2018MNRAS.480.2377B

We report the detection of relativistically broadened iron K α emission in the X-ray spectra of active galactic nuclei detected in the 4Ms CDF-S. Using the Bayesian X-ray analysis (BXA) package, we fit 199 hard band (2-7 keV) selected sources in the redshift range z = 0.5-4 with three models: (i) an absorbed power law, (ii) the first model plus a narrow reflection component, and (iii) the second model with an additional relativistic broadened reflection. The Bayesian evidence for the full sample of sources selects the model with the additional broad component as being 10⁵ times more probable to describe the data better than the second model. For the two brightest sources in our sample, CID 190 (z = 0.734) and CID 104 (z = 0.543), BXA reveals the relativistic signatures in the individual spectra. We estimate the fraction of sources containing a broad component to be 54⁺³⁵_–37 per cent (107/199 sources). Considering that the low signal-to-noise ratio of some spectra prevents the detection of the broad iron K α line, we infer an intrinsic fraction with broad emission of around two thirds. The detection of relativistic signatures in the X-ray spectra of these sources suggests that they are powered by a radiatively efficient accretion disc. Preliminary evidence is found that the spin of the black hole (BH) is high, with a maximally spinning Kerr BH model (a = 1) providing a significantly better fit than a Schwarzschild model (a = 0). Our analysis demonstrates the potential of X-ray spectroscopy to measure this key parameter in typical supermassive black hole systems at the peak of BH growth.