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Exploring the geometry of circumnuclear material in active galactic nuclei through X-ray spectroscopy


I have studied the X-ray spectral properties of active galactic nuclei (AGN) in order to gain a better understanding of the nature of the circumnuclear material surrounding the central black hole in these objects. From the RXTE archive I constructed two survey samples of broad band X-ray spectra. The first was a bright sample of 23 AGN that had high quality spectra up to at least 100 keV, which provided constraints on the high energy rollover expected by models of inverse Comptonization of low energy photons. The average lower limit to E/roll was ̃225 keV for the majority of objects, implying a coronal electron temperature of kBTe≥75 keV for these models. The second sample was an expanded survey of ̃100 AGN for which spectral parameters could be well-determined. I compared Fe line equivalent widths with measured Compton reflection hump strengths and found that on average ̃40% of the Fe line emission comes from reflection off Compton-thick material, with the remainder likely arising in isotropic emission from Compton-thin gas. In the full sample, the distributions of photon indices for Seyfert 1's and 2's were consistent with the idea that Seyferts share a common central engine, however the distributions of Compton reflection hump strengths did not support the classical picture of absorption by a torus and reflection off a Compton-thick disk with type depending only on inclination angle. I have concluded that a more complex reflecting geometry such as a combined disk and torus or clumpy torus is likely a more accurate picture of the Compton-thick material. I have performed additional analyses of individual objects. An occultation event in Cen A, discovered through RXTE monitoring, revealed the clumpy nature of its torus and placed constraints on the amount of material in the vicinity of the black hole in this object. A Suzaku long-look observation of MCG-2-58-22 provided constraints on the location of Fe line emitting material to ≥1200 R/S, likely associated with the torus which was successfully modeled by the MYTorus reflection model. A Suzaku observation of Mkn 590 revealed a disappearing soft excess, possibly providing evidence that the soft excess is associated with thermal disk emission

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