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Supermassive Black Hole Binaries and Transient Radio Events: Studies in Pulsar Astronomy

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Supermassive Black Hole Binaries and Transient Radio Events: Studies in Pulsar Astronomy Supermassive Black Hole Binaries and Transient Radio Events: Studies in Pulsar Astronomy Sarah Burke Spolaor Presented in fulfillment of the requirements of the degree of Doctor of Philosophy 2011 Faculty of Information and Communication Technology Swinburne University i Abstract The field of pulsar astronomy encompasses a rich breadth of astrophysical topics. The research in this thesis contributes to two particular subjects of pulsar astronomy: gravitational wave science, and identifying celestial sources of pulsed radio emission. We first investigated the detection of supermassive black hole (SMBH) binaries, which are the brightest expected source of gravitational waves for pulsar timing. We consid- ered whether two electromagnetic SMBH tracers, velocity-resolved emission lines in active nuclei, and radio galactic nuclei with spatially-resolved, flat-spectrum cores, can reveal systems emitting gravitational waves in the pulsar timing band. We found that there are systems which may in principle be simultaneously detectable by both an electromagnetic signature and gravitational emission, however the probability of actually identifying such a system is low (they will represent 1% of a randomly selected galactic nucleus sample). ≪ This study accents the fact that electromagnetic indicators may be used to explore binary populations down to the “stalling radii” at which binary inspiral evolution may stall indef- initely at radii exceeding those which produce gravitational radiation in the pulsar timing band. We then performed a search for binary SMBH holes in archival Very Long Base- line Interferometry data for 3114 radio-luminous active galactic nuclei. One source was detected as a double nucleus. This result is interpreted in terms of post-merger timescales for SMBH centralisation, implications for “stalling”, and the relationship of radio activity in nuclei to mergers. Our analysis suggested that binary pair evolution of SMBHs (both 8 of masses >10 M⊙) spends less than 500 Myr in progression from the merging of galactic stellar cores to within the purported stalling radius for SMBH pairs, giving no evidence for an excess of stalled binary systems at small separations. Circumstantial evidence showed that the relative state of radio emission between paired SMBHs is correlated within orbital separations of 2.5 kpc. We then searched for transient radio events in two archival pulsar surveys, and in the new High Time Resolution Universe (HTRU) Survey. We present the methodology employed for these searches, noting the novel addition of methods for single-event recog- nition, automatic interference mitigation, and data inspection. 27 new neutron stars were discovered. We discuss the relationship between “rotating radio transient” (RRAT) and pulsar populations, finding that the Galactic z-distribution of RRATs closely resembles the distribution of pulsars, and where measurable, RRAT pulse widths are similar to in- dividual pulses from pulsars of similar period, implying a similar beaming fraction. We postulate that many RRATs may simply represent a tail of extreme-nulling pulsars that are “on” for less than a pulse period; this is supported by the fact that nulling pulsars and single-pulse discoveries exhibit a continuous distribution across null/activity timescales and nulling fractions. We found a drop-off in objects with emissivity cycles longer than ii 300 seconds at intermediate and low nulling fractions which is not readily explained by ∼ selection effects. The HTRU deep low-latitude survey (70- min. pointings at galactic lati- tudes b < 3.5◦ and longitudes 80◦ <l< 30◦) will be capable of exploring whether this | | − deficit is natural or an effect of selection. The intriguing object PSR J0941–39 may rep- resent an evolutionary link between nulling populations; discovered as an sparsely-pulsing RRAT, in follow-up observations it often appeared as a bright (10 mJy) pulsar with a low nulling fraction. It is therefore apparent that a neutron star can oscillate between nulling levels, much like mode-changing pulsars. Crucially, the RRAT and pulsar-mode emission sites are coincident, implying that the two emission mechanisms are linked. We estimate that the full HTRU survey will roughly quadruple the known deep-nulling pulsar popu- lation, allowing statistical studies to be made of extreme-nulling populations. HTRU’s low-latitude survey will explore the neutron star population with null lengths lasting up to several hours. We lastly reported the discovery of 16 pulses, the bulk of which exhibit a frequency sweep with a shape and magnitude resembling the “Lorimer Burst” (Lorimer et al. 2007), which three years ago was reported as a solitary radio burst that was thought to be the first discovery of a rare, impulsive event of unknown extragalactic origin. However, the new events were of clearly terrestrial origin, with properties unlike any known sources of terrestrial broad-band radio emission. The new detections cast doubt on the extragalactic interpretation of the original burst, and call for further sophistication in radio-pulse sur- vey techniques to identify the origin of the anomalous terrestrial signals and definitively distinguish future extragalactic pulse detections from local signals. The ambiguous origin of these seemingly dispersed, swept-frequency signals suggest that radio-pulse searches us- ing multiple detectors will be the only experiments able to provide definitive information about the origin of new swept-frequency radio burst detections. Finally, we summarise our major findings and suggest future work which would expand on the work in this thesis. iii Acknowledgements Many good people have contributed to the completion of this thesis and to the unique period of personal and professional growth that the past few years have represented. I owe a big round of thanks to four great supervisors, Matthew Bailes, Dick Manchester, Simon Johnston, and David Barnes, who each in their way has constantly conveyed an excitement for science and the scientific process that I strive to replicate. I appreciate the scientific and personal support offered by all of them at various times, as well as the space and encouragement they gave to explore my own ideas (and patient support through those “bold claims,” as Matthew would say, that came from left-field ideas!). All of these elements were essential in navigating successfully through this period, and my closer supervisors, Dick, Simon, and Matthew, all made crucial contributions to my education and to developing the scientific ideas explored by this thesis. I would like to particularly thank my primary supervisor, Matthew, for being an important role model for balancing the importance of family with a strong astronomical career. I have a great respect for the leadership he has shown over the course of my PhD. I am tremendously grateful to have had the chance to interact closely with both Ron Ekers and George Hobbs. Ron first introduced me to the Australian scientific community and has always been a valuable sounding board for new ideas, a like-minded brainstorm companion, a constant source of encouragement and way-out-of-the-box thinking, a men- tor, and a good friend. George has likewise been an indespensable part of my candidature, often providing a house filled with good music, food, films, and strange board games, and always providing insightful discussions on pulsar and extragalactic astronomy. I must ex- press appreciation for those who laid the seeds of my scientific career early on through dedicated and creative teaching. This includes Doc Stoneback, Steve Boughn, Walter Smith, and the person who first introduced me to the world of scientific research and to the Universe through cosmology, Prof. Bruce Partridge. A very general thanks go to the staff and scientists at Swinburne Ctr. for Astrophysics, ATNF Marsfield, and Parkes Telescope: rest assured all of you helped make each of these places welcoming, engaging institutions. I hold special thanks for a few: the post-docs at Swinburne who let me wander into their offices to either chat idly, snatch books off their shelves, nag them about code bugs, or deliberate about a range of topics—Willem, Ramesh, Andrew, and Jonathon, thanks for all that. It has been a pleasure to learn from such knowledgeable scientists, and in many cases, have sources of light relief in S. Amy, B. Koribalski, J. Reynolds, G. Carrad, T. Tzioumis, R. Hollow, C. Cliff, J. Caswell, T. Cornwell, R. Smits, R. Jenet, Shirley and Janette, J. Khoo, DJ Champion, E. Sadler, A. Graham, T. Murphy, M. Kesteven, M. Keith, P. Weltevrede, and D. Schnitzeler. Big thanks are owed to John Sarkissian for the countless late-night chats about everything in iv the world, and the tireless help with observing. My fellow students and friends have been necessary stress relievers along the way, particularly my “not-really-band-mates” Joris, Carlos, Chris, and Max, as well as many others in and out of work like Lina, Stefan, Evelyn, Adam, Juan, Adrian, Francesco, Caterina, Andy, Ben, Amanda and Jonathon, Marc, Glenn, Nigel, Nick Jones, Minnie, Katherine Newton-McGee, Peter and Sofie. I hope to see you all again many times in the coming years of our careers. Last and anything but least, I acknowledge the invaluable and ongoing support of my family. From my beginning, my parents have taught and encouraged me to pursue every interest in and out of this world; they, my siblings Hannah and Ryan, JPL, and my extended family have represented a wonderful source of many different kinds of wisdom and friendship in my
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