Twinkle, twinkle, little stars: shedding light on the population of Galactic gravitational wave sources Thesis by Kevin Brian Burdge In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2021 Defended May 17, 2021 ii © 2021 Kevin Brian Burdge ORCID: 0000-0002-7226-836X All rights reserved iii To my wife Maiko, and my son Luke iv Seht die Sterne, die da lehren wie man soll den Meister ehren. Jeder folgt nach Newtons Plan Ewig schweigend seiner Bahn. -Albert Einstein v ACKNOWLEDGEMENTS My journey through graduate school would not have been possible were not not for many people: Firstly, I thank my advisor, Tom Prince, who put up with my visiting his office dozens of times a day (and often came down to visit mine), which helped me thrive. I suspect that I probably communicated more with my advisor than any other graduate student in the building, and I am incredibly fortunate to have had one who was willing to devote so much time to me. Tom has not only been instrumental in developing my research career, but he has also taught me a great deal about how to be a supportive and effective mentor, and I will do my best to apply these lessons if I ever have my own graduate students and postdocs. Shri Kulkarni, who throughout graduate school provided me with valuable advice, effectively acting as secondary advisor. Shri provided me with a broader perspective on how to approach research and life in general. I thank him for his patience, and I value his no-nonsense approach to the world. I particularly appreciate his honesty and willingness to critically discuss my weaknesses as a scientist with me, something which I wish more people were willing to do. Being aware of these weaknesses has given me focus on what to work on, and I appreciate this a great deal; I know that I will grow immensely as a scholar if I can successfully apply Shri’s advice. Jim Fuller, who also acted as a secondary advisor to me (Jim also endured many unannounced visits to his office). I appreciate the ease with which I could approach Jim about any question, and his enthusiasm in understanding the odd systems discovered in this thesis work. Tom Marsh, who I met at the European White Dwarf workshop in 2018. I had heard about Tom prior to meeting him (and read many of his papers), but I never could have anticipated how much I would learn from him and how much I would come to value his friendship. What I value the most about working with Tom is his shared enthusiasm and excitement for every new discovery (those who know me know I am largely motivated vi by my own excitement and enthusiasm, and it was a joyous experience to meet someone who shared my degree of enthusiasm for compact binaries). David Kaplan, a role model whose rigorous and thorough approach to research I greatly admire. His remarkably dense feedback as a co-author greatly enhanced all of the publications presented in this thesis. There are many other faculty working in the field of white dwarfs who had an incredibly positive impact on my career in the last few years, including Warren Brown, Boris Gaen- sicke, JJ Hermes, Mukremin Kilic, who all share my enthusiasm for double degenerates (and white dwarfs in general). Gregg Hallinan, for renewing my enthusiasm for instrumentation, and teaching me a lot about CHIMERA. Matthew Graham, who shared my general enthusiasm for exotic variables and fast algorithms for mining large astronomical datasets. I learned an enormous amount from conversations with Matthew over the years, and grew a lot as a data scientist as a result of his advice. I collaborated with many excellent postdoctoral scholars during my time at Caltech, who I also wish to acknowledge: Michael Coughlin, who has played a major role in much of my work, suffered through many nights of KPED with me, and has always been a supportive friend. Ilaria Caiazzo, whose remarkable knowledge and enthusiasm for white dwarfs continues to amaze me, and has bolstered my enthusiasm for research. Przemek Mroz, whose remarkable programming skills never failed to stun me, and have opened up many new scientific doors to explore. Jan van Roestel, whose enthusiasm for white dwarfs and variable stars in general helped further fuel my own. I thank Jan in particular for all of his help with observing, and the many things he taught me about lightcurve modelling. I also thank Marin Anderson, my office mate for several years, whose friendship made coming to the office something I always enjoyed. vii I thank Kishalay De and Anna Ho, two good friends, and also colleagues within the ZTF project working on very different topics than myself, whose perspectives I always found incredibly valuable. I thank Andrew Tisdale for his generous gift towards my research, which has enabled a significant amount of progress and contributed significantly to the discoveries presented in this thesis. I thank the Palomar and Keck support astronomers who helped me get through many long nights, and the Caltech Optical Observatory staff, in particular Jennifer Millburn, who worked very hard over the years to help me facilitate many upgrades to instruments such as CHIMERA. I thank Ernie and his team, whose food truck supplied a large fraction of my biomass in graduate school, and whose friendship I came to value over the many years we interacted on a daily basis (I was such a consistent patron that if I did not show up to his food truck, he would ask me the next day if everything was ok). Perhaps the saddest day in my time at Caltech was the day Ernie retired, and I enjoyed his tacos for the last time. I thank my parents Christine and Richard for supporting me all my life, and giving me a great deal of autonomy from an early age. I also thank the rest of my family for their support, including my sister Sarina, and my grandparents. I also thank my many close friends from high school, college, and graduate school. Finally, I thank my wife Maiko, who puts up with my eccentric personality, inspires me every day, and always gives me something to look forward to. viii ABSTRACT Time domain surveys are revolutionizing our understanding of compact binary systems containing a white dwarf and another compact object at short orbital periods. These extreme binaries are astrophysical laboratories which can probe compact object physics, the nature of Type Ia supernova progenitors, accretion physics, tidal physics, the process of binary evolution, and they will dominate the population of objects the Laser Interfer- ometer Space Antenna (LISA) will detect. In this thesis, I present substantial advances in the discovery and characterization of compact binaries using the Zwicky Transient Facility (ZTF). This work has resulted in a ten-fold increase in the discovery rate of such binaries compared to previous work in the field, and has helped lay the groundwork for discovering and characterizing these sources using other facilities, such as the the Transiting Exoplanet Survey Satellite (TESS), the upcoming Vera Rubin Observatory (VRO) and eventually LISA itself. ix PUBLISHED CONTENT AND CONTRIBUTIONS Burdge, K. B., M. W. Coughlin, J. Fuller, D. L. Kaplan, et al. (2020). “An 8.8 Minute Orbital Period Eclipsing Detached Double White Dwarf Binary”. In: ApJL 905.1, p. L7. doi: 10.3847/2041-8213/abca91. arXiv: 2010.03555 [astro-ph.SR]. Burdge, K. B., T. A. Prince, et al. (2020). “A Systematic Search of Zwicky Transient Facility Data for Ultracompact Binary LISA-detectable Gravitational-wave Sources”. In: ApJ 905.1, p. 32. doi: 10.3847/1538- 4357/abc261. arXiv: 2009.02567 [astro-ph.SR]. Burdge, K. B., M. W. Coughlin, J. Fuller, T. Kupfer, et al. (2019). “General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system”. In: Nature 571.7766, pp. 528–531. doi: 10.1038/s41586-019-1403-0. arXiv: 1907.11291 [astro-ph.SR]. Burdge, K. B., J. Fuller, et al. (2019). “Orbital Decay in a 20 Minute Orbital Period Detached Binary with a Hydrogen-poor Low-mass White Dwarf”. In: ApJL 886.1, p. L12. doi: 10.3847/2041-8213/ab53e5. arXiv: 1910.11389 [astro-ph.SR]. In all publications included in this thesis (Chapters 2-5): KBB discovered all but one object presented in this thesis (the exception, ZTF J2130+4420, was discovered by Zhuyun Zhuang as she was being mentored by KBB as an undergradu- ate researcher). KBB conducted all of the lightcurve and spectroscopic analyses, eclipse time analyses, and was the primary author of all manuscripts. KBB also collected the majority of the data presented in this thesis, and also reduced most of it. x TABLE OF CONTENTS Acknowledgements . v Abstract . viii Published Content and Contributions . ix Table of Contents . ix Chapter I: Introduction and summary . 1 1.1 Review of previous work . 1 Chapter II: Orbital Decay in a 20-minute-orbital-period detached binary with a hydrogen-poor low-mass white dwarf . 5 Abstract . 6 2.1 Introduction . 7 2.2 Observations . 8 2.3 Analysis and results . 12 2.4 Discussion . 23 2.5 Conclusion . 30 Chapter III: General relativistic orbital decay in a seven-minute-orbital-period eclipsing binary system . 32 Abstract . 33 3.1 Summary of Observations . 44 3.2 Period Finding . 44 3.3 Lightcurve Modelling . 44 3.4 Orbital Decay . 45 3.5 Spectroscopic Analysis . 49 3.6 Mass and Radius Analysis . 52 3.7 Distance Estimate . 53 3.8 Gravitational Wave Strain .