Probing the Inner Accretion Flow Properties Around Black Holes with X-ray Observations Thesis by Yanjun Xu In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2021 Defended May 19, 2021 ii c 2021 Yanjun Xu ORCID: 0000-0003-2443-3698 All rights reserved except where otherwise noted. iii ACKNOWLEDGEMENTS First and foremost, I would like to express gratitude to my parents and grandparents for being extra supportive of me in all aspects of my life, allowing entire freedom for me to explore and pursue paths that I have been interested in ever since my childhood. And special thanks to my mom for being my best friend, and being always interested in my work although she probably would not pay much attention to astrophysics otherwise. I am sincerely grateful to my Ph.D. advisor Prof. Fiona Harrison for her guidance and support of me through my years in graduate school, and for her always being a open-minded and inclusive resource to talk to and discuss scientific problems with. I am also eternally grateful for her constant determination and efforts to lead me to become a confident and independent researcher. I am very grateful to Prof. Jon Miller, Prof. Andy Fabian, and Dr. John Tomsick for carefully reading my papers, always offering insightful and helpful comments and discussions, and being supportive of me to pursue a scientific career. My thanks also go to everyone else in the NuSTAR team at Caltech, from whom I learned a lot when working with them, especially to Dr. Karl Froster for helping schedule my NuSTAR observations. And I want to thank Dr. Dominic Walton, Dr. Mislav Balokovic,´ Dr. Lian Tao, Dr. Felix Fürst, and Dr. Murray Brightman for teaching me about the basics of X-ray data analysis at the start of my Ph.D. I thank Prof. Phil Hopkins, Prof. Chris Martin, and Prof. Alan Weinstein for serving on my thesis committee and providing helpful advice. And I am grateful to my undergraduate research advisors Prof. Tinggui Wang and Prof. Linqing Wen for inspiring my interest in high energy astrophysics and continuing to offer me valuable advice over the years. I am lucky to have many supportive friends, Xiangcheng Ma, Xinghao Zhou, Song- ming Du, Feng Bi, Dan Zhou, and Sanle Hu among others, at Caltech from the beginning of my graduate study. Many thanks to them for kindly picking me up from the airport on my first day at Caltech, helping me settle into the new envi- ronment, and taking me out for dinner every weekend for almost two years until I earned the skill to drive in LA traffic (and helping with that too). And thanks to Jamie Rankin, Aaron Pearlman, Matt Hartley, and Sean Pike for being my good office mates for many years. iv Last, I hold sincere gratitude to the faculty and staff at Caltech for undertaking tireless administrative efforts to make student life as normal as possible during the COVID-19 period, without which this thesis could not have been finished in time. I also hold sincere gratitude to anyone continuing their hard work during this un- precedentedly difficult time, of which I may be aware or not, to keep our everyday life going and strengthen our faith in the good sides of humanity despite all the uncertainties in life. v ABSTRACT Accretion, governed by gravity, is a fundamental source of energy in the universe. Accretion is important to the growth and evolution of black holes, as well as the structure in the universe on larger scales. Accretion disks around supermassive black holes are likely to have produced most of the ionizing radiation in the uni- verse since the epoch of reionization. Outflows launched from the accretion disk of black holes, either in the form of disk winds or jets, provide a feedback mecha- nism that plays an important role in the co-evolution of black holes and their host galaxies. It has been decades since the fundamental theories about black hole ac- cretion were established. Black hole accretion has been widely studied in active galactic nuclei (AGNs) and black hole binaries ever since the early days of X-ray astronomy. However, important questions still remain regarding the fundamental physical properties of black holes, the structure and geometry of accretion disks and coronae, and the nature of disk winds and jets. In this thesis, I report results from recent X-ray observations of an ultra-luminous infrared galaxy with a central AGN, IRAS 05189–2524, and several black hole X-ray binaries (or black hole candidates for those currently lacking dynamical mass determinations), IGR J17091–3624, MAXI J1535–571, Swift J1658.2–4242, MAXI J1631–479, and MAXI J1820+070. Most of the black hole X-ray bina- ries studied in this thesis were uncatalogued sources and were discovered as bright Galactic X-ray transients over the past few years, offering great opportunities for investigating black hole accretion with high quality datasets. The launch of the NuSTAR telescope in 2012 has brought the advanced capabilities of performing high sensitivity observations in the hard X-ray band and remaining free from pile- up for bright Galactic sources, providing new angles for the detailed study of ob- servational phenomena around both supermassive and stellar-mass accreting black holes. I study the inner accretion flow properties around black holes in the above objects by conducting spectral and timing analyses of the NuSTAR observations and the simultaneous soft X-ray band data from the Swift or XMM-Newton telescope. I have searched for and analyzed various accretion related observational features in these systems (disk reflection spectra, ionized absorption caused by disk winds, quasi periodic oscillations (QPOs) in the X-ray light curves, and unusual aperiodic flux variations or accretion state changes), and interpret the results in terms of the physical properties about the inner accretion flows and the central black holes. vi By modeling the relativistic disk reflection spectra, I have found that the inner edge of the optically-thick accretion disk is truncated in IGR J17091–3624 during the bright hard state, in MAXI J1631–479 during the very high state, and in MAXI J1820+070 during the faint hard state, whereas the inner accretion disk is consis- tent with extending down to the ISCO in MAXI J1535–571 and Swift J1658.2–4242 during their bright hard states, and in MAXI J1631–479 during its soft state. Based on all the observational evidence gathered in this thesis, the general picture about the accretion flow geometry at different accretion states seems to be more compli- cated than that from previously well accepted theoretical predictions. In addition, I have measured the black holes spins and inner accretion disk inclinations for IRAS 05189–2524, MAXI J1535–571, Swift J1658.2–4242, and MAXI J1631–479 using the disk reflection modeling method, where the central black holes are all found to be rapidly spinning. There are also evidence for strong disk winds detected in the X-ray spectra of some of the objects studied in this thesis with interesting implica- tions. I summarize my studies of these individual objects at the end of this thesis in a more broader context of the characteristic behaviors of the population and their general physical implications, and discuss about the possibilities of extending my research in this direction with the upcoming new X-ray missions. vii PUBLISHED CONTENT AND CONTRIBUTIONS Xu Y., Harrison F. A., Tomsick J. A., Hare J. et al. (2020). “Evidence for Disk Trun- cation at Low Accretion States of the Black Hole Binary MAXI J1820+070 Observed by NuSTAR and XMM-Newton.” The Astrophysical Journal, 893(1): 42. doi: 10.3847/1538–4357/ab7cdb. Y.X. participated in the conception of the project, proposed for the observa- tions, analyzed the data, and wrote the manuscript. Xu Y., Harrison F. A., Tomsick J. A., Walton D. J. et al. (2020). “Studying the Reflection Spectra of the New Black Hole X-Ray Binary Candidate MAXI J1631–479 Observed by NuSTAR.” The Astrophysical Journal, 893(1):30. doi: 0.3847/1538–4357/ab7dc0. Y.X. participated in the conception of the project, proposed for the observa- tions, analyzed the data, and wrote the manuscript. Xu Y., Harrison F. A., Tomsick J. A., Barret D. et al. (2019). “Broadband X- Ray Spectral and Timing Analyses of the Black Hole Binary Candidate Swift J1658.2–4242: Rapid Flux Variation and the Turn-on of a Transient QPO.” The Astrophysical Journal, 879(2):93. doi: 0.3847/1538–4357/ab24bf. Y.X. participated in the conception of the project, proposed for the observa- tions, performed data analyses, provided physical interpretation of the results, and wrote the manuscript. Xu Y., Harrison F. A., Kennea J. A., Walton D. J. et al. (2018). “The Hard State of the Highly Absorbed High Inclination Black Hole Binary Candidate Swift J1658.2–4242 Observed by NuSTAR and Swift.” The Astrophysical Journal, 865(1):18. doi: 0.3847/1538–4357/aada03. Y.X. participated in the conception of the project, proposed for the observa- tions, analyzed the data, and wrote the manuscript. Xu Y., Harrison F. A., García J. A., Fabian A. C. et al. (2018). “Reflection Spec- tra of the Black Hole Binary Candidate MAXI J1535–571 in the Hard State Observed by NuSTAR.” The Astrophysical Journal Letters, 852(2):L34. doi: 10.3847/2041–8213/aaa4b2. Y.X. participated in the conception of the project, analyzed the data, and wrote the manuscript. Xu Y., García J. A, Fürst F., Harrison F. A. (2017).