X-ray emission from hot hydrogen-deficient white dwarfs
K. WernerLehrstuhl Astrophysik, Universität Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany
The ROSAT observatory has detected X-rays from a small number of hot hydrogen-deficient (pre-) white dwarfs. These PG 1159 stars and helium-rich white dwarfs are hot enough (Teff>=100000K) to emit thermal X-rays from their photospheres. We summarize the results of non-LTE model atmosphere analyses.
One remarkable result is the discovery of two previously unknown and peculiar PG 1159 stars. In a combined analysis with ultraviolet spectroscopic data obtained with the Hubble Space Telescope one of these objects turns out to be the hottest known PG 1159 star. Another surprising result is the first detection of a corona about a white dwarf.
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