Context. Accretion models predict that fluorescence lines broadened by relativistic effects should arise from reflection of X-ray emission onto the inner region of the accretion disc surrounding the central black hole of active galactic nuclei (AGN). The theory behind the origin of relativistic lines is well established, and observational evidence from a moderate number of sources seems to support the existence of these lines.
Aims: The aim of this work is to establish the fraction of AGN with relativistic Fe K_α lines, and study possible correlations with source physical properties.
Methods: An XMM-Newton collection of 149 radio-quiet Type 1 AGN has been systematically and uniformly analysed in order to search for evidence of a relativistically broadened Fe K_α line. To enable statistical studies, an almost complete, flux-limited subsample of 31 sources has been defined by selecting the FERO sources observed by the RXTE all-sky Slew Survey with a count rate in the 3-8 keV energy band greater than 1 cts/sec. The 2-10 keV spectra of the FERO sources where compared with a complex model including most of the physical components observed in the X-ray spectra of Seyfert galaxies: a power law primary continuum modified by non-relativistic Compton reflection and warm absorption, plus a series of narrow Fe line reflection features.
Results: The observed fraction of sources in the flux-limited sample that show strong evidence of a relativistic Fe K_α line is 36%. This number can be interpreted as a lower limit to the fraction of sources that present a relativistic broad Fe K_α line in the wider AGN population. The average line equivalent width (EW) is of the order of 100 eV. The outcome of the fit yields an average disc inclination angle of 28 ± 5° and an average power-law index of the radial disc emissivity law of 2.4 ± 0.4. The spin value is well constrained only in 2 cases (MCG-6-30-15 and MRK 509); in the rest of the cases, whenever a constraint can be placed, it always implies the rejection of the static black hole solution. The Fe K_α line EW does not correlate with disc parameters or with system physical properties, such as black hole mass, accretion rate, and hard X-ray luminosity.

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