Browsing by Author "Baronchelli, L."

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    Relativistic accretion disc reflection in AGN X-ray spectra at z=0.5-4: a study of four Chandra Deep Fields
    (2020) Baronchelli, L.; Nandra, K.; Buchner, J.
    We confirm that the spectra are best fit by a model containing two Compton reflection components, one from distant material, and the other displaying relativistic broadening, most likely from the inner accretion disc. The degree of relativistic broadening indicates a preference for high black hole spin, but the reflection is weaker than that expected for a flat disc illuminated by a point source. We investigate the Compton reflection signatures as a function of luminosity, redshift, and obscuration, confirming an X-ray Baldwin effect for both the narrow and broad components of the iron line. Anticorrelations are also seen with redshift and obscuring column density, but are difficult to disentangle from the Baldwin effect. Our methodology is able to extract information from multiple spectra with low signal-to-noise ratio (SN), and can be applied to future data sets such as eROSITA. We show using simulations, however, that it is necessary to apply an appropriate S/N cut to the samples to ensure the spectra add useful information.
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    Relativistic reflection from accretion discs in the population of active galactic nuclei at z=0.5-4
    (2018) Baronchelli, L.; Nandra, K.; Buchner, J.
    We report the detection of relativistically broadened iron K alpha 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 10(5) 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(-37)(+35) per cent (107/199 sources). Considering that the low signal-to-noise ratio of some spectra prevents the detection of the broad iron K alpha 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.