The transient radio sky observed with the Parkes radio telescope
Emily Brook Petroff
Presented in fulfillment of the requirements of the degree of Doctor of Philosophy
February, 2016
Faculty of Science, Engineering, and Technology Swinburne University
i
Toute la sagesse humaine sera dans ces deux mots: attendre et espérer.
All of human wisdom is summed up in these two words: wait and hope. —The Count of Monte Cristo, Aléxandre Dumas ii Abstract This thesis focuses on the study of time-variable phenomena relating to pulsars and fast radio bursts (FRBs). Pulsars are rapidly rotating neutron stars that produce radio emission at their magnetic poles and are observed throughout the Galaxy. The source of FRBs remains a mystery – their high dispersion measures may imply an extragalactic and possibly cosmological origin; however, their progenitor sources and distances have yet to be verified. We first present the results of a 6-year study of 168 young pulsars to search for changes in the electron density along the line of sight through temporal variations in the pulsar dispersion measure. Only four pulsars exhibited detectable variations over the period of the study; it is argued that these variations are due to the movement of ionized material local to the pulsar. Our upper limits on DM variations in the other pulsars are consistent with the scattering predicted by current models of turbulence in the free electron density along these lines of sight through the interstellar medium (ISM). We also present new results of a search for single pulses from Fast Radio Bursts (FRBs), including a full analysis of the data from the High Time Resolution Universe (HTRU) sur- vey at intermediate and high Galactic latitudes. No new FRBs were found in the intermedi- ate latitude survey and five new bursts were found at high latitudes. The unexpected dearth at intermediate latitudes is found to be inconsistent (with 99% confidence) with an isotropic distribution using previously published rates. From the 9 FRBs at high latitude an all-sky rate can be derived with the largest sample of FRBs to date of R (F 0.6Jyms)> FRB +4.2 3 1 1 +3.2 3 5.7 2.7 10 (95%) FRBs sky day ,orRFRB(F 2Jyms)=2.5 1.6 10 (95%) ⇥ ⇥ 1 1 FRBs sky day . Although lower than previously published estimates, these rates are still inconsistent with results at intermediate latitudes. The FRBs from the HTRU survey were re-observed in a detailed follow-up campaign to place limits on possible repeating sources. No repetition was detected from any of the FRBs; however, a new burst was discovered in real-time during these observations: FRB 140514. Polarization information for the burst was preserved by the real-time search pipeline at Parkes, and the burst was found to be 21 7% circularly polarized. Multi- ± wavelength follow-up was also performed and no variability was detected in the field related to the burst. These observations placed the first limits on an afterglow. The full search of the HTRU intermediate and high latitude survey also resulted in the discovery of 50 “perytons”, seemingly dispersed terrestrial signals of unknown origin. We ⇠ present conclusive evidence that perytons are caused by on-site microwave ovens producing sparks in a non-linear shut down phase. Based on the properties of the perytons and FRBs iii we conclude that the observed FRBs cannot be produced by the microwave ovens on site and an astrophysical origin remains highly favored. iv v Acknowledgments
It’s difficult to find the words to fully express my gratitude to all the people who deserve it. There were a lot of people who helped make this happen from when I decided I wanted a PhD when I was twelve. Early thanks are due to Bill Lamb, Rosa Hemphill, Catherine Garland, and all the people at Oregon Episcopal School who let me do my own thing as well as to Cindy Blaha and everyone in the Carleton College physics department. I owe my love of pulsars and my introduction to radio astronomy to the one and only Joel Weisberg. Thank you, Joel, for working patiently with me for over three years at Carleton and for introducing me to Australia and to Parkes. I would never be here writing this if it weren’t for you. Before going any further enormous and heartfelt thanks go to my amazing supervisors: Willem van Straten, Simon Johnston, and Matthew Bailes. Willem – thank you for pro- viding support, encouragement, counsel, and wisdom over the last three years. Your good judgment has helped me navigate the world of research and made me a better person. Simon – thank you for giving me one of the greatest gifts a supervisor ever could, the space and freedom to speak my mind. I’ve been learning from you since my first trip to the ATNF back in 2009 and it has been an honour. Matthew – thank you for showing me what quality research looks like and giving me something to aspire to. Wisdom doesn’t just come from supervisors. I’m grateful to the friends who I have had along the way who have taught me in one way or another. Jonathan, thank you for the book recommendations, the debates, the coding lessons, the chocolate, and saving me from my terrible cooking. Dave, thank you for your endless positivity, your encyclopedic movie knowledge, and for basically carrying our team at trivia (bonus points for your specific Star Trek theme song knowledge). Jonathan and Dave, thanks for making Scotch Saturday happen. Tyler, thank you for being a friend through thick and thin, laughing with me, and showing me how a good thesis is done. Rebecca, thank you for being you, for having an infectious amount of happiness about the world around you and for picking me up whenever I got down. Evan, thank you for your counsel and for being willing to listen when asked and give advice when greatly needed. Ewan, thank you for always encouraging me to do my best, push outside my comfort zone, not be afraid, and for reminding me that things will turn out OK. Enormous thanks to Andrew for answering my questions – from the embarrassingly simple to the ridiculously technical. And a great big thank you to the rest of the pulsar group for everything along the way: Paul, Stefan, Pablo, Fabian, Manisha, Shivani, Vivek, vi
Vikram, Damien and Ian. I would also like to acknowledge all the mentors who have given me advice, support, direction, and encouragement in one way or another during these past three years (in no particular order): Katie Mack, Bryan Gaensler, Brian Schmidt, Elaine Sadler, Tamara Davis, Naomi McClure-Griffiths, Michael Childress, Fang Yuan, Chris Blake, Alan Duffy, Virginia Kilborn, Karl Glazebrook, Jeff Cooke, Michael Murphy, Sarah Maddison, Tyler Pritchard, George Hobbs, Antonia Rowlinson, Dick Manchester, Keith Bannister, Tara Murphy, Kate Gunn, Sue Lester, Elizabeth Thackray, John Sarkissian, John Reynolds, Phil Edwards, Brett Presig, Mal Smith, JP Macquart, Cath Trott, Ron Ekers, Ben Stap- pers, Andrea Possenti, Jasson Hessels, Aris Karastergiou, Sarah Burke-Spolaor, David Champion, and Michael Kramer. Swinburne, CSIRO, CAASTRO, and Parkes have been supportive and fantastic envi- ronments in which to work and I wish I could thank the entire community individually. You’ve all encouraged, motivated, inspired, and impressed me during my time here. Believe it or not I would also like to thank those out there who caused me pain, both mental and physical – the ones who said I’d never make it. Proving you wrong has been one of the most rewarding experiences of my life. Natasha, thank you for being my best friend for 10 years and reminding me every time I come back why Portland is home. Norma, thank you for being my biffle, my other Wonder Twin, and my robot unicorn. Special thanks to Hallie, Becca, Bizou, Tug, and Sanny, even though you’re not people and you’ll never read this. Pakey, thank you for listening; I aspire to be as good of a person as you. Erica, just thank you. Thank you for being my sister. I’d especially like to thank my mom and dad for supporting me from the very beginning, giving me all a daughter could want, and always being on the other side of the door, or the phone, or the Skype call when I wanted to quit and said I couldn’t do it, saying “Yes you can”. I love you both more than anything. vii viii Declaration
The work presented in this thesis has been carried out in the Centre for Astrophysics & Supercomputing at Swinburne University of Technology (Hawthorn, VIC), the Australia Telescope National Facility/CSIRO Astronomy and Space Sciences (Marsfield, NSW) and the CSIRO Parkes radio telescope (Parkes, NSW) between 2012 and 2015. This thesis contains no material that has been accepted for the award of any other degree or diploma. To the best of my knowledge, this thesis contains no material previously published or written by another author, except where due reference is made in the text of the thesis. The content of the chapters listed below has appeared in refereed journals. Minor alterations have been made to the published papers in order to maintain argument continuity and consistency of spelling and style.
Chapter 3 has been published in Monthly Notices of the Royal Astronomical Society, • 435, 1610, 2013, as “Dispersion measure variations in a sample of 168 pulsars”, au- thored by E. Petroff, M. J. Keith, S. Johnston, W. van Straten, and R. M. Shannon.
Chapter 4 has been published in Astrophysical Journal Letters, 789,L26,2014,as • “An absence of fast radio bursts at intermediate galactic latitudes”, authored by E. Petroff, W. van Straten, S. Johnston, M. Bailes, E. D. Barr, S. D. Bates, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, D. Champion, P. Coster, C. Flynn, E. F. Keane, M. J. Keith, M. Kramer, L. Levin, C. Ng, A. Possenti, B. W. Stappers, C. Tiburzi, and D. Thornton.
Chapter 5 has been published in Monthly Notices of the Royal Astronomical Society, • 451, 3933, 2015, as “Identifying the source of perytons at the Parkes radio telescope”, authored by E. Petroff, E. F. Keane, E. D. Barr, J. E. Reynolds, J. Sarkissian, P. G. Edwards, J. Stevens, C. Brem, A. Jameson, S. Burke-Spolaor, S. Johnston, N. D. R. Bhat, P. Chandra, S. Kudale, S. Bhandari.
Chapter 6 (excluding Section 6.2) has been published in Monthly Notices of the • Royal Astronomical Society, 454. 457, 2015, as “A survey of FRB fields: Limits on repeatability”, authored by E. Petroff, S. Johnston, E. F. Keane, W. van Straten, M. Bailes, E. D. Barr, B. R. Barsdell, S. Burke-Spolaor, M. Caleb, D. J. Champion, C. Flynn, A. Jameson, M. Kramer, C. Ng, A. Possenti, B. W. Stappers.
Chapter 7 has been published in Monthly Notices of the Royal Astronomical Society, • 477, 246, 2015, as “A real-time fast radio burst: polarization and multi-wavelength ix follow-up”, authored by E. Petroff, M. Bailes, E. D. Barr, B. R. Barsdell, N. D. R. Bhat, F. Bian, S. Burke-Spolaor, M. Caleb, D. Champion, P. Chandra, G. Da Costa, C. Delvaux, C. Flynn, N. Gehrels, J. Greiner, A. Jameson, S. Johnston, M. M. Kasliwal, E. F. Keane, S. Keller, J. Kocz, M. Kramer, G. Leloudas, D. Malesani, J. S. Mulchaey, C. Ng, E. O. Ofek, D. A. Perley, A. Possenti, B. P. Schmidt, Yue Shen, B. W. Stappers, P. Tisserand, W. van Straten, C. Wolf.
Emily Petroff Melbourne, Victoria, Australia 2015 x xi
For my family
Contents
Abstract i
Acknowledgements iv
Declaration vii
List of Figures xvi
List of Tables xviii
1Introduction 1 1.1 Transient radio astronomy ...... 1 1.2 Pulsars ...... 5 1.2.1 Dispersion ...... 8 1.2.2 Scattering ...... 10 1.2.3 Interstellar scintillation ...... 12 1.2.4 Faraday rotation ...... 13 1.2.5 Pulsar searches and rotating radio transients ...... 15 1.3 Fast Radio Bursts ...... 16 1.3.1 Distances, energies, and brightness temperature ...... 18 1.3.2 FRB rates and progenitor theories ...... 20 1.3.3 Pulse propagation ...... 23 1.3.4 Perytons ...... 24 1.4 Thesis outline ...... 25
2TechnologicalIntroduction 27 2.1 Data acquisition ...... 27 2.1.1 The Parkes radio telescope ...... 29 2.2 Detecting single pulses ...... 30 2.2.1 The heimdall pipeline ...... 33 2.2.2 Real-time searches ...... 34 2.3 The High Time Resolution Universe survey ...... 38
3Dispersionmeasurevariationsofyoungpulsars 41 3.1 Introduction ...... 41 3.2 Observations ...... 44
xiii xiv Contents
3.3 Analysis ...... 44 3.4 Results - Detections ...... 46 3.4.1 PSR J0835–4510 ...... 47 3.4.2 PSR J0908–4913 ...... 48 3.4.3 PSR J1824–1945 ...... 50 3.4.4 PSR J1833–0827 ...... 51 3.5 Results - Upper limits ...... 53 3.6 Conclusions ...... 57
4 An Absence of Fast Radio Bursts at Intermediate Latitudes 59 4.1 Introduction ...... 59 4.2 Analysis and Results ...... 60 4.3 Discussion ...... 62 4.3.1 Dispersion in the ISM ...... 62 4.3.2 Scattering in the ISM ...... 63 4.3.3 Sky Temperature ...... 64 4.3.4 Scintillation ...... 64 4.3.5 Sensitivity Map ...... 65 4.3.6 Summary ...... 66 4.4 Conclusions ...... 67
5Identifyingthesourceofperytons 69 5.1 Introduction ...... 69 5.2 Observations ...... 70 5.3 Results ...... 72 5.3.1 Three perytons ...... 72 5.3.2 Prevalence of 2.3 2.5 GHz signals at Parkes ...... 74 5.3.3 Archival perytons ...... 74 5.3.4 Generating perytons ...... 77 5.3.5 The Peryton Cluster of 1998 June 23 ...... 79 5.4 Discussion ...... 80 5.5 Relevance to FRBs ...... 81 5.5.1 Differences in observed properties ...... 81 5.5.2 What is FRB 010724? ...... 82 5.5.3 Deciphering new transient events ...... 82 5.6 Conclusions ...... 84 Contents xv
6 Discovery and follow-up of FRBs at high latitudes 85 6.1 Introduction ...... 85 6.2 Search of the HTRU high latitude survey ...... 87 6.2.1 Burst properties ...... 88 6.2.2 Updated FRB rates and Latitude Distribution ...... 91 6.3 Parkes follow-up ...... 92 6.4 Data processing ...... 93 6.5 Follow-up Results ...... 95 6.6 Discussion ...... 96 6.6.1 Total time ...... 96 6.6.2 Multi-day observations of FRB 090625 ...... 96 6.6.3 Follow-up of FRB 140514 ...... 97 6.7 Conclusions ...... 99
7Areal-timefastradioburst 101 7.1 Introduction ...... 101 7.2 Real-Time Transient Pipeline ...... 102 7.3 Parkes real-time detection of FRB 140514 ...... 103 7.4 FRB Follow-up at Other Telescopes ...... 108 7.4.1 Parkes Radio Telescope ...... 108 7.4.2 Australia Telescope Compact Array ...... 108 7.4.3 Giant Metrewave Radio Telescope ...... 109 7.4.4 Swift X-Ray Telescope ...... 109 7.4.5 Gamma-Ray Burst Optical/Near-Infrared Detector ...... 110 7.4.6 Swope Telescope ...... 110 7.4.7 Palomar Transient Factory ...... 110 7.4.8 Magellan Telescope ...... 111 7.4.9 SkyMapper ...... 111 7.4.10 Effelsberg Radio Telescope ...... 111 7.4.11 Keck Spectroscopy ...... 112 7.4.12 Nordic Optical Telescope Spectroscopy ...... 112 7.5 Interpretation and Discussion ...... 112 7.5.1 Polarization ...... 112 7.5.2 Possible connection with FRB 110220 ...... 116 7.5.3 Limits on a varying counterpart ...... 117 7.5.4 Host galaxies ...... 118 xvi Contents
7.6 Conclusions ...... 118
8 Conclusions 121 8.1 Major Findings of the Thesis ...... 121 8.2 Future Directions ...... 123
Bibliography 138
Appendices
AGlossary 139
B Derivation of Bayesian probabilities 141 B.1 Derivation ...... 141 B.2 Justification ...... 142 List of Figures
1.1 The radio transient parameter space ...... 6 1.2 The Galactic distribution of radio pulsars ...... 8 1.3 Spectrum of a radio pulse from a pulsar ...... 9 1.4 Kolmogorov power law of interestellar turbulence ...... 11 1.5 A cartoon of the scatter broadening of a pulse by a thin screen of plasma . . 12 1.6 Spectrum and pulse profile of FRB 010724 (the Lorimer Burst) ...... 17 1.7 Spectrum of a peryton signal detected at the Parkes radio telescope . . . . . 25
2.1 The Parkes multibeam receiver ...... 29 2.2 Recovered signal-to-noise ratio of a pulse using different single pulse search codes...... 32 2.3 Overview plot of heimdall outputs for a single pointing ...... 35 2.4 The heimdall processing pipeline for filterbank data ...... 36
3.1 DM measurements of PSR J0835 4510 over 2000 days ...... 47 3.2 DM measurements of PSR J0835 4510 since 1969 ...... 49 3.3 DM measurements of PSR J0908 4913 ...... 50 3.4 DM measurements of PSR J1824 1945 ...... 51 3.5 DM measurements of PSR J1833 0827 ...... 52 3.6 Upper limits on dDM/dt for pulsars with no detected DM variations . . . . 53
4.1 Galactic effects on a simulated FRB pulse with strong scattering ...... 66 4.2 Galactic effects on a simulated FRB pulse with no scattering ...... 67
5.1 The time–frequency structure of the three January perytons ...... 73 5.2 RFI monitor spectra for the three January perytons ...... 75 5.3 Data from the Parkes and ATCA RFI monitors at the times of the perytons 76 5.4 Histogram of narrow RFI spikes detected at Parkes ...... 76 5.5 A bright peryton detected during tests of the Parkes microwave ovens . . . 78 5.6 Azimuth and elevation positions with direct line of sight between Parkes receiver and the Woolshed microwave oven ...... 79 5.7 FRB and peryton distributions in time of day and DM ...... 83
6.1 Pulse profiles of the 5 new FRBs from the high latitude survey ...... 90 6.2 Probability of detection for repeating progenitors for FRB 090625 ...... 98
xvii xviii List of Figures
7.1 Spectrum and pulse profile of FRB 140514 ...... 106 7.2 Full polarization profile of FRB 140514 ...... 107 7.3 FRB 140514 multi-wavelength couterpart magnitude limits from follow-up observations ...... 119
8.1 All-sky distribution of the known FRBs ...... 124 List of Tables
2.1 Properties of central, inner, and outer rings of the 21-cm multibeam receiver 30 2.2 Survey regions of the low, intermediate, and high latitude components of HTRU South ...... 39
3.1 Pulsars with DM variations over 6 years above 3 levels ...... 46 3.2 Measurements of DM and dDM/dt for all pulsars in the Fermi sample . . . 55
5.1 Properties of the perytons from 2015 January ...... 72
6.1 Repeating progenitor models for FRBs and their timescales ...... 87 6.2 Observed properties of the 9 FRBs from the HTRU high latitude survey . . 89 6.3 Derived cosmological properties of the 9 FRBs from the HTRU high latitude survey ...... 89 6.4 Total hours of follow-up observations for 8 FRBs from HTRU ...... 94 6.5 Summary of targeted observations for FRB 090625 ...... 97
7.1 Observed properties of FRB 140514...... 105 7.2 Derived cosmological properties of FRB 140514 ...... 105 7.3 Follow-up observations conducted at 12 telescopes for FRB 140514 . . . . . 113
xix
1 Introduction
Within the past decade it has become possible to observe time-variable radio sources with unprecedented sensitivity at high time and frequency resolution thanks, in part, to advances in radio astronomical instrumentation and signal processing. The experimental methods of time-variable radio astronomy, such as observations of pulsars and radio transients, provide a window into events and processes not observable through other techniques. In this chapter we describe different types of transient radio sources, some of which will be discussed in-depth in this work, and give some examples of their unique uses as probes of the ionized baryons in the near-vacuum of space. The first section of this chapter details the history of transient radio astronomy and different types of radio transients. The second section discusses the discovery of pulsars and their use as probes of the interstellar medium. The third section introduces the recently discovered fast radio bursts and their exciting potential as probes of the intergalactic medium over cosmological distances.
1.1 Transient radio astronomy
An astronomical transient is an event of short duration relative to the timescales typical of events in the Universe. Different definitions of transients exist but for the purposes of this thesis astronomical transients are defined in the following three ways: sources that appear and disappear on timescales observable by humans that have no known counterpart; sources that appear and disappear on timescales observable by humans that are associated with known one-off events; or sources that emit short-duration emission which may be highly sporadic. Transient phenomena in the skies have been recorded for centuries; early examples in- clude the observation of a ‘guest star’ in the night sky in the year 1054 A.D. The nearby supernova, which created the Crab Nebula, is the birthplace of one of the youngest known
1 2 Chapter 1. Introduction neutron stars (Mayall & Oort, 1942). More recently, telescopes probing the entire electro- magnetic spectrum, from gamma-rays and X-rays down to radio frequencies, have detected fainter and more distant variable and transient phenomena. Beginning in 1933 with the study of the radio background at 20 MHz by Karl Jansky (Jansky, 1933a,b), observations of the radio sky offered a surprising new window into the physics of our own Galaxy and beyond. Radio transients were first observed in the form of non-thermal Solar emission at 10 40 MHz correlating with periods of high sunspot activity (Appleton, 1945). As radio instrumentation improved and larger radio observatories were constructed, sensitivity to the variability of astrophysical sources increased. Refraction of radio frequen- cies in the Earth’s ionosphere was first observed in the late 1940’s as changes in brightness of stable radio sources over time (Hey et al., 1946). However, radio stars observed with the most advanced radio telescopes of the time were seen to fluctuate in brightness on levels that could not be explained with ionospheric effects. It was realized that the cause of these fluctuations was scintillation, or twinkling, of radio light not within the ionosphere but rather in the ionized material in the Solar System produced by the Sun (Hewish, 1955). The discovery of interplanetary scintillation opened the doors to the sub-discipline of time-domain radio astronomy and the first detections of radio pulses. In 1967 a source was discovered that emitted highly periodic, bright pulses, each lasting only 30 ms the discovery of the first pulsar (Hewish et al., 1968). Pulsars are rapidly rotating neutron stars and sources of strong radio emission when fortuitously oriented towards the observer. Pulsar astronomy has become a rich and vibrant field and pulsars themselves can be used as excellent probes of gravitation (Kramer et al., 2006; Hulse & Taylor, 1975), condensed matter physics (Antoniadis et al., 2013), and the interstellar medium of the Galaxy (Keith et al. 2013, see also Section 1.2). In the 50 years since the discovery of pulsars, several classes of radio transients have been discovered and studied. Some of these other radio sources and their defining properties and timescales include the following:
i. Solar bursts. The Sun is a strong source of transient radio emission emitting several types of bursts labeled Types I V on a variety of timescales. Type I solar bursts last for only 1 second and are caused by plasma radiation during periods of sunspot ⇠ activity; Type III bursts last for several seconds as sub-relativistic electrons accelerate away from the solar surface; and Type II flares – emission that originates in outward propagating shock waves – can continue for up to 30 minutes. Storms of burst activity have also been observed on the Sun that can last for days to weeks (Dulk, 1985). Most 1.1. Transient radio astronomy 3
types of solar bursts are caused by instabilities and variations in the solar plasma (Melrose, 1980) and are typically detected below 200 MHz.
ii. Brown dwarfs. Periodic bright transient emission has been observed for a small number of brown dwarfs and low mass stars (Hallinan et al., 2007, 2008). The emission, caused by the electron cyclotron maser instability, originates at the magnetic poles of the star and is up to 100% circularly polarized. For the known pulsing dwarf stars, pulses last for several minutes and repeat on timescales of 2 3 hours, the rotation period of the star (Hallinan et al., 2008). All radio detections of these pulses have been made between 3 5 GHz (Osten et al., 2009; Route & Wolszczan, 2013). iii. Flare stars. Bursts of incoherent emission have been recorded from nearby M dwarfs for over 50 years (Lovell, 1963); however, more recent observations have revealed radio flares of highly circularly polarized coherent emission at frequencies of 1 GHz (Osten, ⇠ 2008). The coherent flares take place on timescales of a few minutes but are also seen to exhibit temporal variations of the order of a millisecond believed to be intrinsic to the source of the flare on the stellar surface (Osten & Bastian, 2008). The emission mechanism of these coherent flares remains an open question, however the electron cyclotron maser instability observed from brown dwarfs is preferred (Osten, 2008). iv. Radio afterglows from supernovae and gamma-ray bursts. Long-lived radio emission has been detected from supernovae (SNe, Weiler et al., 1982; Sramek et al., 1984) and gamma-ray bursts (GRBs, Kulkarni et al., 1998) in the form of a radio afterglow following the peaked emission in the optical (for SNe, Weiler et al., 1982) and the gamma-ray and X-ray (for GRBs, Galama et al., 1998). Radio emission has been observed in connection with Type Ib/c, and Type II (core collapse) events peaking at various timescales with respect to the optical transient. Type Ib/c radio emission is seen to peak nearly simultaneously with the optical peak, whereas Type II emission peaks months after the optical maximum (e.g. Weiler et al., 2002). The source of the radio emission from supernovae is thought to be the collision of the supernova shock wave into the ionized circumstellar medium producing incoherent synchrotron radiation visible across a range of radio frequencies (Weiler et al., 1982).
Whereas optical brightening of supernovae can be observed for many days before and after the peak, gamma-ray bursts are of much shorter duration; e.g. as a canonical long gamma-ray burst, caused by the rapid collapse of a high-mass star into a black hole (Woosley & Bloom, 2006), is visible only on second timescales at gamma-ray frequen- cies (Weiler et al., 2002). Only a subset of GRBs have observed radio counterparts; 4 Chapter 1. Introduction
based on available data, Chandra & Frail (2012) estimate approximately 31% of GRBs have detectable radio afterglows. Afterglows of GRBs are observed in X-ray, optical, and radio as the columnized jet from the GRB moves through the interstellar medium near the source and produces incoherent synchrotron radiation. As the jet travels further away from the source the energy of the emission decreases resulting in an af- terglow spectrum that peaks first in X-rays then optical and finally radio frequencies (van Paradijs et al., 2000). Radio afterglows are seen anywhere from 0 to 80 days ⇠ post-burst (Chandra & Frail, 2012).
v. Unknown origin. A subset of radio transients have been detected only once, and no rigorous physical model exists for their progenitors. Of this small subset, the “Wow signal” is likely the most widely known. The signal was detected as a 72-second peak of radio emission at 1.4 GHz during a drift scan of the sky for signals as part of the Search for Extraterrestrial Intelligence (SETI) at Ohio State University (Kraus, 1979). A terrestrial origin was determined to be unlikely based on the detection in only one of two beams on the sky. A proposed explanation of the signal was artificial boosting of the source brightness due to interstellar scintillation, but a VLA search of the detection region revealed no sources at that frequency in a search with 100 times the sensitivity of the original detection receiver making scintillation an unlikely cause (Gray & Marvel, 2001). Ultimately no conclusive evidence has been put forward for the signal’s astrophysical or terrestrial origin and it remains an anomaly.
For each of these sources we can calculate a brightness temperature, or the temperature required to produce the observed radio intensity from a black body radiating in the classical Raleigh-Jeans part of the Planck spectrum (Lorimer & Kramer, 2004, 3.4). For a source § at a distance D with a width (or duration) W and peak flux S emitted at a frequency ⌫ the brightness temperature will be
2 2 23 S.D ⌫.W TB & 10 K. (1.1) Jy.kpc2 GHz.ms ✓ ◆✓ ◆ It has been shown by Kellermann et al. (1969) that brightness temperatures above T B 1012 K cannot be produced by incoherent emission processes such as synchrotron radiation and must instead be produced through coherent emission processes, although coherent synchrotron emission may still be possible at these high brightness temperatures (Caroff & Scargle, 1970). Typical brightness temperatures for classes of short-duration radio transients are of the order of 108 1010 K for solar bursts, 7 109 K for pulses from brown dwarfs and 1014 ⇥ 1.2. Pulsars 5
K for the coherent radio flares from flare stars (Dulk, 1985; Hallinan et al., 2008; Osten, 2008). Single pulses from pulsars stand out from the transient classes listed above in their extremely short duration (second to nanosecond timescales) and relatively high brightness temperatures. The observed single pulses from pulsars span several orders of magnitudes ranging from T 1016 K for the least energetic observable pulses to 1035 1037 Kfor B ⇠ the nanosecond bursts of coherent emission from the Crab pulsar (Hankins et al., 2003; Lorimer & Kramer, 2004, 3.4). § The radio transient parameter space is presented in Figure 1.1, from Macquart et al. (2015). The single pulses from pulsars occupy an extreme area of this parameter space high brightness temperatures and short transient timescales; however the pulses from the emerging population of sources called ‘Fast Radio Bursts’ (FRBs) are more extreme still. Since these pulses are thought to originate from outside the Galaxy (see Section 1.3) the inferred brightness temperatures and peak luminosities of the progenitors are required to be 10 orders of magnitude greater than the pulses from normal pulsars. These two classes of extreme phenomena, pulsars and fast radio bursts, are discussed in the following sections.
1.2 Pulsars
Pulsars were discovered through their periodic single pulses in 1967 (Hewish et al., 1968) and since then a population of over 2000 pulsars has been discovered1 (Manchester et al., 2005). Soon after their discovery the link was made between the observed periodic pulses and the theoretical objects called neutron stars (NSs) thought to be formed in the gravita- tional collapse of an intermediate mass star during a supernova (Baade & Zwicky, 1934). It is now widely accepted that the pulsar emission is from rapidly rotating, highly mag- netized neutron stars and is generated in the open magnetic field line region at the star’s magnetic poles. As the star spins, the highly focused beams of radiation from the poles sweep across the sky like a lighthouse and if one of these beams intersects the line of sight with Earth the star is seen as a pulsar. The pulsar emission mechanism is poorly understood (Melrose, 1992). The radio emis- sion is thought to be generated by charged particles accelerated along open magnetic field lines at the polar caps; however, the actual process by which this occurs is unknown. Pul- sar emission is also relatively broadband – most pulsars are observable from GHz down to 100s and even 10s of MHz in frequency with a spectral index of between 1.4 and 2 (i.e. 1For a complete list of known pulsars see the pulsar catalogue http://www.atnf.csiro.au/people/pulsar/psrcat/ 6 Chapter 1. Introduction
Figure 1.1 The parameter space of radio transients reproduced from Macquart et al. (2015) illustrating the wide range of transient radio phenomena. Several classes of transients are labeled including solar bursts, flare stars, single pulses from pulsars, and fast radio bursts. Lines of constant brightness temperature are shown diagonally. Sources in the blue triangle produce radio emission through incoherent emission processes, at T 1012 K and coherent B emission processes occur above this boundary. The 1 kpc and 1 Gpc sensitivity curves are shown for Parkes (black), and two components of the Square Kilometre Array(SKA): SKA1-LOW (pink), and SKA1-MID (grey). 1.2. Pulsars 7 pulsars are brighter at lower frequencies), although some exhibit spectral turnover and have lower flux densities than expected at lower frequencies (Karastergiou et al., 2014). More recently, gamma-ray emission has been detected from both young and old pulsars (Abdo et al., 2009). The gamma-ray emission peak is typically offset from the peak radio pulse suggesting that emission at high energies takes place in the pulsar magnetosphere, further from the surface than the narrowly-beamed radio emission (Weltevrede et al., 2010a).
The pulsar population can be divided into two general categories – the normal (or canonical) pulsars and the millisecond pulsars (MSPs). Canonical pulsars represent the majority of the pulsar population and have spin periods P 500 ms and period deriva- ⇠ tives P˙ 10 15 ss 1; in contrast millisecond pulsars have typical values of P 5 ms ⇠ ⇠ and P˙ 10 20 ss 1 (Lorimer & Kramer, 2004, 1.3). The evolutionary histories of these ⇠ § two classes are also very different. MSPs are formed when a canonical pulsar in a binary system accretes mass from a companion into a disk around the neutron star; the accre- tion from this disk onto the NS surface is responsible for ‘spinning up’ the pulsar (Alpar et al., 1982). Approximately 80% of MSPs still reside within binary systems (Grégoire & Knödlseder, 2013). As pulsars are descendants of main sequence stars, their population is highly concentrated in the plane of the Galaxy (see Figure 1.2); however, many pulsars are moving at high velocities out of the plane, possibly due to natal kicks (van den Heuvel & van Paradijs, 1997). Consequently the older age, and by extension longer travel time, of the MSP population results in a more uniform distribution on the sky than the canonical pulsar population.
The incredibly stable periods of pulsars, especially of the MSPs, make pulsars excel- lent probes of gravity and extreme physics (Matsakis et al., 1997; Lattimer & Prakash, 2007; Hobbs et al., 2012). Through precision timing of pulsars the dynamics of the bi- nary systems, the parameters of their orbits, and the masses of their companions can be measured extremely accurately. For example, the energy lost in the decaying orbit of the NS-NS system observed via the pulsar PSR B1913+16 agrees with the expected emission of gravitational waves predicted by general relativity to within 0.3%: the first detection, although indirect, of gravitational radiation (Taylor et al., 1979; Weisberg et al., 2010). Similarly, precise determination of neutron star masses in the double pulsar system PSRs J0737 3039A&B allowed the most stringent test of general relativity in the strong-field regime (to 99.5% precision; Kramer et al., 2006) by comparing the observed properties of the relativistic orbit with the parameters predicted by general relativity.
Besides being excellent probes of gravity, pulsars are powerful tools for studying the interstellar medium (ISM) of our Galaxy. As radio pulses travel through the Galaxy they 8 Chapter 1. Introduction
Figure 1.2 The distribution of known pulsars in Galactic longitude and latitude in an Aitoff projection. The figure includes normal pulsars (black circles), MSPs (blue triangles), and RRATs (red stars). experience propagation effects caused by the ionized material in the ISM. The four main effects on a radio pulse traveling through the ISM are dispersion, scintillation, scattering, and Faraday rotation which we will discuss in the next four subsections. Dispersive effects on plane waves are most pronounced at low frequencies, thus astrophysical radio pulses are ideal for studies of the magnetoionic medium along the line of sight to the source.
1.2.1 Dispersion
Dispersion is observed as the frequency-dependent group velocity of radio waves. In the interstellar medium, radio signals experience a delay that is proportional to the integrated electron column density along the line of sight, such that the time delay between two frequencies ⌫high and ⌫low will be
d 2 2 ⌫low ⌫high 0 ned` t =4.148808 3 ms (1.2) GHz GHz ⇥ pc cm ⇣ ⌘ ⇣ ⌘ R d where the integral 0 ned` is the integrated electron density ne along the line of sight to a source at distance dR (Ekers & Moffet, 1968). This frequency-dependent delay can be seen in the frequency-time spectrum of a pulsar in Figure 1.3. The integral in Equation 1.2 is commonly defined as the dispersion measure (DM) which can be determined for a pulse 1.2. Pulsars 9
Figure 1.3 The frequency-time spectrum of the radio pulsar J1644 4599 over a range of 64 frequency channels between 710 and 760 MHz as a function of pulse phase. The pulse experiences a frequency dependent delay as it travels through the ionized interstellar medium causing the pulsar pulse to arrive later in lower frequency channels. The pulsar signal has been summed over several pulse periods to achieve a high signal to noise ratio. recorded over a finite bandwidth by measuring the time delay as a function of frequency. DM measurements for pulsars combined with electron density models of the Galaxy are used to estimate distances to Galactic pulsars. The most commonly used model of this type is NE2001, developed by Cordes & Lazio (2002), which models an elaborate, multi- component Galaxy accounting for spiral arms, thin and thick disks, and an outer halo. Distances estimated via DM are calibrated against parallax distances; however, these mea- surements are possible for only about 2% of the pulsar population (Brisken et al., 2002). As such this model is highly uncertain and errors of the order of a factor of two in the model-derived distances are to be expected, especially in regions such as the Galactic halo where the population of pulsars is sparse. 3 DM is measured in units of pc cm and typical values for Galactic pulsars range from 3 3 only 2 pc cm for the closest sources to >1700 pc cm for pulsars near the centre of our Galaxy (Eatough et al., 2013). For most purposes, a pulsar’s dispersion measure is treated as a constant, however some studies over multi-year timescales have detected dispersion measure variations due to changes in the amount of ionized material along the line of sight (Hamilton et al., 1985; Backer et al., 1993). Most variations reported in the literature have been attributed to supernova remnants or pulsar wind nebulae local to the pulsar; however, 10 Chapter 1. Introduction some studies have shown variations on timescales of a few years due to the structure of the ISM on larger scales (Keith et al., 2013). The interstellar medium is believed to contain turbulence within the larger smooth distribution of ionized material. The spectral energy density scale of interstellar turbulence is thought to obey a power law between large (1018 m) and small (106 m) spatial scales, q,suchthat
2 P (q) C q (1.3) 3N ⇡ N represents the power spectrum of the electron density P3N with a spectral index and a 2 structure coefficient CN (Armstrong et al., 1981). This spectrum is thought to be consistent with a Kolmogorov power law described by = 11/3, shown in Figure 1.4. Turbulent interstellar material is probed on various scales by different types of observable phenomena, from rotation measure variations at the largest scales, to dispersion measure variations on intermediate scales, to weak interstellar scintillation at the smallest scales (Armstrong et al., 1995). Variations in pulsar dispersion properties on day to decade timescales are able to probe various regimes of this power law. The turbulent plasma in the ISM is also the cause of the scattering and scintillation phenomena described in the following sections.
1.2.2 Scattering
Multi-path propagation through a turbulent ISM can produce an exponential scattering tail on the trailing edge of the pulsar pulse (Armstrong et al., 1995). The simplest model that can reproduce the observed results of scattering in the ISM is that of a single thin screen (Williamson, 1972). In this approximation, a plane wave experiences distortions due to propagation through a screen of inhomogeneous plasma. The scattering effect is maximized when the scattering screen is located halfway between the source and the observer. In this model the wave experiences a number of phase variations as it travels through the plasma; these arise from the deflection of light by an angle ✓0, which also produces a broadened image of the source with angular radius ✓d such that
2 e ne pd ✓d = ✓0/2= 2 (1.4) 2⇡me pa ⌫ where ne is the perturbation in electron density, a is the width of the screen, d is the distance to the source, and ⌫ is the frequency. The intensity distribution of the light coming from the pulsar through the screen has an angular dependence which also corresponds to a geometric time delay t. The intensity of the scattered pulse as a function of time is 1.2. Pulsars 11
Figure 1.4 The power law relation between spectral energy density and spatial scale for 1 turbulence in the ionized ISM in terms of spatial wavenumber, or (spatial scale) ,and spectral density. A line representing a spectral index of 4 (dot-dashed) and a Kolmogorov spectral index of 11/3 (dotted) are shown. Figure reproduced from Armstrong et al. (1995).
given by
2 t/⌧ I( t) exp( c t/✓ d) e d (1.5) / d ⌘ where ⌧d is defined as the scattering timescale. From Eq. 1.4:
2 4 2 ✓dd e ne 4 4 ⌧d = = 2 2 d⌫ d⌫ . (1.6) c 4⇡ me a /
From this equation we see that the scattering timescale is dependent on distance and heavily dependent on observing frequency. Thus scattering is stronger at larger distances and often correlates with DM. The relation in Eq. 1.6 is a theoretical approximation based upon a single thin screen. In reality the interstellar medium is made up of a large number of scattering screens; however, the observational data from pulsars agrees well with the thin screen model which, to close approximation, can reproduce the exponential scattering tails observed for many pulsars (Sutton, 1971; Williamson, 1972). The observed scattering relation for pulsars has been approximated by Bhat et al. (2004) by 12 Chapter 1. Introduction
Figure 1.5 A cartoon of the thin screen scattering model for radio pulses from a pulsar. A wave encountering a plasma screen half way between source and observer is distorted and deflected through the screen with propagation delays away from the direct line of sight. The result is a scatter-broadened image of the source which appears to have a radius ✓d. Figure reproduced from Lorimer & Kramer (2004).
log⌧ = a + b (logDM) + c (logDM)2 ↵ log⌫ (1.7) d where ⌧d is the scattering timescale in ms, ⌫ is the observing frequency in GHz, and DM is the dispersion measure. The coefficients a, b, c, as well as the scattering index ↵ were fit to the data for Galactic pulsars to obtain values of a = 6.46, b =0.154, c =1.07,and ↵ =3.86 0.16. This relation is useful when the scattering timescale cannot be directly ± measured observationally for a pulsar with a known DM; however, in observational data there are several orders of magnitude of scatter around this relation, especially at higher DMs (Bhat et al., 2004).
1.2.3 Interstellar scintillation
Multi-path propagation through turbulent regions leads to patterns of constructive and destructive interference observed as brightness variability that makes the pulsar appear to scintillate, or twinkle (Armstrong et al., 1995). Scintillation is seen as the observer travels through the interference pattern created by the thin screen. This interference pattern (shown on the right hand side of Figure 1.5) arises because the paths of light through the medium have a range of phases such that the characteristic phase difference is (Rickett, 1977)