X‐Ray Lines in Active Galactic Nuclei and Photoionized Gases

Open Access
Publisher Website
Google Scholar
Abstract
This paper addresses the observed 0.1-10 keV spectrum in active galactic nuclei (AGNs) and other photoionized gases. Detailed calculations, encompassing a large range in density (10-1014 cm–3), column density (1021-1023.5 cm–2), and level of ionization (3 orders of magnitude in ionization parameter), are described and discussed. The results are presented as line intensity and three types of equivalent width over the entire range of physical conditions.We identify the most likely conditions in the X-ray-absorbing gas in AGNs and argue that its temperature is below 2 × 105 K and that it is thermally stable. The strongest 0.1-10 keV lines, when observed against the central source continuum, have typical equivalent widths of 5-50 eV and are thus at the limit of present-day detection. The equivalent width can be significantly stronger when the line is observed against the absorbed continuum. Much larger equivalent widths, of 100-1000 eV, are expected when the central continuum is obscured, as in Seyfert 2 galaxies. The importance of line-continuum fluorescence diminishes with line optical depth, and we do not find this process to be important in warm AGN absorbers. The X-ray line intensities depend on the gas density, especially for very low (N < 103 cm–3) and very high (N > 1012 cm–3) densities. They are also sensitive to the geometry and covering fraction.We discuss the ultraviolet and extreme-ultraviolet spectrum and argue that the highly ionized gas contributes significantly to the observed intensity of Ne VIII λ774 and, in some cases, also to O VI λ1035. We confirm that observed ultraviolet absorption lines, in several AGNs, originate from gas with conditions similar to the X-ray-absorbing gas and show the expected column density and absorption equivalent width for the strongest species. If indeed the same physical component, wherever C IV λ1549 absorption is visible, we predict substantially greater absorption in the line O VI λ1035. None of the observed Extreme Ultraviolet Explorer lines in the spectrum of NGC 5548 are consistent with the ionized gas component, and there is no satisfactory model to explain these observations.A new model is presented, in which evaporating "seeds" (high-density clouds or bloated stars) in the broad-line region are filling the intercloud medium with material that is streaming outward. The material's location, density, column density, covering fraction, and perhaps also dynamical state are consistent with observed properties of X-ray and ultraviolet absorbers.