Evidence for Ultra-Fast Outflows in radio-quiet active galactic nuclei. II. Detailed photoionization modeling of Fe K-shell absorption lines

Abstract

X-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 10,000kms-1 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 FeK 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% and that the majority of the FeK absorbers are indeed associated with UFOs. The outflow velocity distribution spans from ∼10,000kms-1 (∼0.03c) up to ∼100,000kms-1 (∼0.3c), with a peak and mean value of ∼42,000kms-1 (∼0.14c). The ionization parameter is very high and in the range log ξ ∼3-6ergs-1cm, with a mean value of log ξ ∼4.2ergs -1cm. The associated column densities are also large, in the range NH ∼1022-1024cm-2, with a mean value of NH∼ 1023cm-2. We discuss and estimate how selection effects, such as those related to the limited instrumental sensitivity at energies above 7keV, may hamper the detection of even higher velocities and higher ionization absorbers. We argue that, overall, these results point to the presence of extremely ionized and possibly almost Compton-thick outflowing material in the innermost regions of AGNs. This also suggests that UFOs may potentially play a significant role in the expected cosmological feedback from AGNs and their study can provide important clues on the connection between accretion disks, winds, and jets. © 2011. The American Astronomical Society. All rights reserved.