Evidence for ultra-fast outflows in radio-quiet Active Galactic Nuclei. II. Detailed photoionization modeling of Fe K-shell absorption lines
Publication date: 03 November 2011
Authors: Tombesi, F., et al.
Journal: The Astrophysical Journal
Copyright: IOP ScienceX-ray absorption line spectroscopy has recently shown evidence for previously unknown Ultra-fast Outflows (UFOs) in radio-quiet active galactic nuclei (AGNs). These have been detected essentially through blueshifted Fe XXV/XXVI K-shell transitions. In the previous paper of this series we defined UFOs as those highly ionized absorbers with an outflow velocity higher than 10,000 km/s and assessed the statistical significance of the associated blueshifted absorption lines in a large sample of 42 local radio-quiet AGNs observed with XMM-Newton. The present paper is an extension of that work. First, we report a detailed curve of growth analysis of the main Fe XXV/XXVI transitions in photoionized plasmas. Then, we estimate an average spectral energy distribution for the sample sources and directly model the Fe K absorbers in the XMM-Newton spectra with the detailed Xstar photoionization code. We confirm that the frequency of sources in the radio-quiet sample showing UFOs is >35 per cent and that the majority of the Fe K absorbers are indeed associated with UFOs. The outflow velocity distribution spans from ~10,000 km/s (~0.03c) up to ~100,000 km/s (~0.3c), with a peak and mean value of ~42,000 km/s (~0.14c). The ionization parameter is very high and in the range log xi~3-6 erg s-1 cm, with a mean value of log xi~4.2 erg s-1 cm. The associated column densities are also large, in the range NH~1022-1024 cm-2, with a mean value of NH~1023 cm-2. We discuss and estimate how selection effects, such as those related to the limited instrumental sensitivity at energies above 7 keV, may hamper the detection of even higher velocities and higher ionization absorbers.
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