We present models for the X-ray spectrum of the Seyfert 2 galaxy NGC 1068. These are fitted to data obtained using the High Energy Transmission Grating (HETG) on the Chandra X-ray observatory. The data show line and radiative recombination continuum
(RRC) emission from a broad range of ions and elements. The models explore the importance of excitation processes for these lines including photoionization followed by recombination, radiative excitation by absorption of continuum radiation and inner shell fluorescence. The models show that the relative importance of these processes depends on the conditions in the emitting gas, and that no single emitting component can fit the entire spectrum. In particular, the relative importance of radiative excitation and photoionization/recombination differs according to the element and ion stage emitting the line. This in turn implies a diversity of values for the ionization parameter of the various components of gas responsible for the emission, ranging from log(xi)=1 -- 3. Using this, we obtain an estimate for the total amount of gas responsible for the observed emission. The mass flux through the region included in the HETG extraction region is approximately 0.3 Msun/yr assuming ordered flow at the speed characterizing the line widths. This can be compared with what is known about this object from other techniques.
The spectrum from the black hole X-ray transient GRO J1655-40. obtained using the $Chandra$ High Energy Transmission Grating (HETG) in 2005 is notable as a laboratory for the study of warm absorbers, and for the presence of many lines from odd-$Z$ el
ements between Na and Co (and Ti and Cr) not previously observed in X-rays. We present synthetic spectral models which can be used to constrain these element abundances and other parameters describing the outflow from the warm absorber in this object. We present results of fitting to the spectrum using various tools and techniques, including automated line fitting, phenomenological models, and photoionization modeling. We show that the behavior of the curves of growth of lines from H-like and Li-like ions indicate that the lines are either saturated or affected by filling-in from scattered or a partially covered continuum source. We confirm the conclusion of previous work by cite{Mill06} and cite{Mill08} which shows that the ionization conditions are not consistent with wind driving due to thermal expansion. The spectrum provides the opportunity to measure abundances for several elements not typically observable in the X-ray band. These show a pattern of enhancement for iron peak elements, and solar or sub-solar values for elements lighter than calcium. Models show that this is consistent with enrichment by a core-collapse supernova. We discuss the implications these values for the evolutionary history of this system.
