Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-11-27
Astrophys.J. 604 (2004) 551-555
Astronomy and Astrophysics
Astrophysics
5 pages, 4 figures, accepted by ApJ
Scientific paper
10.1086/382038
The unresolved transition array (UTA) of iron M-shell ions is a prominent absorption feature in the X-ray spectrum of many active galactic nuclei (AGNs). Modeling photoionized plasmas in attempt to match the observed silicon and oxygen lines fail to predict the level of ionization of iron as inferred by this feature. It is suggested that the discrepancy is due to underestimation of the low-temperature dielectronic recombination rates for iron M-shell ions. Modified ionization balance calculations, based on new (guessed) atomic data, support this idea. The results are shown and compared to the global properties of several observed UTAs. Implications for AGN absorbing gas are discussed including an analysis of the ionization parameter distribution in such sources. The need for real calculations of such atomic data is stressed.
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