The broad-band x-ray spectra of heavily obscured AGN
2014-12-15T10:40:48Z (GMT) by
This thesis presents a study of heavily obscured AGN using data from the Ginga, ASCA and ROSAT X-ray observatories. Chapter 1 provides a brief introduction into the classification of AGN, the Seyfert Unification Theory, the X-ray properties of Seyfert galaxies and possible emission mechanisms that occur in these objects. This is followed by a short discussion of the performance characteristics of the instruments on-board the observatories mentioned above (chapter 2). Chapter 3 focuses on the X-ray properties of the well-known Seyfert 2 galaxy Mrk 3. The utilization of non-simultaneous Ginga, ASCA and ROSAT observations allows the broad-band (0.1- 30.0 keV) X-ray spectrum of this object to be investigated, including the slope and level of the hard continuum, the presence of Compton reflection, iron Kq, emission and the relatively complex soft X-ray spectrum which comprises an emission feature near 0.9 keV. Following on from this, non-simultaneous Ginga and ASCA observations are employed to analyse the mid-hard X-ray spectrum of a sample of five heavily obscured (with column density Njy > 1023 cm-2) Seyfert 2 galaxies (chapter 4). The main result of these investigations is evidence for large variations in the line-of-sight gas column density in three out of the five objects. The X-ray spectrum of the Seyfert 1.5 galaxy NGC 4151 is also considered in this chapter. Multiple ASCA observations of this source show that the X-ray continuum, in the 1-7 keV band, varies with time in a confusing manner that can only be explained as an effect of the complex absorption that is present in the spectrum. A sharp emission feature present in the X-ray spectrum of NGC 4151 at 0.9 keV is investigated in chapter 5. Using models produced by the photoionization code xstar, this feature is explained as recombination of free electrons directly into the ground state of fully ionized oxygen. In addition, evidence for a feature at 0.9 keV is found for Mrk 348, NGC 4507 and NGC 7582. Chapter 6 discusses the potential of future X-ray satellites such as Chandra, ASTRO-E and in more detail XMM to further the investigations carried out in this thesis.