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Stellar Radio Transients: from Brown Dwarfs to Pulsars and Beyond

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Stellar Radio Transients: from Brown Dwarfs to Pulsars and Beyond Stellar Radio Transients: From Brown Dwarfs to Pulsars And Beyond Vikram Ravi A thesis submitted for the degree of Bachelor of Science with Honours in Astronomy & Astrophysics at The Australian National University Research School of Astronomy & Astrophysics October 2009 ii iii Declaration This thesis is an account of research undertaken between February 2009 and October 2009 at The Research School of Astronomy & Astrophysics, The Aus- tralian National University, Canberra, Australia. Except where acknowledged in the customary manner, the material presented in this thesis is, to the best of my knowledge, original and has not been submitted in whole or in part towards a degree at any university. Vikram Ravi iv v Acknowledgements My honours year, and this thesis, has been among the most happily hectic experiences of my life. I will start with the formal/semi-formal thank you’s and finish with some shout-outs. First, a huge thank-you to my supervisors: Dayal Wickramasinghe for his guidance through (and away from) the sometimes crazy ideas involved in the thesis; Lilia Ferrario, for her constant interest and help with the basics of setting up an honours project; and finally George Hobbs. Thank you, George, for the drama, the (few) losses at ping-pong, the calls of incompetence, and the Ashes. But most of all, thank you (and now Sha) for always offering me a place at your house, for going along with the, sometimes silly, ideas I spout, and for being a good friend. To the Astronomy & Astrophysics Honours coordinator Helmut Jerjen, thank you so much for making the Honours year as simple as possible, and for organising the RSAA travel funds (and also for the fun times in class). I would also like to thank Baerbel Koribalski for making me an informal ATNF student. Next, I would like to acknowledge the Mathematical Sciences Institute at the ANU for offering me such conveniently located desk space, and great office mates who put up with my somewhat focused working habits. Thank you to everyone who helped and collaborated with me on the project: Dr. David Champion (ATNF/MPIfR), Dr. Mike Keith (ATNF), Prof. Ray Nor- ris (ATNF), Prof. Don Melrose (USyd), Dr. Dick Manchester (ATNF). Mr. Justin Bray (Adelaide), Dr. Simon Johnston (ATNF), Dr. Patrick Weltevrede (ATNF/JBO), Dr. Jamie Stevens (ATNF), Dr. Martin Cohen (UC Berkeley), Prof. Saul Adelman (Citadel), Prof. Joel Weisberg (Carleton), and Dr. Gyula Josza (ASTRON). There’s not enough room to say all I want about everyone individually, but, in any case, thanks, it’s been a grand learning experience and a great time. For the observing trips: thanks to Jamie Stevens, Robin Wark and Phil Ed- wards for putting up with my antics at Narrabri and for making the system work so well, and to my DAs Rajan Chhetri and Emil Lenc. Thank you David for the great drive, all the other fun at Sydney, and your constant presence on Skype. Also, thank you to Ettore Caretti and John Reynolds for the help with Parkes. Finally, Jonathan Khoo − thanks mate, it’s been a blast...mate. To my parents: thank you for everything Now for the shoutouts: Burgmann in general, for the fun and support; Will, it’s going to be amazing; Tom, for IB; Zemma, for astro and tea; Don, for being a mate; all my friends, for being there; and to the ANU and Berkeley, for the last four years. vi vii Preface The research that comprises this thesis has been an extraordinary experience, in that it involved a large collaboration of interested researchers from a variety of institutions, has led to more than 90 hours of telescope time and a publication being submitted to MNRAS, with a few more in preparation. The body of work described here was the result of collaboratons with and advice from a number of scientists, from the Australian National University (ANU), the Australia Telescope National Facility (ATNF), the University of Sydney (USyd), the University of Adelaide (Adelaide) and the Max Planck Institut f¨ur Radioas- tronomie (MPIfR). Included in this collaboration were my supervisors Prof. Dayal Wickramasinghe (ANU), Dr. Lilia Ferrario (ANU) and Dr. George Hobbs (ATNF), as well as collaborators and advisors Dr. David Champion (ATNF/MPIfR), Dr. Mike Keith (ATNF), Prof. Ray Norris (ATNF), Prof. Don Melrose (USyd), Dr. Dick Manchester (ATNF) and Mr. Justin Bray (Adelaide). A number of other people made useful and significant contributions: they are listed in the acknowl- edgements. The observational data that was used in this thesis partly resulted from obser- vations undertaken in October and November 2008, with the Australia Telescope Compact Array, before the start of my honours year. However, I found it necessary to obtain more data, some of which is included in this thesis. I was first author on proposals to the Westerbork Synthesis Radio Telescope (R09B/010 - 36 hours) and the Australia Telescope Compact Array (C2114 - 24 hours), and used a large amount of unallocated time at the Parkes Radio Telescope (∼ 20 hours) and some at the Australia Telescope Compact Array (∼ 10 hours). The data that was not processed in time for this thesis will be looked at shortly after. Finally, we, as a collaboration, have submitted a paper to the Monthly Notices of the Royal Astronomical Society (MNRAS) (Ravi et al. 2009, submitted), which consists of the work described in Chapter 3 of this thesis. Other papers, partic- ularly on the work described in Chapter 5, are being written, and we anticipate that the data that has not yet been processed will lead to exciting results as well. viii ix Abstract A variety of magnetic stars, from brown dwarfs to Ap stars to white dwarfs, have recently been observed to produce transient radio emission, sometimes peri- odic. These pulses and flares are, in general, 100% circularly polarised, and occur on timescales ranging between a few minutes and an hour. In all cases, the emis- sion is thought to be the result of a coherent mechanism and magnetospheric in origin, with the periodicities linked to the stellar rotation periods. In this thesis, I present observational and modelling studies of a select sample of previously detected transient emitters from each of the aforementioned stellar classes − the magnetic Ap star CU Virginis, the brown dwarfs DENIS 1048−3956 and LHS 3003 and the white dwarfs RX J0317−853 and PG 1031+234 − and con- sider them as defining examples of populations of stellar radio transient sources. I use the modelling to constrain the properties of the magnetospheres of these stars using observational data obtained at the Australia Telescope Compact Ar- ray. Twin pulses are detected from CU Virginis every rotation period: here, these are shown to be caused by electron-cyclotron maser emission. The CU Virginis magnetosphere and non-thermal electron population are modelled by fitting the quiescent lightcurves. I find that the quiescent radio spectrum of the flaring brown dwarf DENIS 1048−3956 obeys a negative power law, the first such result for a brown dwarf. This implies a power law non-thermal electron distribution similar to that of CU Virginis. No emission is detected from the white dwarfs, and these null results are used to suggest that white dwarf winds must closely resemble pulsar winds in their anisotropic electron pitch angle distributions. I also discuss unified characteristics of the emission models of CU Virginis and the brown dwarfs, with the aim of linking the modelling ideas to analogous pulsar phenomena. Finally, I discuss and analyse the populations of the transient emitters from both physical and emperical perspectives, in order to assess their detectability in upcoming radio surveys with next-generation telescopes. The study of the transient radio sky is poised to be among the most exciting and prolific areas in the future of radio astronomy. x xi Note on Units and Constants SI units are used throughout this thesis. The only major exception is in the case of radio flux density, where the more commonly used Jansky (Jy; 1 Jy = 10−26 Wm−2 Hz−1) is applied. The magnetic field strength (or magnetic flux den- sity) is always expressed in Teslas (T), rather than Gauss (G). For reference, 1 T =104 G. Also, parsecs (pc) are usually used as astronomical units of distance: 1 pc =3.08568 × 1016 m. Symbols for fundamental constants are not defined in the text, but are listed here: Fundamental unit of charge, or proton charge: qe. Electron mass: me. Mass of a hydrogen atom: mH. Speed of light in vacuum: c. Boltzmann’s constant: kB . Universal gravitational constant: G. Note that ‘e’ always represents the mathematical constant (Euler’s number). Also, commonly used symbols are: M: mass of a star. R: radius of a star. ν: frequency of electromagnetic radiation. xii CONTENTS CHAPTER 1 − Introduction and Motivation ............... 1 1.1Introduction............................... 1 1.1.1 Abriefhistoricaloverview................... 2 1.2Summaryofthisthesis......................... 4 CHAPTER 2 − Observational Techniques ................. 7 2.1Introduction............................... 7 2.2Measuresofradioemission....................... 9 2.3Radiointerferometry.......................... 10 2.3.1 Atwoelementinterferometer................. 10 2.3.2 Aperturesynthesisinterferometry............... 13 2.3.3 The real world: imaging, interference and data analysis . 13 CHAPTER 3 − Characterising Periodic Transient Radio Emission from CU Virginis ............................. 17 3.1Introduction............................... 17 3.2Observinganddataanalysis...................... 20 3.3Results.................................. 21 3.3.1 Propertiesoftheradiopulses................. 21 3.3.2 Quiescent emission from CU Vir at 20 and 13 cm . 22 3.3.3 The correlated flux variability of the pulses and quiescence . 31 3.3.4 A rotational anomaly in the pulse arrival times . 31 3.4AnECMmodelforthepulsedemission................ 35 3.5TheincidenceofCUVir-typeradiotransients............ 41 3.6Conclusions............................... 45 CHAPTER 4 − Modelling the CU Virginis Magnetosphere ...... 47 4.1Introduction............................... 47 4.2Descriptionofthenewmodel..................... 51 4.2.1 Thedeadzone.........................
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