Thermal soft X-ray emission is detected from many hot hydrogen-rich white dwarfs (spectral type DA) with an effective temperature in excess of 20 000 K. Most of the objects with Teff <40000 K have virtually pure hydrogen atmospheres while the majority of the hotter ones emit X-ray fluxes lower than predicted by hydrogen model atmospheres and, therefore, must contain heavier elements as absorbers. From ROSAT and EUVE observations it was concluded that trace metals sustained by radiative levitation increase the atmospheric X-ray opacity.
ROSAT also detected a smaller number of helium-rich white dwarfs (spectral type DO), as well as some of their immediate progenitors, the PG1159 stars, whose photospheres are dominated by C, He, and O. Their effective temperatures exceed 100 000 K and 140 000 K, respectively. The hottest object (H1504+65, Teff = 200 000 K) is unique and possibly a naked C-O stellar core. It exhibits an extremely rich absorption-line spectrum as revealed by EUVE and Chandra.
The origin of hard X-ray emission possibly detected in a number of hot white dwarfs remains unclear.
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