailed look at the photospheric X-ray spectrum of these objects. The shape of the soft X-ray spectrum is highly sensitive to the spectral parameters; we use this sensitivity to derive accurate estimates of the effective temperature, surface gravity, and chemical composition of the atmosphere, for two of the three white dwarfs for which X-ray spectra were obtained with EXOSAT (HZ 43 and Sirius B). The accuracy can be increased by combining all available spectral data for these stars, from the optical to the X-ray band. A fully consistent description of the complete electromagnetic spectrum, in terms of a single model atmospheres spectrum, is the result. The third hot white dwarf for which we have EXOSAT spectroscopy, Feige 24, shows an EUV spectrum that defies interpretation in terms of `simple' model atmospheres spectra appropriate to hot white dwarfs.