Date of Award
5-2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Physics and Astronomy
Committee Chair/Advisor
Marco Ajello
Committee Member
Jeffrey Fung
Committee Member
Stephen Kaeppler
Committee Member
Stefano Marchesi
Abstract
It is important to understand the nature of the gas and dust immediately surrounding the supermassive black hole (SMBH) in active galactic nuclei (AGN), because this material feeds the accretion disk and couples the activities going on at the AGN scale to the host galaxy. However, there remain large uncertainties on basic properties about this material such as its location and distribution. This is primarily due to the difficulties in resolving spatial information on these scales at extragalactic distances.
AGN emit a tremendous amount of radiation all across the electromagnetic spectrum, including X-rays. The X-rays come from a very small region close to the central SMBH, and they interact with the surrounding material in multiple ways. These interactions leave specific signatures on the observed X-ray spectrum that can be analyzed and tied back to the physical properties of the gas and dust. One of the most important interactions that can occur is photoelectric absorption. Photoelectric absorption preferentially absorbs low-energy X-rays relative to high-energy X-rays resulting in an observable low-energy cutoff in the X-ray spectrum. The location of this cutoff can be used to determine the amount of material present between the X-ray source and the observer. This is quantified by the hydrogen column density, NH.
The NH is a useful quantity because it probes a specific line of sight in material that is otherwise spatially unresolved. Furthermore, there is evidence of NH variability in a significant fraction of AGN. By studying the variability in NH over time, the distribution of material may be mapped as various overdensities and underdensities move in and out of the line of sight. Unfortunately, there are very few AGN (~0) with a large enough number of observations required to perform this kind of analysis, so population statistics are likely the best way to obtain constraints. Furthermore, the analysis itself is time consuming and requires a great deal of hands-on testing to ensure the robustness of the NH determinations. This is not ideal since many sources end up being non-variable.
This work presents a method to quickly determine whether two observations are likely to display NH variability. It is based on hardness ratios, which are model independent and not affected significantly by many other things besides NH. The hardness ratio bands were carefully chosen so as to maximize sensitivity in the absorption range NH~1022-24cm-2 which are typical values for nearby Seyfert 2 galaxies. This method was subjected to rigorous testing to understand the reliability of the predictions at various sensitivity levels. This method was applied to a sample of 79 suitable AGN with multiple archival Chandra observations, and a sample of 43 AGN were found to be variable. In addition to identifying this large sample of AGN for follow up analysis, this work represents the first relatively unbiased estimate of the fraction of NH-variable AGN in the local Universe. More importantly, this work shows that the probability of detecting absorption variability in a source heavily depends on the number of observations a source has, and it is here argued that this observable has constraining power on the distribution of gas and dust.
This work also makes significant contributions to understanding the reliability of NH determinations in spectral modeling. It achieves this by comparing the NH results using several different models and spectral fitting procedures. It is found that while models can disagree quite significantly in some parameters, the NH remains in much better agreement. The apparent robustness of this particular parameter across different model assumptions and fitting algorithms helps to solidify the results shown in this work and other works, both past and future.
Recommended Citation
Cox, Isaiah S., "X-raying the Material Surrounding Active Supermassive Black Holes" (2026). All Dissertations. 4287.
https://open.clemson.edu/all_dissertations/4287
Author ORCID Identifier
0000-0003-2287-0325