X-RAY-LINES IN ACTIVE GALACTIC NUCLEI AND PHOTOIONIZED GASES

This paper addresses the observed 0.1-10 keV spectrum in active galact ic nuclei (AGNs) and other photoionized gases. Detailed calculations, encompassing a large range in density (10-10(14) cm(-3)), column densi ty (10(21)-10(23.5) cm(-2)), and level of ionization (3 orders of magn itude in ionization parameter), are described and discussed. The resul ts are presented as line intensity and three types of equivalent width over the entire range of physical conditions. We identify the most li kely conditions in the X-ray-absorbing gas in AGNs and argue that its temperature is below 2 x 10(5) K and that it is thermally stable. The strongest 0.1-10 keV lines, when observed against the central source c ontinuum, have typical equivalent widths of 5-50 eV and are thus at th e limit of present-day detection. The equivalent width can be signific antly stronger when the line is observed against the absorbed continuu m. Much larger equivalent widths, of 100-1000 eV, are expected when th e central continuum is obscured, as in Seyfert 2 galaxies. The importa nce 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 < 10(3) cm(-3)) and very high (N > 10(12) cm(-3)) densiti es. They are also sensitive to the geometry and covering fraction. We discuss the ultraviolet and extreme-ultraviolet spectrum and argue tha t the highly ionized gas contributes significantly to the observed int ensity of Ne VIII lambda 774 and, in some cases, also to O VI lambda 1 035. We confirm that observed ultraviolet absorption lines, in several AGNs, originate from gas with conditions similar to the X-ray-absorbi ng gas and show the expected column density and absorption equivalent width for the strongest species. If indeed the same physical component , wherever C Iv lambda 1549 absorption is visible, we predict substant ially greater absorption in the line O VI lambda 1035. None of the obs erved Extreme Ultraviolet Explorer lines in the spectrum of NGC 5548 a re consistent with the ionized gas component, and there is no satisfac tory 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 d ensity, covering fraction, and perhaps also dynamical state are consis tent with observed properties of X-ray and ultraviolet absorbers.