Analysis of ROSAT pointed observations of 15 hot DA white dwarfs.

Abstract

We present the results of pointed observations of hot white dwarfs with the ROSAT PSPC detector, and through the P1 filter of the Wide Field Camera. Model atmospheres containing hydrogen and helium, either homogeneously mixed or chemically stratified, are used to analyze the new data together with observations of the same objects by EXOSAT and the ROSAT all-sky survey. We confirm the results of previous studies, that objects between about 20000 and 40000 K have almost pure hydrogen atmospheres while additional absorbers are present in all three objects at higher temperatures. As a by-product we found evidence that the ROSAT PSPC detector is a factor 2.0+/-0.3 more sensitive to very soft X-ray photons compared to the standard calibration.