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Constraining the Sources of Ultra-High Energy Cosmic Rays with Multi-Messenger Data

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Constraining the Sources of Ultra-High Energy Cosmic Rays with Multi-Messenger Data UNIVERSITY COLLEGE LONDON Faculty of Mathematics and Physical Sciences Department of Physics & Astronomy CONSTRAINING THE SOURCES OF ULTRA-HIGH ENERGY COSMIC RAYS WITH MULTI-MESSENGER DATA Thesis submitted for the Degree of Doctor of Philosophy of the University of London by Foteini Oikonomou Supervisors: Examiners: Prof. Ofer Lahav Prof. Jim Hinton Prof. David Waters Dr Ryan Nichol 15th March 2014 To Dimitra and Kostas I, Foteini Oikonomou, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. All the research has been conducted in collaboration with my supervisors Ofer Lahav and David Waters as well as Filipe Abdalla, Amy Connolly and Kumiko Kotera. The work presented in chapter 4 has been published in The Journal of Cosmology and Astroparticle Physics JCAP05(2013)015 (Oikonomou et al. 2013). The work presented in chapter 5 has been submitted for publication to Astronomy & Astrophysics (Oikonomou et al. 2014b). The material in chapter 6 is presently in preparation for submission to The Journal of Cosmology and Astroparticle Physics (Oikonomou et al. 2014a). Abstract Ultra-high energy cosmic rays (UHECRs) are cosmic rays with energy exceeding 1018 electronvolts. The sources of these particles remain unknown despite decades of research. This thesis presents a series of studies aimed at constraining the sources of UHECRs both directly by studying their observed arrival directions and indirectly through their expected secondary gamma-ray signatures. An analysis of the arrival direction distribution of the highest energy cosmic rays detected at the Pierre Auger Observatory is presented. The aim of the study was to determine whether the arrival directions of observed UHECRs follow the distribution of nearby extragalactic sources, which is expected if UHECRs are light nuclei of extragalactic origin. A departure from isotropy at the 95% level is observed but no clear correlation with the extragalactic matter distribution is found. The sensitivity of upcoming UHECR experiments, with an order of magnitude higher annual exposure than current experiments, to the expected UHECR anisotropy has been investigated through simulations. It is shown, that with five years of data from such a detector an anisotropy should be detectable at the 99% level as long as the composition is proton dominated. In a scenario where the UHECR source distribution is strongly clustered, similar to the distribution of galaxy clusters, an anisotropy at the 99:9% level is expected even if the fraction of protons at the highest energies is as low as 30%. Constraints on the sources of UHECRs may also come from the secondary particles that UHECRs produce during their propagation. A study of the expected secondary gamma-ray signatures of UHECR accelerators embedded in magnetised environments is presented. The secondary gamma-ray emission expected in this model is shown to be consistent with the spectra of a number of extreme blazars. It is shown that this model is 3 more robust to variations of the overall extragalactic magnetic field strength than other proposed scenarios, which is appealing in view of the large uncertainty surrounding the strength and configuration of extragalactic magnetic fields. 4 Acknowledgements During the years over which this research was conducted, I have had the fortune of working alongside a number of exceptional people that have generously contributed to this work. First I'd like to express my gratitude to Ofer Lahav and David Waters. `Are you a happy customer?' Ofer used to ask every once in a while, when our meeting was about to end; for the always encouraging and supportive mentoring, thank you Ofer. Thank you Dave for always providing wisdom and invaluable support, for making time for all of my questions and always providing me with excellent feedback and advice. I am deeply grateful to Kumiko Kotera, the UHECR expert, who co-supervised me during the final year of this work, for the immense fortune of her arrival in London and for the chance I've had to work with her since. For the tireless support and advice and tireless encouragement, for the incredible opportunities you've given me and for so generously sharing your insight, thank you Kumiko! It has been a real pleasure working with you. A very big thank you to Filipe Abdalla, who always supported my progress during this PhD. For the support and honest advice, for always helping out with whatever the issue: science, coding, broken cluster queues, nerves, life. You have been a vital source of support, thank you Fil. I am very grateful to Amy Connolly, who co-supervised me during the first year of this work, before she had to leave for the US. Thank you for your enthusiastic guidance and encouragement Amy. It was wonderful working with you and I am very happy that you'll be right next door in the US for the next few years. Thank you to my incredible collaborators, Kohta Murase and Eli Waxman; it has been a privilege working with you. Thank you also to Shaun Thomas for the help at the early stage of this thesis, when we overlapped at UCL. I would also like to thank 5 the other scientists that I've had the fortune to meet and discuss the mysteries of the non-thermal Universe with. In particular, I acknowledge helpful discussions with Markus Ahlers, John Beacom, Pirin Erdogdu, Eichiro Komatsu, Daniel Mortlock, Angela Olinto, Etienne Parizot, Hiranya Peiris, Dmitri Semikoz, Joe Silk and Hajime Takami. Thank you also to Glen Cowan and Matthew Wing for two very useful and interesting sets of lectures at the very beginning of this PhD. My research in UCL was supported by the UCL Institute of Origins. I also thank the Institut Lagrange de Paris and the Institut d'Astrophysique de Paris for the three months of kind hospitality in Paris and all the staff and students at the IAP for the very warm and welcoming environment. To the wonderful people in the UCL Astrophysics Group that I've shared these years with: it has been amazing working near you in this uniquely pleasant and friendly atmo- sphere and I will miss all of you very very much! Thank you for all the great moments, your support and the general craziness. Finally thank you to Dimitra and Kostas, my parents. Thank you for your uncondi- tional support and tireless encouragement. The great thing about science is that you can get it wrong over and over again because what you're after - call it truth or understanding - waits patiently for you. Ultimately, you'll find the answer because it doesn't change. -Dudley Herschbach, shared 1986 Nobel Prize in Chemistry Contents Table of Contents 7 List of Figures 11 List of Tables 15 1 Ultra-high energy cosmic rays 17 1.1 Cosmic ray energy spectrum . 17 1.2 Detection . 19 1.2.1 Early experiments . 20 1.2.2 Current and upcoming experiments . 20 1.3 Sources and acceleration of UHECRs . 22 1.3.1 Acceleration . 24 1.3.2 Candidate sources . 26 1.4 Mass composition . 28 2 Interactions of UHECRs in the intergalactic medium and secondary products 31 2.1 Cosmic background radiation fields . 32 2.1.1 Cosmic microwave background . 32 2.1.2 Extragalactic background light . 33 2.1.3 Universal radio background . 34 2.2 Proton propagation and interactions . 35 2.2.1 Pair production . 36 2.2.2 Pion photoproduction . 37 7 Contents 8 2.2.3 Redshift energy losses . 38 2.2.4 Neutrons and nuclei . 39 2.2.5 Secondary products of UHECRs . 40 2.3 Gamma-rays . 42 2.3.1 Instruments for the detection of gamma-rays . 43 2.3.2 Intergalactic electromagnetic cascades . 46 3 Propagation of UHECRs in a magnetised universe and anisotropies 53 3.1 Magnetic fields . 53 3.1.1 Measurement techniques and observations . 54 3.1.2 galactic magnetic field . 56 3.1.3 Extragalactic magnetic fields . 57 3.1.4 Numerical simulations . 59 3.1.5 UHECR deflections . 60 3.2 UHECR anisotropies . 63 4 Search for correlation of UHECRs with the local galaxy distribution 67 4.1 Introduction . 67 4.2 Galaxy surveys . 68 4.3 Auger exposure . 70 4.4 Model of UHECR source distribution . 70 4.4.1 Source density . 71 4.4.2 UHECR energy losses during propagation . 73 4.4.3 Galaxy survey completeness . 74 4.4.4 Galaxy peculiar velocities . 77 4.4.5 Magnetic fields . 78 4.5 Statistical approach . 79 4.6 Results . 81 4.6.1 Cross-correlation of UHECRs and nearby LSS . 82 4.6.2 Cross-correlation in equal predicted flux radial shells . 89 4.6.3 Magnetic deflections . 94 4.6.4 Systematic uncertainties . 95 4.7 Discussion . 96 Contents 9 5 Gamma-ray spectra of Blazars 101 5.1 Introduction . 102 5.1.1 Blazars . 102 5.1.2 Leptonic origin . 103 5.1.3 Hadronic origin . 105 5.1.4 The role of magnetic fields . 109 5.2 Methodology . 110 5.2.1 Selection of sources . 110 5.2.2 Magnetic field model . 112 5.2.3 Simulations . 114 5.3 Robustness of synchrotron signal with application to specific sources . 117 5.3.1 1ES 0229+200 . 119 5.3.2 Other sources . 120 5.3.3 UHE photons . 121 5.4 Discussion . 127 6 Simulations for a next-generation UHECR observatory 135 6.1 Introduction . 135 6.2 Model building . 139 6.2.1 Bias prescription of UHECR source clustering . 139 6.2.2 UHECR intensity maps and simulations . 141 6.3 Results . 148 6.3.1 Isotropic fraction . 148 6.3.2 Sensitivity to the UHECR source density .
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