Evidence for ultra-fast outflows in radio-quiet AGNs^*

I. Detection and statistical incidence of Fe K-shell absorption lines

¹ INAF-IASF Bologna, Via Gobetti 101, 40129 Bologna, Italy e-mail: tombesi@iasfbo.inaf.it
² Dipartimento di Astronomia, Università degli Studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Department of Physics and Astronomy, Johns Hopkins University, 3400, Baltimore, MD 21218, USA
⁴ Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Astrophysics Group, School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire ST5 5BG, UK
⁶ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

Received: 9 October 2009
Accepted: 14 June 2010

Abstract

Context. Blue-shifted Fe K absorption lines have been detected in recent years between 7 and 10 keV in the X-ray spectra of several radio-quiet AGNs. The derived blue-shifted velocities of the lines can often reach mildly relativistic values, up to 0.2–0.4c. These findings are important because they suggest the presence of a previously unknown massive and highly ionized absorbing material outflowing from their nuclei, possibly connected with accretion disk winds/outflows.

Aims. The scope of the present work is to statistically quantify the parameters and incidence of the blue-shifted Fe K absorption lines through a uniform analysis on a large sample of radio-quiet AGNs. This allows us to assess their global detection significance and to overcome any possible publication bias.

Methods. We performed a blind search for narrow absorption features at energies greater than 6.4 keV in a sample of 42 radio-quiet AGNs observed with  XMM-Newton. A simple uniform model composed by an absorbed power-law plus Gaussian emission and absorption lines provided a good fit for all the data sets. We derived the absorption lines parameters and calculated their detailed detection significance making use of the classical F-test and extensive Monte Carlo simulations.

Results. We detect 36 narrow absorption lines on a total of 101  XMM-Newton EPIC pn observations. The number of absorption lines at rest-frame energies higher than 7 keV is 22. Their global probability to be generated by random fluctuations is very low, less than 3 × 10^-8, and their detection have been independently confirmed by a spectral analysis of the MOS data, with associated random probability <10^-7. We identify the lines as Fe XXV and Fe XXVI K-shell resonant absorption. They are systematically blue-shifted, with a velocity distribution ranging from zero up to ~0.3c, with a peak and mean value at ~0.1c. We detect variability of the lines on both EWs and blue-shifted velocities among different  XMM-Newton observations even on time-scales as short as a few days, possibly suggesting somewhat compact absorbers. Moreover, we find no significant correlation between the cosmological red-shifts of the sources and the lines blue-shifted velocities, ruling out any systematic contamination by local absorption. If we define ultra-fast outflows (UFOs) those highly ionized absorbers with outflow velocities higher than 10⁴ km s^-1, then the majority of the lines are consistent with being associated to UFOs and the fraction of objects with detected UFOs in the whole sample is at least ~35%. This fraction is similar for type 1 and type 2 sources. The global covering fraction of the absorbers is consequently estimated to be in the range C ∼ 0.4-0.6, thereby implying large opening angles.

Conclusions. From our systematic X-ray spectral analysis on a large sample of radio-quiet AGNs we have been able to clearly assess the global veracity of the blue-shifted Fe K absorption lines at E > 7 keV and to overcome their publication bias. These lines indicate that UFOs are a rather common phenomenon observable in the central regions of these sources and they are probably the direct signature of AGN accretion disk winds/ejecta. The detailed photo-ionization modeling of these absorbers is presented in a companion paper.