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M. A. Jimenez-Garate (LLNL and Columbia University), D. A. Liedahl (LLNL), J. C. Raymond (CfA), C. J. Hailey (Columbia University)
Since X-ray line emission depends strongly on the temperature structure of the emitting gas, high resolution spectra will test our understanding of the viscous dissipation and heat transport in accretion disk atmospheres. The multi-temperature atmosphere has non-negligible optical depth, which can result in observable radiative transfer effects. By imposing hydrostatic equilibrium and thermal balance, we use a 1-D multi-group radiative transfer code, an improved version of the model by Raymond (1993), to calculate the ion abundances and vertical structure of the centrally illuminated disk. We model the line emission from the entire disk, considering each disk annulus individually, using the HULLAC atomic code to calculate local emission spectra. Over the range of temperatures where X-ray lines from high-ionization species are formed, the photoionized gas is thermally unstable. The predicted strengths of the lines depend on how this instability is resolved. Two stable solutions exist. We consider separately the high and low temperature solutions. We further introduce a phenomenological model where enhanced vertical heat transport suppresses the instability and enhances X-ray line emission. Our models also show that gaussian-like line profiles result from the distribution of the emission over a large range of disk radii. Comparisons with data now being obtained with Chandra and XMM/Newton are discussed.
Work at LLNL was performed under the auspices of the U.S. Department of Energy, Contract No. W-7405-Eng-48.
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