Mathematics – Probability
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........55k&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF MARYLAND COLLEGE PARK, 1993.Source: Dissertation Abstracts International, Volume: 54-10, Section:
Mathematics
Probability
Accretion Disk Corona
Scientific paper
We investigate the structure of accretion disks illuminated by X-rays from a central compact object in a binary system. For the optically thick disk photosphere at radii larger than a certain value, X-ray heating can dominate the viscous heating and determine the disk structure. We calculate the disk photosphere structure resulting from X-ray heating both analytically and numerically based on the standard alpha-disk model for low mass X-ray binaries as well as for active galactic nuclei. We show that X-ray illumination does have important effects on the disk structure under a variety of circumstances and that instabilities may arise under significant X-ray heating. X-rays can also photoionize the upper atmosphere of the disk and form an accretion disk corona (ADC) where emission lines can form. We construct a model to calculate the vertical structure and the emission spectrum of the ADC with parameters appropriate to the low mass X-ray binaries. These models are made by non-local thermodynamic equilibrium calculations and include a large number of atomic processes for ten cosmically abundant elements. Transfer of radiation is treated by using the escape probability formalism. We self-consistently calculate the temperature, density, ionization, ion level populations and the emitted spectrum in such a gas and more accurately account for the complicated microphysics for a wide range of densities than has been done before. The temperature profile of the ADC consists of a Compton heated region and a mid-T zone where the temperature is approximately 10^6 K. A thermal instability occurs above and close to the disk photosphere and causes the temperature of the ADC to drop abruptly from 10 ^6 K to several times 10^6 K. The emission spectrum in the optical, ultraviolet, extreme ultraviolet and X-ray range is discussed and compared with the observations. The amount of illumination at different radii of the accretion disk implied by the comparison between the models and the observational data is also discussed.
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