Broad-band spectral analysis of a complete sample of type 1 AGN detected by INTEGRAL

Abstract

This thesis contains results on type 1 Active Galactic Nuclei detected and studied with INTEGRAL. The sample of sources analysed in the present thesis is based on a complete sample of type 1 AGN extracted from the 3rd IBIS/ISGRI catalogue. Archival X-ray data taken from several observatories, such as XMM-Newton, Chandra, Swift/XRT and ASCA, have been combined together with IBIS spectra, providing for the first time a broad-band spectral analysis of a hard X-ray detected complete sample of type 1 AGN. The principal aim of this work is to study the continuum properties of type 1 AGN, i.e. power law slope, reflection fraction and high energy cut-off, and their relation to spectral modelling of AGN and to synthesis models of the Cosmic Diffuse X-ray Background. The analysis presented in this work covers two subclasses of type 1 AGN: Broad Line Radio Galaxies (BLRG) and Radio Quiet (RQ) type 1 sources. In particular, among BLRG, IGR J21247+5058 is studied in great depth (see chapter 5). This is in fact a very peculiar source, displaying very complex absorption, in the form of two layers partially covering the central emitting source. So far, only one other BLRG was known to require such complexity regarding absorption (namely 4C 445), making IGR 21247+5058 indeed a very peculiar and almost unique AGN. Broad-band spectra of the other seven BLRG present in the INTEGRAL complete sample have been analysed in search for a dichotomy between this population (chapter 6) and their radio quiet counterparts (chapter 7). Several studies have in fact shown in the past that BLRG might behave differently than radio quiet AGN, displaying weaker reprocessing features (i.e. the reflection component and the strength of the iron line), possibly –i– associated with the presence of the jet. In the present study, however, such diversity is not found to be very striking, with the reprocessing features of the analysed sources not being as weak as expected. This could imply a different scenario for the dilution of these features, not involving a jet but rather a different geometry and/or accretion flow efficiency in the accretion disk. The analysis of the complete sample of type 1 AGN also allows a general picture of the average properties of this class of sources to be obtained. The mean power law slope is found to be ?=1.86±0.01, in good agreement with the generally accepted canonical spectral index of 1.9. The average cut-off energy is Ec=104 keV, lower than previously found in other works; the mean reflection fraction is 1.08±0.14. Correlations between the spectral parameters have also been investigated, but most of them remain still to be proven. When considering these results in the framework of AGN spectral modelling, we found that the average Comptonising plasma temperature is typically ?50 keV with an optical depth ranging from 2 to 5, i.e. the plasma is not too thick. As far as the Cosmic X-ray Diffuse Background is concerned, synthesis models have so far assumed a spectral shape for the CXB with ?=1.9 with no dispersion in values and a high energy cut-off of at least 200 keV. However, the value of ?=1.86 with a dispersion of 0.2 and a cut-off energy with a narrow range of values around 100 keV, as found in the present thesis, could provide a self-consistent modelling of the CXB. The implementation of the information provided in this thesis is clearly very important for CXB synthesis models and represents the next step of this work.

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