Probing Circumnuclear Material in Seyfert Galaxies with X-ray Spectroscopy

by Jonathan Gelbord

The Johns Hopkins University
Department of Physics and Astronomy

Defended on 16 October 2002;
degree conferred in May 2003.

Thesis advisors: Kim Weaver, Tahir Yaqoob & Tim Heckman

Abstract

Using archival X-ray observations of Seyfert galaxies, we test the limits of the unified model for Seyfert galaxies. By considering the line-of-sight attenuating columns within the context of the relative arrangement of the Seyfert structural components, we find evidence suggesting that there are two distinct structures which are capable of absorbing the broad-line emission and giving rise to type 2 spectral characteristics. One absorbing system has high column densities (N_H > 10²³ cm^-2) and is uncorrelated with the orientation of the host galaxy, while the other has lower column densities and is more closely associated with the host galactic plane. In addition to establishing the presence of two discrete absorbing structures, we are able to place some constraints on the geometry of these structures.

In a separate survey of type 1 Seyferts with repeated observations in the ASCA archive, we find that variability of the iron line commonly occurs on a wide range of timescales and is generally uncorrelated with changes in the continuum. The disappearance of the iron feature puts a strong upper limit on the contribution of large-scale structures to the line flux in some systems, raising the question of whether putative tori are present in all Seyfert galaxies. In addition, the lack of correlated variability is similar to results obtained for shorter timescales and is difficult to reconcile with simple models of reflection. Taken together, these results call into question both the completeness and the universality of the unified model for Seyfert galaxies.

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The entire thesis. (252 pages, gzipped postscript, 1.5 Mb).
Abstract page, available either as a PDF (17 kb) or a PS.GZ (192 kb) file.
Individual chapters (all gzipped postscript):
- Frontmatter. Includes title page, abstract, acknowledgements, table of contents, etc. (13 pages, 195 kb)
- Chapter 1: Introduction to Seyfert galaxies. Sections cover observational history and phenomenology, the unified model paradigm, and some limitations of this paradigm. (23 pages, 244 kb)
- Chapter 2: Introduction to ASCA. Satellite instrumentation and data issues. (15 pages, 609 kb)
- Chapter 3: X-ray data processing. A quick general introduction to X-ray data analysis, followed by gory details about the automated processing pipeline developed for the X-ray absorber study in chapter 4. (16 pages, 275 kb)
- Chapter 4: Dual absorbing systems in Seyfert 2 galaxies. We present evidence that there are two distinct absorbing systems in Seyfert 2 galaxies: one that is (or is nearly) Compton-thick and is arbitrarily misaligned with the host galactic plane, and one that has lower column densities and is more closely aligned with the host galactic plane. (46 pages, 599 kb)
  The material in this chapter is currently being prepared for publication.
- Chapter 5: Fe K line variability in Seyfert 1 galaxies on long timescales. We find evidence from X-ray emission line variability suggesting that X-ray reprocessing takes place in structures on a range of physical size scales. Line variability on timescales of days to years is a common attribute, but is difficult to correlate to changes in the illuminating continuum. (41 pages, 443 kb)
  Much of the results presented in this chapter have been published in Weaver, Gelbord & Yaqoob 2001 (ApJ 550, 261).
- Chapter 6: Conclusions. A summary of the findings in chapters 4 & 5, and a brief look towards future work. (8 pages, 205 kb)
- Appendix A: Data processing notes. Details and notes on the extraction and processing of approximately 200 observations of over 80 Seyfert galaxies. (34 pages, 220 kb)
- Appendix B: Active galaxies observed by ASCA. A tabulation of active galaxies observed by ASCA, including all AGNs that were the subject of pointed observations and a number of additional serendipitously-observed systems. (36 pages, 227 kb)
- Bibliography. The complete reference list for the entire thesis. (13 pages, 198 kb)

Note that the version of my thesis provided here differs from the official version submitted on Halloween, 2002. This revised version fixes various errata and was completed in February 2003.

Jonathan Gelbord / MIT Kavli Inst. for Astrophysics & Space Research / jonathan [AT] space [DOT] mit [DOT] edu