Irradiated Accretion Disc Models

Details Irradiated Accretion Disc Models Irradiated Accretion Disc Models

Aug 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004mst..........1h&link_type=abstract

Masters Thesis, Tübingen, Germany, 2004.

Computer Science

Accretion Disc, Irradiated, Nlte, Synthetic Spectrum, Lmxb

Scientific paper

Most of the known accretion discs in binary systems are composed of hydrogen rich material. A few years ago, a new type of binary system was discovered, in which the disc is obviously free of hydrogen and helium. Nowadays six systems are known, with an accretion disc mainly composed of C, O, Ne and Mg (Juett, Psaltis & Chakrabarty 2001; Schulz et al. 2001). The most prominent representative of these ultra compact binary systems is the LMXB 4U 1626-67. The system is consisting of a 7.66 s X-ray pulsar as primary component and a 0.02 solar mass white dwarf as secondary (Chakrabarty 1998). Having a binary separation of only 300 0000 km (Schulz et al. 2001) one can not neglect the external irradiation of the accretion disc by the neutron star. The external irradiation dominates the disc spectrum over a wide range. In the UV one sees emission lines of highly ionised elements, like OV or CIV.
One of the biggest numerical problems, occurring during the calculation of an irradiated accretion disc model, is the extreme temperature stratification and structure. This causes severe problems with the occupation numbers of some atomic levels. These problems should be solved by implementing the depth dependent model atom, during my thesis work. During his PhD thesis, Nagel (2003) has developed the numeric code package AcDc. Therewith one can calculate the vertical structure and the emergent synthetic spectrum of an accretion disc. During the work presented here the depth dependent model atom (DEPTOM) was developed and implemented in AcDc as a code module. DEPTOM identifies the trouble making ionisation stages and deactivates them. Thus the calculation is numerically more stable and, hence, makes AcDc more powerful.

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