X-ray Lines and Gas Composition in NGC 1068

Abstract

Previous X-ray and ultraviolet spectroscopy suggested that the Fe/O abundance ratio in NGC 1068 may be abnormally high. We have measured some 15 X-ray lines in NGC 1068, to an accuracy better than a factor 2, and modeled the continuum in two different ways. The first assumes that the hard X-ray continuum is the reflection of the nuclear source by two, extended, photoionized gas components; a warm (T$\sim 1.5 \times 10^5$ K) gas and a hot (T$\sim 3 \times 10^6$ K) gas. All the observed emission lines are produced in this gas and there is an additional, extended 0.6--3 keV pure continuum component. The second model is a combination of a hard reflected continuum with a soft thermal plasma component. The calculations show that the emission lines in the photoionized gas model are in very good agreement with the observed ones assuming solar metallicity for all elements except for iron, which is more than twice solar, and oxygen, which is less than 0.25 solar. Models with solar oxygen are possible if the 0.5--1 keV continuum is weaker but they do not explain the magnesium and silicon lines. The thermal model fit requires extremely low metallicity (0.04 solar) for all elements. We discuss these findings and compare them with the ASCA spectra of recently observed starburst galaxies. Given the strong H-like and He-like lines, as well as the prominent Fe-L emission features, the origin of the soft X-ray lines in this source is more likely photoionized gas. We compare our model with the recent Iwasawa etal (1997) paper. We also show that fluorescence lines of low-Z element lines in AGN is likely to be larger than previously assumed.