Unveiling the Structure of Active Galactic Nuclei with Hard X-ray Spectroscopy Thesis by Mislav Baloković In Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2017 Defended May 25, 2017 ii © 2017 Mislav Baloković ORCID: 0000-0003-0476-6647 All rights reserved except where otherwise noted. iii ACKNOWLEDGEMENTS Many good people helped me on my way to this point in life. First and foremost, my loving family, Snježana, Darije, Jakov and Luka Baloković, and my amaz- ing wife, Antonija Oklopčić. I am in great debt of gratitude to my parents for their unconditional support stretching over (more than!) 30 years and across more than one continent. They taught me to be inquisitive, open-minded, persistent, and hard-working in my pursuit of knowledge. I will always be grateful for their encouragement and their unwavering belief in me. I owe so much of my success and happiness to Antonija that I cannot mention her in just a single paragraph. Words cannot fairly express how grateful I am that our lives are intertwined in their every aspect. I am truly thankful her continued love, support, encouragement, and understanding since before we even considered going to graduate school. She made the last 6 years in Pasadena, our home away from home, the most wonderful in my life. I would not have made it all the way to Pasadena without my many friends, teachers and mentors. Special thanks go to Mario Pallua, my best-est man, for introducing me to the wonders of the night sky and amateur astronomy, in addition to the music and the resulting life-long friendships. Our shared enthusiasm fueled our many nights under the open sky with a telescope, our small astronomy projects, and our early efforts to share the excitement of astronomy with everyone around us. Ana Bedalov, Dejan Vinković, Mario Jurić, Željko Ivezić, and others have helped me more than they know in defining my path toward a career in astronomy. Their encouragement and support led me to spend my undergraduate summers at top-class astronomy institutions around the world, which I enjoyed immensely. I am indebted to Martin Kürster, Anton Koekemoer, and Dominik Riechers, for investing their time to show me what it means to be a professional astronomer. Along the way I met Vernesa Smolčić, to whom I owe many thanks for being my undergraduate thesis advisor, and for providing valuable support in preparing both Antonija and me for graduate school abroad. I am sincerely grateful to my thesis advisor, Fiona Harrison, for her sustained support, and the freedom I was given to explore scientific questions outside of my thesis topic. I consider myself fortunate to have had an advisor who provided help and guidance whenever it was needed despite her countless other responsibilities, iv while letting me develop my own research plan and set my own pace. I appreciate learning from her about the importance of being concise and clear, both in expressing scientific ideas, as well as in being an efficient leader. Thanks to Fiona and NuSTAR I had the privilege to work with some of the greatest experts in my field, Andrea Comastri, Greg Madejski, Dave Alexander, Giorgio Matt, and Mike Koss, among others. It continues to be a pleasure to share research ideas with such excellent scientists. Much of my work would not have been possible without the dedication and hard work of many people in the NuSTAR collaboration, at Caltech and around the world. I appreciate the help, patience, and kindness from all who have been a part of Fiona’s research group over the past 6 years, especially Dan Stern, Brian Grefenstette, Kristin Madsen, Dom Walton, Felix Fürst, Murray Brightman, Karl Forster, Liz Rivers, and Marianne Heida. I learned so much about so many different things from them. My time in graduate school would have been much less fun without the good friends I found there. I really appreciate the many times spent with Ben Montet, Allison Strom, Trevor David, Yi Cao, Abhilash Mishra, and Antonija, be it at work (working or not) or far from campus, discussing greatest scientific achievements or airline miles, studying or partying, celebrating or commiserating... I am also grateful for company, advice, and shared experiences with many other Caltech students; in particular, older colleagues like Gwen Rudie and Drew Newman, who helped me a lot in navigating the graduate school life and the postdoc job market. I would also like to thank the rest of my thesis committee: Phil Hopkins, the committee chair, Lynne Hillenbrand, Tony Readhead, and Nick Scoville. I admire their foresight in giving me good advice on my thesis plan at my candidacy exam 3 years prior, which came to be crucial in the end although to me it did not appear so at the time. More generally, along with other faculty, staff, and administrators, I thank them for keeping the life in the Astronomy Department so supportive and enjoyable. Finally, I gratefully acknowledge the Fulbright International Science and Technology Fellowship, which generously supported my first three years in graduate school, and led to many good times spent getting to know Fulbright fellows from all corners of the world. I also acknowledge the NASA Earth and Space Science Fellowship for the support of the latter three years in graduate school. At the very end, I would like to thank Caltech as a whole, and the many people who make it such a magnificent place to do science! v ABSTRACT Despite the long history of studies of active galactic nuclei (AGN), details of the structure of the accretion flow onto supermassive black holes are far from clear. Work presented in this thesis is directed at unveiling properties of AGN structure through broadband X-ray spectroscopy, with particular emphasis on the hard X- ray band (photon energies > 10 keV). With its unprecedented sensitivity in this energy band, the NuSTAR telescope provides the key observational diagnostics of the properties of the AGN X-ray source, the corona, and the surrounding gas in the accretion disk, the broad-line region, and the torus. The first study presented in this thesis focuses on measurements of the optical depth and the temperature of the plasma in the corona of an obscured AGN. Fitting theoretical spectral models for coronal emission to the NuSTAR data constrained these two basic physical parameters under the assumption of either spherical or disk-like geometry for the corona. The remainder of the thesis is dedicated to studies of the anisotropic obscuring structure broadly referred to as the torus. One of them is a case study of three heavily obscured AGN with spectra dominated by the X-ray light scattered and reprocessed in the torus, where it is possible to constrain one of the basic torus properties – its globally averaged column density. The following study presents the calculation of a new spectral model for reprocessing of the intrinsic X-ray continuum within the torus. Its added flexibility compared to previously available models allows for both the average column density of the torus and its covering factor to be constrained from broadband X-ray spectra of a wide variety of AGN. The final part of the thesis in based on a large survey of the local obscured AGN population performed with NuSTAR. Spectral modeling of more than a hundred individual AGN, including both old and new spectral models, is presented. From analyses of the X-ray data for a large and representative AGN sample, for the first time, it is found that their tori preferentially have high covering factors and average column densities close to unity optical depth for Compton scattering. vi PUBLISHED CONTENT AND CONTRIBUTIONS Baloković, M., Brightman, M., Harrison, F. A., et al., 2017, New Spectral Model for Measuring Torus Covering Factors from Broadband X- ray Spectra of Active Galactic Nuclei, submitted to The Astrophysical Journal (in peer review as of May 25, 2017) Contributions: I formulated the idea for this project, wrote the computer code for calculation of the new model, selected the data to demonstrate the application of the model, performed spectral analyses, and wrote the paper. The second author contributed to computation of the new model, while the third author helped with writing the paper. All authors were involved in discussions, and provided advice on various aspects of model building and application of the model to real data. Baloković, M., Matt, G., Harrison, F. A., et al., 2015, Coronal Properties of the Seyfert 1.9 Galaxy MCG-05-23-016 Determined from Hard X-Ray Spectroscopy with NuSTAR, The Astrophysical Journal, 800, 62, DOI: 10.1088/0004-637X/800/1/62 Contributions: The project was conceived by the second and the third author. I performed the majority of the data processing and analysis with input from these two authors. I wrote the paper reporting the observational results and their interpretation. The rest of the authors contributed to discussion of the results, and/or to the NuSTAR mission in general. Baloković, M., Comastri, A., Harrison, F. A., et al., 2014, The NuSTAR View of Nearby Compton-thick Active Galactic Nuclei: The Cases of NGC 424, NGC 1320, and IC 2560, The Astrophysical Journal, 798, 111, DOI: 10.1088/0004-637X/794/2/111 Contributions: The first three authors collaborated on the selection of the NuSTAR data for this publication. I performed all data processing and spectral analyses, and I wrote the paper with help from the second and third authors. The rest of the authors contributed to discussion of the results and their implications, and/or to the NuSTAR mission in general.