PDF hosted at the Radboud Repository of the Radboud University Nijmegen The following full text is a publisher's version. For additional information about this publication click this link. http://hdl.handle.net/2066/147186 Please be advised that this information was generated on 2021-10-08 and may be subject to change. Searching for Fast Radio Transients with LOFAR Sander ter Veen Searching for Fast Radio Transients with LOFAR Proefschrift ter verkrijging van de graad van doctor aan de Radboud Universiteit Nijmegen op gezag van de rector magnificus prof. dr. Th.L.M. Engelen, volgens besluit van het college van decanen in het openbaar te verdedigen op donderdag 26 november 2015 om 14:30 uur precies door Sander ter Veen geboren op 21 maart 1984 te Hoogkerk Promotor: Prof. dr. Heino Falcke Manuscriptcommissie: Prof. dr. G. J. Heckman (voorzitter) Prof. Dr. O. Scholten Rijksuniversiteit Groningen Prof. dr. F. W. M. Verbunt Dr. P. C. C. Freire Max Planck Institute for Radio Astronomy, Bonn, Germany Dr. A. G. J. van Leeuwen ASTRON, Dwingeloo © 2015, Sander ter Veen Searching for Fast Radio Transients with LOFAR Thesis, Radboud University Nijmegen Illustrated; with bibliographic information and Dutch summary ISBN: 978-94-6259-906-2 Contents 1 Introduction 1 1.1 Pulsars......................................... 1 1.2 PulsarsinGlobularClusters. ........ 3 1.3 RotatingRadioTransients. ....... 4 1.4 FastRadioBursts ................................. ... 5 1.5 CosmicRays ...................................... 7 1.6 LOFAR .......................................... 8 1.7 Thisthesis...................................... 10 2 Transient Buffer Board Data Acquisition 13 2.1 Introduction.................................... 13 2.2 EarlyTBBdata .................................... 14 2.3 CosmicRayAirShowers............................. 15 2.4 Dataacquisition ................................. 15 2.4.1 tbbctl ....................................... 16 2.4.2 datawriter.................................... 19 2.4.3 triggercenter ................................. 20 2.5 Futureimplementation. ...... 21 3 FRATS: Real-Time searching for Fast Radio Transients with LOFAR (real-time pipeline description and first results) 23 3.1 Introduction.................................... 24 3.2 Method .......................................... 25 3.2.1 LOFAR ...................................... 25 3.2.2 TheFRATSproject ............................... 26 3.3 Observations .................................... 30 3.3.1 Surveydescription . .. .. .. .. .. .. .. .. 30 3.3.2 Dedispersionefficiency . 31 3.3.3 Effective observing time and beam size . ....... 35 i Contents 3.3.4 Noiselevel.................................... 35 3.4 Results......................................... 37 3.4.1 Pulsarrediscoveries . 37 3.4.2 Isolated Fast Radio Transients . ...... 38 3.4.3 Non-dispersedevents. 39 3.5 Discussion...................................... 39 3.5.1 Skyrate...................................... 40 3.5.2 RRATS ...................................... 41 3.5.3 Perytons...................................... 41 3.5.4 OtherLOFARsurveys. .. .. .. .. .. .. .. .. 41 3.6 Conclusions ..................................... 42 4 FRATS: Locating and Identifying Fast Radio Transients with the LOFAR Tran- sient Buffer Boards 45 4.1 Introduction.................................... 45 4.2 Projectbackground............................... ..... 46 4.2.1 LOFARandTBBs ................................ 46 4.2.2 TheFRATSproject ............................... 48 4.3 Validation and localization of Fast Radio Transients . ............... 48 4.3.1 FRATS TBB Analysis Pipeline structure . ...... 50 4.3.2 Initial validation of LOFAR triggers . ........ 50 4.3.3 Standard processing: at station level . ......... 52 4.3.4 Pulseidentification. 56 4.3.5 Standard processing: pulse localization and characterization . 58 4.4 Results......................................... 60 4.4.1 Bad antenna detection . 61 4.4.2 Side-lobe detection: Solar flare . ....... 62 4.4.3 High angular resolution: Pulsars . ....... 63 4.4.4 Pulse characterization . ..... 64 4.5 Conclusion ...................................... 66 5 FRATS: A real-time low frequency search for nearby Fast Radio Bursts up to a DM of 500 pc cm−3 67 5.1 Introduction.................................... 67 5.2 MethodandDataAnalysis . .. .. .. .. .. .. .. .. 68 5.2.1 DMrangeandDMstep ............................. 68 5.2.2 System Equivalent Flux Density . ..... 68 5.2.3 Trigger verification . 69 5.2.4 Volume rate limit for extra-galactic sources . ........... 70 5.3 Results......................................... 71 5.3.1 Pulsars ...................................... 72 5.3.2 Limits on isolated dispersed pulses . ........ 73 5.4 Discussion...................................... 74 ii Contents 5.4.1 Volume rate, comparison and expectations . ......... 74 5.4.2 StarFormationHistory . 75 5.5 Conclusion ...................................... 76 6 Searching for pulsars in Globular Cluster M13 with LOFAR 79 6.1 Introduction.................................... 79 6.2 Method .......................................... 81 6.2.1 Observations .................................. 81 6.2.2 RFImitigation ................................. 81 6.2.3 Dedispersion .................................. 82 6.2.4 Searching ..................................... 83 6.2.5 Scattering.................................... 84 6.3 Results......................................... 85 6.3.1 KnownpulsarsinM13............................. 85 6.3.2 Pulsarsearches ................................ 85 6.3.3 Discussion.................................... 88 6.4 Conclusion ...................................... 91 7 A new limit on the Ultra-High-Energy Cosmic-Ray flux with the Westerbork Synthesis Radio Telescope 101 7.1 Introduction.................................... 102 7.2 ShallowShowers .................................. 103 7.2.1 Čerenkov radiation from finite particle tracks . ...........104 7.2.2 Treatment for deep cascades . 105 7.2.3 Emissioninavacuum ............................. 106 7.3 Exact calculation for near-surface showers . .............107 7.3.1 Checkinglimitingcases . 109 7.3.2 Formation zone effects . 111 7.4 NewlimitontheUHECRflux ........................... 111 7.5 Conclusions ..................................... 115 Summary and Outlook 117 Bibliography 121 Samenvatting 127 Introductie ........................................ 127 LOFAR .............................................128 Snelle variabele radiosignalen . ..........128 Dispersieendedispersie . 129 FastRadioTransientSearch(FRATS) . 130 Pulsarsinbolhopen .................................. 131 Ultrahoogenergetische kosmische straling . ...........132 iii Contents Publication list 133 Statement on the originality of the work 137 Curriculum vitæ 139 Acknowledgements 141 iv Chapter 1 Introduction The topic of this thesis is searching for fast radio transients, sub-second flashes of radio waves from astrophysical phenomena. There are several sources of these flashes, from stars, planets, neutron stars, cosmic particles to the yet unknown origin of the fast radio bursts. In the next sections of this introduction the sources that are most relevant for this thesis are described. This is followed with a section about the new radio telescope LOFAR, that is used for this research. The only important information needed beforehand is that it observes between 10 and 250 MHz, lower than the typical observing frequencies used in fast transient searches in the last decades. This introduction concludes with a section about this thesis itself. 1.1 Pulsars The first detected and most studied class of fast radio transients are the pulsars. Pulsars are highly magnetized rotating neutron stars, that emit radio waves continuously, but are only detected when they point to Earth, like a lighthouse. They were first detected by Jocelyn Bell and Anthony Hewish in 1967, during an experiment to detect inter-stellar scintillation [Hewish et al., 1968] using the Interplanetary Scintillation Array of the UK Mullard Radio Astronomy Observatory. The telescope’s time resolution of 0.1s made it possible to time-resolve some "scruff", as Jocelyn Bell put it, and detect very regular pulsations with a period of 1337 milliseconds at a specific sidereal time, 4 minutes earlier every day, and thus from a source outside our solar system. The short duration of the pulsations hinted to a compact emission region, and therefore initially white dwarfs and neutron stars were suggested as possible sources. Later discoveries of even faster pulsars concluded that the object should be a neutron star, an object that was first predicted by Baade & Zwicky [1934], but was not observed before. The possibility of pulsations from neutron stars was first predicted by Gold [1968]. Since this first discovery more than 2400 pulsars have been detected according to the ATNF pulsar catalog1 [Manchester et al., 2005]. The periods of these pulsars range from 1.4 ms to 11.8 s, and while the typical mass of a neutron star is about 1.4 solar mass, also heavier pulsars have been observed with a mass of 2.0 solar mass. The radius of these objects is only 10-15 km, and 1For the current catalog see http://www.atnf.csiro.au/people/pulsar/psrcat/ 1 Chapter 1 : Introduction therefore these are very dense objects with a density of 1015 g/cm3. In addition they also have a strong magnetic field of 108 1015 Gauss. This makes them important laboratories for physics ∼ − under extreme conditions unaccessible at earth. One important