Ionized Absorbers in AGN: the role of collisional ionization and time-evolving photoionization


In this paper we explore collisional ionization and time-evolving photoionization in the, X-ray discovered, ionized absorbers in Seyfert galaxies. These absorbers show temporal changes inconsistent with simple equilibrium models. We develop a simple code to follow the temporal evolution of non-equilibrium photoionized gas. As a result several effects appear that are easily observable; and which, in fact, may explain otherwise paradoxical behavior. Specifically we find that: 1. In many important astrophysical conditions (OVII, OVIII dominant; and high ( ∼ 10cm) column density) pure collisional and photoionization equilibria can be distinguished with moderate spectral resolution observations, due to a strong absorption structure between 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 absorption structure may be misinterpreted as a flattening of the intrinsic emission spectrum above ∼ 1 keV in low resolution data. 2. In time-evolving non-equilibrium photoionization models the response of the ionization state of the gas to sudden changes of the ionizing continuum is smoothed and delayed at low gas densities (usually up to 10 cm), even when the luminosity increases. The recombination time can be much longer (up to orders of magnitude) than the photoionization timescale. Hence a

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@inproceedings{Nicastro1998IonizedAI, title={Ionized Absorbers in AGN: the role of collisional ionization and time-evolving photoionization}, author={Fabrizio Nicastro and Fabrizio Fiore and G. Cesare Perola and Martin S. Elvis}, year={1998} }