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Chaudhary, Pooja (2011): Properties of the integrated spectrum of active galactic nuclei. Dissertation, LMU München: Faculty of Physics
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Abstract

The aim of this thesis is to characterize the properties of the integrated spectrum of active galactic nuclei(AGNs), such as the ubiquity of the Fe Kα emission in AGNs, the dependence of the spectral parameters(e.g. the power law photon index and the Fe Kα equivalent width etc.) on the X-ray luminosity and redshift, and mean properties (including the line profile) of the Fe Kα line which can be addressed via stacking in the source rest-frame. I selected 2646 point sources observed with XMM-Newton at high galactic latitude (|BII| > 25 degrees) and with the sum of EPIC-PN and EPIC-MOS 0.2–12 keV counts greater than 1000. Redshifts were obtained for 919 sources from the Nasa’s Extragalactic Database. In order to prepare a clean sample, I retained only reliable sources (those for which the detection and the spectral products do not show obvious problems). I also excluded sources classified as HII regions, groups/clusters, star-forming/starburst galaxies. The final sample consists of 507 AGNs. To examine the ubiquity of the Fe Kα emission in AGNs and the dependence of the spectral parameters on the X-ray luminosity and redshift, I employed the observed-frame stacking technique, in which individual source spectra were summed in the observed-frame to compute the integrated spectra in different redshift and luminosity bins over the range 0 < z < 5. Detailed analysis of these spectra used appropriately normalized background spectra and exposure time weighted response and ancillary files. I find that the narrow Fe Kα line at 6.4 keV is significantly detected up to z = 1. The line equivalent width decreases with increasing the X-ray luminosity in the 2–10 keV band (“Iwasawa-Taniguchi effect”). The anticorrelation is characterized by the relation log(EW_Fe) = (1.66 ± 0.09) + (−0.43 ± 0.07) log(LX,44), where EW_Fe is the rest-frame equivalent width of the neutral Fe Kα line in eV and LX,44 is the 2–10 keV X-ray luminosity in units of 10^44 erg/s. The equivalent width is nearly independent of redshift up to z ∼ 0.8 with an average value of 101 ± 40 (rms dispersion) eV in the luminosity range 43.5 ≤ log LX ≤ 44.5. This analysis also confirms the hardening of the spectral indices at low luminosities, implying a dependence of obscuration on luminosity. To derive the mean properties of the Fe Kα line in the source rest-frame, I refined the final sample of 507 AGNs and selected 248 AGNs with the EPIC-PN net 2–10 keV rest-frame counts ≥200 and power law photon indices in the range 1.5–2.2. I computed the integrated spectrum of these 248 AGNs in the source rest-frame using two different rest-frame stacking procedures. I carried out a detailed spectral analysis of these integrated spectra using various models comprising simple/complex continuum and Fe Kα line components. I find that the average Fe Kα line profile in our sample is best represented by a combination of a narrow and a broad line. The equivalent widths of the narrow and broad components are ∼30 eV and ∼100 eV, respectively. The broad line parameters such as its shape, equivalent width and its detection significance in the integrated spectrum of the 248 AGNs are observed to be very sensitive to the assumed continuum and adopted stacking method. However, despite having a well-defined sample with reasonable statistics (net counts ∼ 198000) in the integrated spectrum, I do not detect a clear extended red-wing, and the measured equivalent width of the broad feature is always lower than 170 eV, implying that most black holes are not maximally rotating.