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 _ { \alpha } 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 _ { \alpha } 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 3-8Â keV energy band greater than 1 cts/sec .
The 2-10Â 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 _ { \alpha } line is 36 % .
This number can be interpreted as a lower limit to the fraction of sources that present a relativistic broad Fe K _ { \alpha } line in the wider AGN population .
The average line equivalent width ( EW ) is of the order of 100Â eV .
The outcome of the fit yields an average disc inclination angle of 28 \pm 5 ^ { \circ } and an average power-law index of the radial disc emissivity law of 2.4 \pm 0.4 .
The spin value is well constrained only in 2 cases ( MCG-6-30-15 and MRKÂ 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 _ { \alpha } 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 .

Conclusions :