TY - THES A1 - Antonelli, Andrea T1 - Accurate waveform models for gravitational-wave astrophysics: synergetic approaches from analytical relativity N2 - Gravitational-wave (GW) astrophysics is a field in full blossom. Since the landmark detection of GWs from a binary black hole on September 14th 2015, fifty-two compact-object binaries have been reported by the LIGO-Virgo collaboration. Such events carry astrophysical and cosmological information ranging from an understanding of how black holes and neutron stars are formed, what neutron stars are composed of, how the Universe expands, and allow testing general relativity in the highly-dynamical strong-field regime. It is the goal of GW astrophysics to extract such information as accurately as possible. Yet, this is only possible if the tools and technology used to detect and analyze GWs are advanced enough. A key aspect of GW searches are waveform models, which encapsulate our best predictions for the gravitational radiation under a certain set of parameters, and that need to be cross-correlated with data to extract GW signals. Waveforms must be very accurate to avoid missing important physics in the data, which might be the key to answer the fundamental questions of GW astrophysics. The continuous improvements of the current LIGO-Virgo detectors, the development of next-generation ground-based detectors such as the Einstein Telescope or the Cosmic Explorer, as well as the development of the Laser Interferometer Space Antenna (LISA), demand accurate waveform models. While available models are enough to capture the low spins, comparable-mass binaries routinely detected in LIGO-Virgo searches, those for sources from both current and next-generation ground-based and spaceborne detectors must be accurate enough to detect binaries with large spins and asymmetry in the masses. Moreover, the thousands of sources that we expect to detect with future detectors demand accurate waveforms to mitigate biases in the estimation of signals’ parameters due to the presence of a foreground of many sources that overlap in the frequency band. This is recognized as one of the biggest challenges for the analysis of future-detectors’ data, since biases might hinder the extraction of important astrophysical and cosmological information from future detectors’ data. In the first part of this thesis, we discuss how to improve waveform models for binaries with high spins and asymmetry in the masses. In the second, we present the first generic metrics that have been proposed to predict biases in the presence of a foreground of many overlapping signals in GW data. For the first task, we will focus on several classes of analytical techniques. Current models for LIGO and Virgo studies are based on the post-Newtonian (PN, weak-field, small velocities) approximation that is most natural for the bound orbits that are routinely detected in GW searches. However, two other approximations have risen in prominence, the post-Minkowskian (PM, weak- field only) approximation natural for unbound (scattering) orbits and the small-mass-ratio (SMR) approximation typical of binaries in which the mass of one body is much bigger than the other. These are most appropriate to binaries with high asymmetry in the masses that challenge current waveform models. Moreover, they allow one to “cover” regions of the parameter space of coalescing binaries, thereby improving the interpolation (and faithfulness) of waveform models. The analytical approximations to the relativistic two-body problem can synergically be included within the effective-one-body (EOB) formalism, in which the two-body information from each approximation can be recast into an effective problem of a mass orbiting a deformed Schwarzschild (or Kerr) black hole. The hope is that the resultant models can cover both the low-spin comparable-mass binaries that are routinely detected, and the ones that challenge current models. The first part of this thesis is dedicated to a study about how to best incorporate information from the PN, PM, SMR and EOB approaches in a synergistic way. We also discuss how accurate the resulting waveforms are, as compared against numerical-relativity (NR) simulations. We begin by comparing PM models, whether alone or recast in the EOB framework, against PN models and NR simulations. We will show that PM information has the potential to improve currently-employed models for LIGO and Virgo, especially if recast within the EOB formalism. This is very important, as the PM approximation comes with a host of new computational techniques from particle physics to exploit. Then, we show how a combination of PM and SMR approximations can be employed to access previously-unknown PN orders, deriving the third subleading PN dynamics for spin-orbit and (aligned) spin1-spin2 couplings. Such new results can then be included in the EOB models currently used in GW searches and parameter estimation studies, thereby improving them when the binaries have high spins. Finally, we build an EOB model for quasi-circular nonspinning binaries based on the SMR approximation (rather than the PN one as usually done). We show how this is done in detail without incurring in the divergences that had affected previous attempts, and compare the resultant model against NR simulations. We find that the SMR approximation is an excellent approximation for all (quasi-circular nonspinning) binaries, including both the equal-mass binaries that are routinely detected in GW searches and the ones with highly asymmetric masses. In particular, the SMR-based models compare much better than the PN models, suggesting that SMR-informed EOB models might be the key to model binaries in the future. In the second task of this thesis, we work within the linear-signal ap- proximation and describe generic metrics to predict inference biases on the parameters of a GW source of interest in the presence of confusion noise from unfitted foregrounds and from residuals of other signals that have been incorrectly fitted out. We illustrate the formalism with simple (yet realistic) LISA sources, and demonstrate its validity against Monte-Carlo simulations. The metrics we describe pave the way for more realistic studies to quantify the biases with future ground-based and spaceborne detectors. N2 - Wenn zwei kompakte Objekte wie Schwarze Löcher oder Neutronensterne kollidieren, wird der Raum und die Zeit um sie herum stark gekrümmt. Der effekt sind Störungen der Raumzeit, sogenannte Gravitationswellen, die sich im gesamten Universum ausbreiten. Mit den leistungsstarken und präzisen Netzwerken von Detektoren und der Arbeit vieler Wissenschaftler rund um den Globus kann man Gravitationswellen auf der Erde messen. Gravitationswellen tragen Informationen über das System, das sie erzeugt hat. Insbesondere kann man erfahren, wie sich die kompakten Objekte gebildet haben und woraus sie bestehen. Daraus lässt sich ableiten, wie sich das Universum ausdehnt, und man kann die Allgemeine Relativitätstheorie in Regionen mit starker Gravitation testen. Um diese Informationen zu extrahieren, werden genaue Modelle benötigt. Modelle können entweder numerisch durch Lösen der berühmten Einstein-Gleichungen oder analytisch durch Annäherung an deren Lösungen gewonnen werden. In meiner Arbeit haben wir den zweiten Ansatz verfolgt, um sehr genaue Vorhersagen für die Signale zu erhalten, die bei kommenden Beobachtungen durch Gravitationswellendetektoren verwendet werden können. KW - gravitational waves KW - Gravitationswellen KW - general relativity KW - allgemeine Relativitätstheorie KW - data analysis KW - Datenanalyse Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-576671 ER - TY - THES A1 - Cotesta, Roberto T1 - Multipolar gravitational waveforms for spinning binary black holes and their impact on source characterization T1 - Multipolare Gravitationswellenformen für binäre Schwarze Löcher mit Spin und deren Einfluss auf die Quellencharakterisierung N2 - In the last five years, gravitational-wave astronomy has gone from a purerly theoretical field into a thriving experimental science. Several gravitational- wave signals, emitted by stellar-mass binary black holes and binary neutron stars, have been detected, and many more are expected in the future as consequence of the planned upgrades in the gravitational-wave detectors. The observation of the gravitational-wave signals from these systems, and the characterization of their sources, heavily relies on the precise models for the emitted gravitational waveforms. To take full advantage of the increased detector sensitivity, it is then necessary to also improve the accuracy of the gravitational-waveform models. In this work, I present an updated version of the waveform models for spinning binary black holes within the effective-one-body formalism. This formalism is based on the notion that the solution to the relativistic two- body problem varies smoothly with the mass ratio of the binary system, from the equal-mass regime to the test-particle limit. For this reason, it provides an elegant method to combine, under a unique framework, the solution to the relativistic two-body problem in different regimes. The main two regimes that are combined under the effective-one-body formalism are the slow-motion, weak field limit (accessible through the post-Newtonian theory), and the extreme mass-ratio regime (described using the black-hole- perturbation theory). This formalism is nevertheless flexible enough to integrate information about the solution to the relativistic two-body problem obtained using other techniques, such as numerical relativity. The novelty of the waveform models presented in this work is the inclusion of beyond-quadupolar terms in the waveforms emitted by spinning binary black holes. In fact, while the time variation of the source quadupole moment is the leading contribution to the waveforms emitted by binary black holes observable by LIGO and Virgo detectors, beyond-quadupolar terms can be important for binary systems with asymmetric masses, large total mass, or observed with large inclination angle with respect to the orbital angular momentum of the binary. For this purpose, I combine the approximate analytic expressions of these beyond-quadupolar terms, with their calculations from numerical relativity, to develop an accurate waveform model including inspiral, merger and ringdown for spinning binary black holes. I first construct this model in the simplified case of black holes with spins aligned with the orbital angular momentum of the binary, then I extend it to the case of generic spin orientations. Finally, I test the accuracy of both these models against a large number of waveforms obtained from numerical relativity. The waveform models I present in this work are the state of the art for spinning binary black holes, without restrictions in the allowed values for the masses and the spins of the system. The measurement of the source properties of a binary system emitting gravitational waves requires to compute O(107 − 109) different waveforms. Since the waveform models mentioned before can require O(1 − 10)s to generate a single waveform, they can be difficult to use in data-analysis studies given the increasing number of sources observed by the LIGO and Virgo detectors. To overcome this obstacle, I use the reduced-order-modeling technique to develop a faster version of the waveform model for black holes with spins aligned to the orbital angular momentum of the binary. This version of the model is as accurate as the original and reduces the time for evaluating a waveform by two orders of magnitude. The waveform models developed in this thesis have been used by the LIGO and Virgo collaborations in the inference of the source parameters of the gravitational-wave signals detected during the second observing run (O2), and first half of the third observing run (O3a) of LIGO and Virgo detectors. Here, I present a study on the source properties of the signals GW170729 and GW190412, for which I have been directly involved in the analysis. In addition, these models have been used by the LIGO and Virgo collaborations to perform tests on General Relativity employing the gravitational-wave signals detected during O3a, and to analyze the population of the observed binary black holes. N2 - Das Feld der Gravitationswellenastronomie hat sich in den letzten fünf Jahren von einem ausschließlich theoretischen Forschungsgebiet zu einer aufstrebenden experimentellen Wissenschaft entwickelt. Gravitationswellendetektoren haben bereits mehrere Signale gemessen die von binären Schwarzen Löchern und Neutronensternen im Sonnenmassenbereich entstanden sind. Viele weitere solcher Signale werden erwartet sobald die geplanten Aufrüstungen der Detektoren vollzogen wurden. Sowohl der Messung dieser Signale als auch der Charakterisierung ihrer astronomischen Quellen liegen größtenteils Modelle der erwarteten Gravitationswellenformen zugrunde. Um die steigende Sensibilität der Gravitationswellendetektoren voll ausschöpfen zu können ist es daher auch notwendig die Genauigkeit der Gravitationswellenmodelle zu steigern. In dieser Arbeit stelle ich aktualisierte Gravitationswellenmodelle für binäre Schwarze Löcher mit Spin vor die auf dem "effective-onebody" Formalismus beruhen. Dieser Formalismus basiert auf der Annahme, dass die Lösung des relativistischen Zweikörperproblems glatt mit dem Massenverhältnis des Binärsystems variiert, welches sich vom Regime gleicher Massen bis zum Testteilchenlimit erstreckt. Damit stellt der Formalismus eine elegante Methode dar die Lösungen des relativistischen Zweikörperproblems in verschiedenen Regimen zu verbinden. Er findet größtenteils Anwendung um das Regime langsamer Bewegung in schwacher Gravitation (beschrieben durch postnewtonische Theorie) mit dem Regime extremer Massenverhältnisse (beschrieben durch Störungstheorie) zu verbinden. Der Formalismus ist jedoch ebenso anwendbar um Informationen zur Lösung des relativistischen Zweikörperproblems von anderen Methoden zu integrieren, wie beispielsweise Ergebnisse der Numerischen Relativität. Die Neuheiten die diese Arbeit präsentiert umfassen zuvorderst die Einbindung von multipolaren Beiträgen in den Gravitationswellenformen von binären Schwarzen Löchern mit Spin die über den quadrupolaren Beitrag hinausgehen. Während die Zeitentwicklung des Quadrupolmoments typischerweise die Wellenformen von binären Schwarzen Löchern dominiert, die von den LIGO und Virgo Detektoren gemessen werden können, werden Beiträge höherer Multipolmomente insbesondere für Binärsysteme wichtig die asymmetrischen Massen, eine hohe Gesamtmasse oder einen Beobachtungswinkel der stark von der Drehimpulsachse des Orbits abweicht aufweisen. Für diesen Anwendungsbereich kombiniere ich Näherungen der analytischen Ausdrücke dieser höheren Multipolbeiträge mit Ergebnissen aus der Numerischen Relativität um ein Gravitationswellenmodell zu entwickeln welches die Phasen der Einspiralung, der Verschmelzung und des Abklingens von binären Schwarzen Löchern mit Spin mit hoher Genauigkeit beschreibt. Zunächst konstruiere ich das Modell für den einfacheren Fall von Schwarzen Löchern, deren Spins entlang des Drehimpulses ihres Orbits orientiert sind, und verallgemeinere das Modell anschließend für beliebige Orientierungen der Spins. Abschließend teste ich die Genauigkeit dieser beider Modelle anhand einer großen Zahl von Wellenformen aus der Numerischen Relativität. Die Wellenformen, die ich in dieser Arbeit präsentiere, entsprechen dem aktuellen Forschungsstand für binäre Schwarze Löcher mit Spin und enthalten keine Beschränkungen der Massen oder Spins. Um von einem gemessenen Gravitationswellensignal auf Eigenschaften des Binärsystems, welches das Signal produziert hat, schließen zu können ist es notwendig O(107 - 109) verschiedene Wellenformen zu berechnen. Da die oben beschriebenen Modelle jedoch O(1 - 10)s benötigen können um eine einzige Wellenform zu berechnen sind sie in dieser Form für die Datenanalyse der steigenden Zahl von Signalen, die von den LIGO und Virgo Detektoren gemessen werden, nur eingeschränkt brauchbar. Um diese Hürde zu überwinden entwickele ich mithilfe der "reduced-order-modeling" Methode eine schnellere Version der Wellenformmodelle für binäre Schwarze Löcher, deren Spins entlang des Drehimpulses ihres Orbits orientiert sind. Diese Version der Modelle ist ebenso genau wie das Original, reduziert jedoch die Evaluationszeit einer Wellenform um zwei Größenordnungen. Die in dieser Arbeit entwickelten Wellenformmodelle wurden bereits von den LIGO und Virgo Kollaborationen verwendet um Gravitationswellensignale zu analysieren die während des zweiten Beobachtungslaufs (O2) und der ersten Hälfte des dritten Beobachtungslaufs (O3a) der LIGO und Virgo Detektoren gemessen wurden. Hier präsentiere ich eine Studie der Quellencharakterisierung der Signale GW170729 und GW190412, bei denen ich direkt an der Analyse beteiligt war. Die Modelle wurden außerdem bereits von den LIGO und Virgo Kollaborationen verwendet um Test der Allgemeinen Relativitätstheorie mithilfe von Gravitationswellensignalen des O3a durchzuführen, und um die Population der gemessenen binären Schwarzen Löcher. KW - gravitational waves KW - binary black holes KW - binäre Schwarze Löcher KW - Gravitationswellen KW - gravitational-wave astronomy KW - Gravitationswellenastronomie KW - LIGO KW - LIGO KW - Virgo KW - Virgo KW - multipolar gravitational waves KW - Multipolare Gravitationswellenformen Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-508236 ER - TY - JOUR A1 - Nedora, Vsevolod A1 - Radice, David A1 - Bernuzzi, Sebastiano A1 - Perego, Albino A1 - Daszuta, Boris A1 - Endrizzi, Andrea A1 - Prakash, Aviral A1 - Schianchi, Federico T1 - Dynamical ejecta synchrotron emission as a possible contributor to the changing behaviour of GRB170817A afterglow JF - Monthly notices of the Royal Astronomical Society N2 - Over the past 3 yr, the fading non-thermal emission from the GW170817 remained generally consistent with the afterglow powered by synchrotron radiation produced by the interaction of the structured jet with the ambient medium. Recent observations by Hajela et al. indicate the change in temporal and spectral behaviour in the X-ray band. We show that the new observations are compatible with the emergence of a new component due to non-thermal emission from the fast tail of the dynamical ejecta of ab-initio binary neutron star merger simulations. This provides a new avenue to constrain binary parameters. Specifically, we find that equal mass models with soft equations of state (EOSs) and high-mass ratio models with stiff EOSs are disfavoured as they typically predict afterglows that peak too early to explain the recent observations. Moderate stiffness and mass ratio models, instead, tend to be in good overall agreement with the data. KW - equation of state KW - gravitational waves KW - neutron star mergers Y1 - 2021 U6 - https://doi.org/10.1093/mnras/stab2004 SN - 0035-8711 SN - 1365-2966 VL - 506 IS - 4 SP - 5908 EP - 5915 PB - Oxford Univ. Press CY - Oxford ER -