X-ray spectroscopy of H1504+65 comma an extremely hot bare stellar core

H1504+65 is the hottest known white dwarf (Teff200,000 K). Its photosphere ismainly composed of carbon and oxygen, hence this unique white dwarf resemblesthe bare stellar core of a former red giant. Because of its high effectivetemperature H1504+65 is the only star at all which shows a photosphericabsorption line spectrum in the soft Xray region. We have analysed a Chandraspectrum and identified a wealth of spectral lines from highly ionized O, Ne,and Mg. We also detect nickel lines but, surprisingly, fail to detect iron. Thereduced iron opacity allows thermal radiation to escape from deep hot layers.Our models predict that XMMNewton could for the first time acquire a stellarphotospheric spectrum in the 1030 Angstroem (1.20.4 KeV) region. This enables a precise temperature determination. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]