Properties of the integrated spectrum of active galactic nuclei

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.