Ionized absorbers in active galactic nuclei: The role of collisional ionization and time-evolving photoionization

In this paper we explore collisional ionization and time-evolving photoioni zation in the X-ray-discovered, ionized absorbers in Seyfert galaxies. Thes e absorbers show temporal changes inconsistent with simple equilibrium mode ls. We develop a simple code to follow the temporal evolution of nonequilib rium photoionized gas. As a result several effects appear that are easily o bservable and that, in fact, may explain otherwise paradoxical behavior. Sp ecifically, we find the following: 1. In many important astrophysical conditions (O VII, O VIII dominant and h igh [greater than or similar to 10(22.5) cm(-2)] column density) pure colli sional and photoionization equilibria can be distinguished with moderate sp ectral resolution observations, because of a strong absorption structure be tween 1 and 3 keV. This feature is due mainly to iron L XVII-XIX and neon K IX-X absorption, which is much stronger in collisional models. This absorp tion structure may be misinterpreted as a flattening of the intrinsic emiss ion spectrum above similar to 1 keV in low-resolution data. 2. In time-evolving nonequilibrium photoionization models the response of t he ionization state of the gas to sudden changes of the ionizing continuum is smoothed and delayed at low gas densities (usually up to 10(8) cm(-3)), even when the luminosity increases. The recombination time can be much long er (up to orders of magnitude) than the photoionization timescale. Hence, a photoionized absorber subject to frequent, quick, and consistent changes o f ionizing luminosity is likely to be overionized with respect to the equil ibrium ionization state. 3. If the changes of the ionizing luminosity are not instantaneous, and the electron density is low enough (the limit depends on the average ionizatio n state of the gas but is usually similar to 10(7) to similar to 10(8) cm(- 3)), the ionization state of the gas can continue to increase while the sou rce luminosity decreases, so a maximum in the ionization state of a given e lement may occur during a minimum of the ionizing intensity (the opposite o f the prediction of equilibrium models). 4. Different ions of different elements reach their equilibrium configurati on on different timescales, so models in which all ions of all elements are in photoionization equilibrium so often fail to describe active galactic n ucleus (AGI) spectral evolution. These properties are similar to those seen in several ionized absorbers in AGNs, properties that had hitherto been puzzling. We applied these models t o a high signal-to-noise ratio ROSAT PSPC observation of the Seyfert 1 gala xy NGC 4051. The compressed dynamical range of variation of the ionization parameter U and the ionization delays seen in the ROSAT observations of NGC 4051 may be simply explained by a nonequilibrium photoionization model, gi ving well-constrained parameters: n(e) = (1.0(-0.5)(+1.2)) x 10(8) cm(-3) a nd R = (0.74(-0.40)(+0.80)) x 10(16) cm (similar to 3 light-days).