Spectral Formation in X-Ray Illuminated Gas Clouds.
Abstract
The last ten years have seen a very rapid growth in the study of cosmic X-ray sources. As a result, the X-ray UV, and optical spectra from many sources have now been observed at high resolution, and a diagnostic theory is needed to help interpret these observations. This thesis investigates several aspects of the formation of spectral features in one important class of cosmic X-ray sources --the compact sources. We first study the formation of optical and UV emission lines via the Bowen resonance fluorescence mechanism. According to this mechanism, a pair of frequency coincidences allows the L(alpha) line of He II to pump excited levels of O III and N III, which may then decay with the emission of Bowen fluorescence lines. We present a simplified theory of Bowen fluorescence, based on an escape probability treatment of resonance line transfer. The efficiency of conversion from the He II L(alpha) to the O III and N III Bowen lines is calculated as a function of He II L(alpha) optical depth and of continuum absorber abundance, for the case of a uniform slab. Bowen conversion efficiencies are also calculated for non-uniform slabs, such as planetary nebulae and X -ray illuminated nebulae, and the model results are compared with available observations. The results of our study of the Bowen mechanism are incorporated into the second investigation, which describes the heating and ionization of spherical gas clouds surrounding compact X-ray sources, together with the radiation field escaping from the clouds. The cloud structure is determined by assuming a local balance between ionization and recombination and between heating and cooling in the gas, and the radiation field is found by solving a simplified equation of transfer. The calculations include the effects of a large number of emission lines, including the most recent excitation rate data, together with the effects of the destruction of resonance line radiation during trapping. Model results are presented for various gas densities, and for various X-ray source spectra. Finally, we study the effects of Compton scattering on an X-ray spectrum, in the limit that the electron temperature is small. An analytic theory is presented for the change in the X-ray spectrum after a given number of scatterings, and this theory is used to determine the spectrum as a function of time for X-rays scattering in an infinite homogeneous medium, and for X-rays escaping from spherical clouds. The effects of relativistic corrections and non-zero electron temperatures are also discussed.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1980
- Bibcode:
- 1980PhDT.........5K
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Compton Effect;
- Emission Spectra;
- Resonance Fluorescence;
- X Ray Sources;
- Electron Energy;
- Radiation Distribution;
- Space Radiation