Large-Scale surveys for continuous gravitational waves: from data preparation to multi-stage hierarchical follow-ups

Publikation: Qualifikations-/Studienabschlussarbeit › Dissertation

Autoren

  • Benjamin Steltner

Organisationseinheiten

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Details

OriginalspracheEnglisch
QualifikationDoctor rerum naturalium
Gradverleihende Hochschule
Betreut von
  • Maria Alessandra Papa, Betreuer*in
Datum der Verleihung des Grades10 Feb. 2023
ErscheinungsortHannover
PublikationsstatusVeröffentlicht - 2023

Abstract

Das Gravitationswellenereignis GW150914 war der erste direkte Nachweis von Gravitationswellen rund 100 Jahre nach deren Vorhersage durch Albert Einstein. Die Entdeckung war ein Durchbruch und eröffnete einen weiteren Kanal zur Beobachtung des Universums. Seitdem wurden über 90 weitere verschmelzende kompakte Objekte entdeckt, die meisten binäre schwarze Löcher unterschiedlicher Masse, aber auch zweimal verschmelzende Schwarze Löcher mit Neutronensternen und zwei Verschmelzungen von binären Neutronensternen. Ein weiterer Durchbruch war die Beobachtung der ersten Verschmelzung zweier Neutronensterne, GW170817, die mit einer Reihe von elektromagnetischen Beobachtungen einherging, darunter ein Gammastrahlenausbruch 1.7s nach der Verschmelzung. Bei der Verschmelzung kompakter Objekte handelt es sich um kataklysmische Ereignisse, bei denen innerhalb von ~Sekunden mehrere Sonnenmassen in Form von Gravitationswellen ausgestoßen werden. Ihr Nachweis erfordert jedoch hochentwickelte Messgeräte: Laserinterferometer im Kilometermaßstab. Eine weitere, noch nicht nachgewiesene Form der Gravitationsstrahlung sind kontinuierliche Gravitationswellen, die z.B., aber nicht nur, von schnell rotierenden Neutronensternen ausgehen, die relativ zu ihrer Rotationsachse nicht achsensymmetrisch sind. Die Amplitude der kontinuierlichen Gravitationswellen auf der Erde ist um Größenordnungen schwächer als die der verschmelzenden kompakten Objekte, wird aber im Fall des nicht achsensymmetrischen Neutronensterns so lange abgestrahlt, wie der Neutronenstern rotiert und die Deformation aufrechterhält, was Monate bis Jahre sein können. Die Gravitationswelle wird meist mit der doppelten Rotationsfrequenz ausgestrahlt, wobei eine Frequenzentwicklung (Spin-down) aufgrund der von Gravitationswellen ausgesandten Energie, sowie anderer Bremsmechanismen möglich ist. Diese nahezu monochromatische, kontinuierliche Welle wird von einem Beobachter auf der Erde Doppler-moduliert durch die Erdumlaufbahn und die Erddrehung empfangen. Obwohl die Wellenform scheinbar einfach ist, ist das Problem des Nachweises von Signalen aus unbekannten Quellen eine große Herausforderung. Die in dieser Arbeit beschriebene Suche nach unbekannten Neutronensternen in unserer Galaxie über den kompletten Himmel verwendete über mehrere Monate hinweg das Volunteer-Computing-Projekt Einstein@Home und den ATLAS-Supercomputer und benötigte insgesamt Zehntausende von Jahren an Rechenzeit. In dieser Arbeit beschreibe ich das vollständige Datenanalyseverfahren einschließlich der Datenvorbereitung, der Optimierung der Suchparameter und der Nachbearbeitung der Suchergebnisse, dessen Entwurf und Implementierung das Kernstück meiner Doktorarbeit darstellt. Außerdem stelle ich eine Reihe von Beobachtungsergebnissen vor, welche die praktische Anwendung der von mir entwickelten Methoden demonstrieren.

Zitieren

Large-Scale surveys for continuous gravitational waves: from data preparation to multi-stage hierarchical follow-ups. / Steltner, Benjamin.
Hannover, 2023. 129 S.

Publikation: Qualifikations-/Studienabschlussarbeit › Dissertation

Steltner, B 2023, 'Large-Scale surveys for continuous gravitational waves: from data preparation to multi-stage hierarchical follow-ups', Doctor rerum naturalium, Gottfried Wilhelm Leibniz Universität Hannover, Hannover. https://doi.org/10.15488/13266
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abstract = "The gravitational wave event GW150914 was the first direct detection of gravitational waves roughly 100 years after their prediction by Albert Einstein. The detection was a breakthrough, opening another channel to observe the Universe. Since then over 90 detections of merging compact objects have been made, most of them coalescences of binary black holes of different masses. There have been two black hole-neutron star, and two binary neutron-star mergers. Another breakthrough was the first binary neutron-star merger, GW170817, associated with a slew of electromagnetic observations, including a gamma-ray burst 1.7s after the merger. Compact binary coalescence events are cataclysmic events in which multiple solar masses are emitted in gravitational waves in ~seconds. Still, their gravitational wave detection requires sophisticated measuring devices: kilometer-scale laser interferometers. Another not yet detected form of gravitational radiation are continuous gravitational waves from e.g., but not limited to, fast-spinning neutron stars nonaxisymmetric relatively to their rotational axis. The gravitational wave amplitude on Earth is orders of magnitude weaker than the compact binary coalescence events, but, in the case of the nonaxisymmetric neutron star, is emitted as long as the neutron star is spinning and sustaining the deformation, which may be months to years. The gravitational wave is mostly emitted at twice the rotational frequency, with a possible frequency evolution (spin-down) due to the energy emitted by gravitational waves, as well as other braking mechanisms. This nearly monochromatic continuous wave is received by observers on Earth Doppler modulated by Earth's orbit and spin. Although the waveform is seemingly simple, the detection problem for signals from unknown sources is very challenging. The all-sky search for unknown neutron stars in our galaxy detailed in this work used the volunteer distributed computing project Einstein@Home and the ATLAS supercomputer for several months, taking tens of thousands of total CPU-time years to complete. In this work I describe the full-scale data analysis procedure, including data preparation, search set-up optimization and post-processing of search results, whose design and implementation is the core of my doctoral research work. I also present a number of observational results that demonstrate the real-world application of the methodologies that I designed.",
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