2004ApJ...607...76M

Non-LTE line formation is studied in order to obtain the curve of growth of absorption K lines in X-ray wavelengths. We solve radiation transfer coupling with the statistical balance of atomic level populations. Our formulation covers level populations from the coronal limit through LTE, and can describe line formation from emission through absorption regimes consistently, depending on the optical thickness. It also seamlessly covers the classical division of lines into absorption and scattering categories. Using the Voigt function for the line profile, we calculate the absorption equivalent width and find that the curve of growth of X-ray lines deviates from the typical curve of optical or longer wavelength lines, although the deviation becomes small when the radiation field of the absorber is highly anisotropic. This is due to extremely large rates of spontaneous emission for X-ray lines, and photoexcitation is followed immediately by spontaneous emission. The level populations are far from their LTE values, and the absorption lines are dominated by scattering; while in optical lines, quasi-LTE can be attained and most of photons absorbed are destroyed (thermalized). Our concepts will be helpful for understanding line formation and in the quantitative study of absorption-line spectra of intervening gas with weak thermal coupling.