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A Non-Radial Oscillation Model for Radio Pulsars

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A Non-Radial Oscillation Model for Radio Pulsars A Non-Radial Oscillation Model for Radio Pulsars by Rachel Rosen A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Physics & Astronomy. Chapel Hill 2007 Approved by: Dr. J. Christoper Clemens, Advisor Dr. Wayne Christensen, Reader Dr. Charles Evans, Reader Dr. Robert McMahan, Reader Dr. Dan Reichart, Reader Dr. Sean Washburn, Reader c 2007 Rachel Rosen All Rights Reserved ii ABSTRACT Rachel Rosen: A Non-Radial Oscillation Model for Radio Pulsars (Under the Direction of Dr. J. Christoper Clemens) We present a non-radial oscillation model that can successfully reproduce many properties of drifting subpulses in pulsars. By demonstrating the presence of oscillation modes in pulsars, we hope to pave the way for astroseismology of neutron stars. This model is an alternative to the drifting spark model of Ruderman & Sutherland (1975). Our non-radial oscillation model reproduces pulsar morphology in terms of the total intensity and linear polarization. We associate the displacement and velocity of the oscillations to two orthogonal polarization modes. From this, we recreate the Stokes’ parameters to model the intensity, linear polarization, and polarization angle. We use our model to qualitatively reproduce the behavior of several pulsars, including PSR 1919+21, PSR 1237+25, and PSR 0809+74. This behavior includes the time-averaged pulse properties, the individual pulse properties, and the subpulse phase jumps. In addition, we are able to reproduce the orthogonal polarization modes observed in most pulsars. We then show that our model can be successfully fit to the data by conducting a quantitative analysis of PSR 0943+10. iii Dedicated to: My parents, Ellen and Isaac. iv ACKNOWLEDGMENTS In the late 1980s and early 1990s, Time-Life books published a series called Voyage Through the Universe. As a early teenager at the time, I would scavenge the used book stores and the library for copies of the books. At the time, I was too young to understand all the text and its implications, nor did I have the patience to read each book in its entirety. Yet, the vibrant pictures fascinated me. My interest in astronomy began many years before this, but these books solidified in my mind what was to be the obvious course of my education. The completion of this dissertation marks the culmination of my formal education (at least for the immediate future) and, like most things in my life, I did not accomplish this on my own. My parents have been a constant source of support, encouragement, and inspiration. My father, a wonderful physicist in his own right, fostered in me a love and interest for science at an early age. After a bout of bronchitis in fifth grade caused me to miss the lecture on atoms, I remember with vivid clarity sitting with my father at the kitchen table, explaining to me about protons, electrons, and neutrons. Many science fairs and high school classes did not diminish his patience for my curiosity, his endless knowledge on any topic (science or otherwise), and his willingness to make time for me whether for homework help (even if I did leave it for the last minute) or to have fun. My mother, at the opposite end of the academic spectrum, enlightened me in literature, reading, and writing, and to my astonishment, I found that I loved this as much as science. To this day, I still prefer reading as a pastime to anything else. She instilled in me, to my chagrin at the time, the rules of proper writing and the necessity of being v articulate, along with the importance of communication and good interpersonal skills. I now recognize the importance of these qualities, and even if I lack them sometimes, I am eternally thankful for her foresight. In addition to their academic contributions to my life, my parents also taught me the value of hard work, the importance of moral values and being honest, the necessity of fun and laughter, and the true meaning of unconditional love. The transformation from a childhood dream of studying astronomy into a reality occurred in the hands of my advisor and friend, Dr. Chris Clemens. When I first started working for him as an undergraduate, I had no background in astronomy, mediocre grades, and much to prove. But I had a willingness to learn and was eager to work. Chris agreed to take me as a graduate student after I finished my undergraduate degree, and by this time, I knew that he was someone that I worked well with professionally, liked personally, and admired greatly. Near the end of my first year of graduate school, we went one Tuesday evening to Miami Subs to discuss possible projects for my thesis. The question he asked himself – which I will no doubt ask myself at various points in the future – was: if I could work on anything, what would it be? He gave me two options: to continue on his work on pulsations in white dwarf stars or to determine if drifting subpulses in pulsars was evidence of pulsations. Despite his knowing very little about pulsars, and the likelihood of a difficult and possibly unsuccessful road ahead, we tackled this question, and this dissertation was born. I could not have asked for a better mentor than Chris. He has put as much work into this project as I have, and this dissertation is a product of both of our efforts. He has met my ignorance, my errors, and my frustrations with grace and patience. His enthusiasm is contagious and instead of being tired of pulsars and astronomy after six years of graduate school, I am just as eager as the day I started. He taught me the importance of thinking a logical argument through to its conclusion, playing the devil’s advocate, and having moral integrity in all parts of life, scientific or otherwise. On a more personal level, both he and his vi wife, Georgie, have welcomed me into their home and have adopted me into their family. The occasional feelings of homesickness and solitude that I experience from being away from my own family are usually eased by time spent with Chris, Georgie, and whichever of their wonderful children happen to be home at the time. I am eternally thankful to Chris and his family for everything they have done for me and words cannot express my gratitude. Along with Chris, I have had the pleasure of being the student of some fan- tastic professors. Chuck Evans, in addition to being a valuable advisor in regards to my research, has been a role model for me as a teacher. I have attended every class he taught and enjoyed them all thoroughly. I appreciate his well-organized lectures, his time and patience for his students, his clear explanations, and his devotion to teaching. He has inspired me to be a better student, and hopefully one day, a great teacher. Sean Washburn advised me both academically and personally during my undergraduate years. I am gratefully for his patience and time, and for seeing potential in me, even when I did not. Through this arduous journey of graduate school, I have relied on my friends for emotional support and to help me grow and mature in other areas not related to science. After living in Chapel Hill for 11 years, my friends have become my family. Even though I have told them repeatedly home much I love them and how important they are to me, it seems fitting that I should acknowledge them here, because without them, I would not have kept my sanity long enough to finish school. Two of my closest friends are in actuality, my family: my sisters, Laura and Emily. Despite time and distance, they know me better than anyone else and no explanations or excuses are ever necessary. Their constant love and support, and their ability to always make me smile, have been a great source of comfort. Sarah Honer has listened to all my triumphs, my disappointments, my struggles, and my joys, and through them all, she has always been full of wisdom and sound advice, and had faith in my abilities, even when I did not. Michelle Manges has supported my body as well as my mind, making sure I ate and slept vii as well constantly reminding me by example that hard work and perseverance lead to great rewards. I would also like to express my gratitude to Adam Brooks, Adam Crain, Erika Ceseare, John Daniels, Terri Exum, Rachel Garrison, Toby Helmstetter, Mark Kosiewski, Paul Marshall, Atsuko Negishi, Leslie Prochaska, Kimberlee Shaw, and Sara Williamson. Rachel Rosen. July 13, 2007 viii CONTENTS Page LIST OF TABLES ............................... xii LIST OF FIGURES .............................. xiii I. INTRODUCTION ........................... 1 1.1 Introduction ............................ 1 1.2 Direct Observations ........................ 3 1.3 Magnetic Field Strength ..................... 6 1.4 Emission Properties ........................ 8 1.5 The Standard Pulsar Model ................... 12 1.6 Average Pulse Shapes ....................... 19 1.7 Polarization Properties ...................... 23 1.8 Drifting Subpulses ......................... 29 1.9 Non-radial Oscillation Modes ................... 33 1.10 Scope of This Project ....................... 37 1.11 Plan of This Work ........................ 38 Chapter II. OBSERVATIONS OF NON-RADIAL PULSATIONS ........ 42 2.1 Introduction ............................ 42 2.2 Morphology of Non-radial Pulsations .............. 46 2.2.1 Fixed and Variable Pulse Structure ........... 46 2.2.2 Requirements of the Model ................ 53 2.3 Comparison to Observations ................... 56 2.3.1 Subpulse Phase Jumps .................. 56 ix 2.3.2 Pulse Components and Their Separations ........ 62 2.3.3 The Radio Frequency Dependence of Drifting Subpulses 65 2.3.4 Mode Changes and Nulls ................
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