During my diploma thesis I developed a dynamical depth dependent model atom for the accretion disc model spectra code AcDc. The goal was to get rid of problems, with very low occupied atomic levels, when caculating synthetic model spectra for ultracompact Low Mass X-ray Binaries.
Recently, a new class of ultracompact binary systems has been proposed, in which the disc is free of hydrogen and helium, but mainly composed of metals such as oxygen end neon, due to the nature of the secondary (Schulz et al. 2001, Juett et al. 2001). The progenitors of the donor stars have been suggested to be white dwarfs with C/O cores. The excess of neon in the transferred matter is due to chemical fractionation of the white dwarf whic occured prior to the Roche lobe overflow (Yungelson et al. 2002). To study the evolutionary history of the binary system and the interior structure of the white dwarf progenitor it is necessary to determine the metal abundances from quantitative spectral analysis of disc spectra.
With the accretion disc model code AcDc we calculated the vertical structure and the emergent spectrum of the accretion disc of the soft x-ray source 4U1626-67, one of these ultracompact binaries. 4U1626-67 differs from other neon enriched binaries by the presence of a 7.7s x-ray pulsar with strong magnetic fields estimated form cyclotron emission (≈ 3 E12 Gauss, Orlandini et al. 1998). The very short binary period (42min) indicates that the companion must be extremly hydrogen poor (Paczynski & Sienkiewicz 1981). HST-UV and Chandra X-ray spectroscopy reveal emission lines of highly ionized C, O, Ne and Si (Schulz et al. 2001, Homer et al. 2002). The double peaked line profiles probably form in the disc.
