Theses and Dissertations from UMD
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Item New Messengers & New Physics: A Survey of the High-energy Universe(2023) Crnogorcevic, Milena; Ricotti, Massimo; Caputo, Regina; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Studying the origins of the high-energy emission in the Universe can profoundly affect our fundamental understanding of the cosmic origin and its evolution at the most extreme scales. In this dissertation, I explore the high-energy observations of different astrophysical systems to inform our understanding of the theoretical frameworks used to describe them. I harness the current multimessenger infrastructure to investigate questions ranging from new physics and transient astronomy to compact objects and extended emission in the gamma-ray, gravitational-wave, and neutrino skies. The focus in the first part of this dissertation is on utilizing the Fermi Large Area Telescope (LAT) low-energy (LLE) technique to search for the light axion-like-particle (ALP) within the MeV gamma-ray emission of long gamma-ray bursts (GRBs). We perform a data-driven sensitivity analysis to determine distances for which detection of an ALP signal is possible with the LLE technique, which, in contrast to the standard LAT analysis, allows for a larger effective area for energies down to 30 MeV. Assuming an ALP mass $m_a \lesssim 10^{-10}$~eV and ALP-photon coupling $g_{a\gamma} = 5.3\times 10^{-12}$ GeV$^{-1}$, we find that the distance limit ranges from $\sim\!0.5$ to $\sim\!10$~Mpc. We demonstrate that the sensitivity of the LLE technique to detecting light ALPs is comparable to the standard LAT analysis, making it an excellent complementary---yet independent---way to search for ALPs with \textit{Fermi}. Next, we select a candidate sample of twenty-four GRBs and conduct a model comparison analysis in which we consider different GRB spectral models with and without an ALP signal component. We find that including an ALP contribution does not result in any statistically significant improvement of the fits to the data. Motivated by the delay between the ALP emission time and the time of the jet break-out associated with its ordinary long-GRB emission, we conduct a novel search for ALPs within time windows that precede the main-episode gamma-ray emission of a long GRB, focusing on the sample of sources with known precursor emission detected with LAT and LLE. We report no statistically significant detection of ALPs within the GRB precursor emission and discuss the parts of the ALP parameter space probed with this method. Multimessenger astronomy is at the heart of the remainder of this dissertation. First, I present a follow-up search for excess emission of X-rays with the Swift Burst Alert Telescope (Swift-BAT) and that of gamma rays with the Fermi Gamma-ray Burst Monitor (Fermi-GBM), in spatial and temporal correspondence to gravitational-wave events reported by the LIGO/Virgo/Kagra (LVK) Collaboration. In collaboration with the Fermi-GBM Team, we combine the observations from these two instruments to determine whether there is any statistically significant excess emission around the given gravitational-wave trigger. We report no new joint detections but present the joint flux upper limits. Finally, I present the results of the cross-correlation studies between the unresolved Fermi-LAT gamma-ray and the IceCube neutrino skies. We report no positive cross-correlation in the real-data sky maps. We then combine simulation and observation techniques to place upper limits on the fraction of neutrinos produced in proton-proton or proton-gamma interactions that occur in blazars. Assuming all gamma rays from unresolved blazars are produced from neutral pions via proton-proton interactions, we find that---for energies above 10~GeV---up to 60 % of the unresolved blazar population may contribute to the diffuse neutrino background (the fraction is 30 % for proton-gamma interactions). We also include predictions for the improved sensitivity considering 20 years of IceCube data.Item Analytical modeling of compact binaries in general relativity and modified gravity theories(2022) Khalil, Mohammed M.; Buonanno, Alessandra; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Gravitational-wave (GW) signals from the coalescence of almost a hundred binary systems have been detected over the past few years. These observations have improved our understanding of binary black holes and neutron stars, their properties, and astrophysical formation channels. GWs also probe gravity in the nonlinear, strong-field regime, thus allowing us to search for, or constrain, deviations from general relativity. The focus of this dissertation is improving the analytical description of binary dynamics, which is important for producing accurate waveform models that can be used in searching for GW signals, inferring their parameters, and testing gravity. The research presented here can be divided into three complementary parts: 1) extending the post-Newtonian (PN) approximation for spinning binaries to higher orders, 2) improving effective-one-body (EOB) waveform models, and 3) identifying some signatures of modified gravity theories in waveforms. The PN approximation, valid for slow motion and weak gravitational field, is widely used to model the dynamics of comparable-mass binaries, which are the main GW sources for ground-based detectors. We derive PN results for spinning binaries at the third- and fourth-subleading PN orders for the spin-orbit coupling, and at the third-subleading order for the spin(1)-spin(2) coupling. We adopt an approach that combines several analytical approximation methods to obtain PN results valid for arbitrary mass ratios from gravitational self-force results at first order in the mass ratio. This is possible due to the simple mass dependence of the scattering angle in the post-Minkowskian approximation (weak field but arbitrary velocities). The EOB formalism produces accurate waveforms by combining analytical results for the binary dynamics with numerical relativity information, while recovering the strong-field test-body limit. To improve EOB models, we include spin-precession effects in the Hamiltonian up to the fourth PN order, and extend the radiation-reaction force and waveform to eccentric orbits. We also assess the accuracy of post-Minkowskian results, for both bound and scattering orbits, and incorporate them in EOB Hamiltonians. In the context of modified gravity theories, we derive the conservative and dissipative dynamics in Einstein-Maxwell-dilaton theory at the next-to-leading PN order, and compute the Fourier-domain gravitational waveform. We also develop a theory-agnostic effective-field-theory approach for describing spontaneous and dynamical scalarization: non-perturbative phenomena in which compact objects can undergo a phase transition and acquire scalar charge. We apply this approach to binary black holes in Einstein-Maxwell-scalar theory using a quasi-stationary approximation, then extend it to account for the dynamical evolution of the scalar charge, and apply it to binary neutron stars in a class of scalar-tensor theories. Improving waveform models is important for current-generation GW detectors and necessary for future detectors, such as LISA, the Einstein telescope, and Cosmic Explorer. The results obtained in this work are important steps towards that goal.Item Cosmological Phase Transition of Composite Higgs Confinement(2021) Ekhterachian, Majid; Agashe, Kaustubh; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We study the cosmological confinement-deconfinement phase transition (PT) of nearly conformal, strongly coupled large N field theories, applicable to composite Higgs models. We find that despite strong coupling, aspects of the PT can be analyzed when the confinement is predominantly spontaneous. In this scenario, the leading contribution to the transition rate can be computed within effective field theory of dilaton-- the pseudo Nambu-Goldstone boson associated with the spontaneous breaking of conformal symmetry. We then show how the holographic dual formulation in terms of 5D warped compactifications allows for qualitative understanding of the missing pieces of the earlier described 4D picture and a quantitative improvement of the calculations. In this description the PT is from a high-temperature black-brane phase to the low-temperature Randall-Sundrum I phase, and the transition proceeds by percolation of bubbles of IR-brane nucleating from the black-brane horizon. We show that the bubble configuration interpolating between the two phases can be smooth enough to be described within 5D effective field theory. We find that cosmological PT in the minimal models can complete only after a large period of supercooling, potentially resulting in excessive dilution of primordial matter abundances. We then show how generic modifications of the minimal models can result in a much faster completion of the PT. We also study the stochastic gravitational wave background produced by the violent bubble dynamics and discuss the implications of the PT for baryogenesis.Item SEARCH FOR GAMMA-RAY COUNTERPARTS OF GRAVITATIONAL WAVE EVENTS AND OTHER TRANSIENT SIGNALS WITH HAWC(2019) Martinez Castellanos, Israel; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent years we have seen major advances in multi-messenger astronomy. A milestone was achieved by identifying the electromagnetic counterpart of the gravitational wave event GW170817 detected by LIGO and Virgo. Similar efforts led to a set of neutrinos detected by IceCube to be associated with the blazar TXS 0506+056. Both demonstrate the potential of using multiple types of probes to study an astrophysical source. The High-Altitude Water Cherenkov Observatory (HAWC), located in the state of Puebla, Mexico, is a wide field instrument (~2 sr) sensitive to very-high-energy gamma rays (~0.1-100 TeV) which can operate with a large duty cycle (>95%). These characteristics make it well suited to look for transient events correlated with other astronomical messengers. In this work we present a maximum likelihood analysis framework developed to search and analyze signals in HAWC data of arbitrary timescales. We apply this method to search for very-high-energy gamma-ray counterparts of gravitational waves in short timescales (0.3-1000 s). We show that we would be able to either detect or meaningfully constrain the very-high-energy component of a gamma-ray burst within the binary neutron star merger horizon of current gravitational wave detectors if it occurs in our field of view. We did not find evidence for emission for any of the events analyzed. The source location of GW170817 was not observable by HAWC at the time of the merger. We also set flux upper bounds for TXS 0506+056 during the periods when the neutrino flares were identified. For the flare between September 2014 and March 2015 these are the only available limits at very high energy, and are consistent with the low state in high-energy gamma rays reported by the Fermi-LAT Collaboration.Item Low-Latency Searches for Gravitational Waves and their Electromagnetic Counterparts with Advanced LIGO and Virgo(2019) Cho, Min-A; Shawhan, Peter S; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)For the first time in history, advanced detectors are available to observe the stretching and squeezing of space---gravitational waves---from violent astrophysical events. This opens up the prospect of joint detections with instruments of traditional astronomy, creating the new field of multi-messenger astrophysics. Joint detections allow us to form a coherent picture of the unfolding event as told by the various channels of information: mass and energy dynamics from gravitational waves, charged particle environments (along with magnetic field and specific element environments) from electromagnetic radiation, and thermonuclear reactions/relativistic particle outflows from neutrinos. In this work, I motivate low-latency electromagnetic and neutrino follow-up of sources known to emit gravitational radiation in the sensitivity band of ground-based interferometric detectors, Advanced LIGO and Advanced Virgo. To this end, I describe the low-latency infrastructure I developed with colleagues to select and enable successful follow-up of the first few gravitational-wave candidate events in history, including the first binary black hole merger, named GW150914, and binary neutron star coalescence, named GW170817, from the first and second observing runs. As a review, I outline the theory behind gravitational waves and explain how the advanced detectors, low-latency searches, and data quality vetting procedures work. To highlight the newness of the field, I also share results from an offline search for a more speculative source of gravitational waves, intersecting cosmic strings, from the second observing run. Finally, I address how LIGO/Virgo is prepared to adapt to challenges that will arise during the upcoming third observing run, an era that will be marked by near-weekly binary black hole candidate events and near-monthly binary neutron star candidate events. To handle this load, we made several improvements to our low-latency infrastructure, including a new, streamlined candidate event selection process, expansions I helped develop for temporal coincidence searches with electromagnetic/neutrino triggers, and data quality products on source classification and probability of astrophysical origin to provide to our observing partners for potential compact binary coalescences. These measures will further our prospects for multi-messenger astrophysics and increase our science returns.Item Inspiral-merger-ringdown models for spinning black-hole binaries at the interface between analytical and numerical relativity(2014) Taracchini, Andrea; Buonanno, Alessandra; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The long-sought direct detection of gravitational waves may only be a few years away, as a new generation of interferometric experiments of unprecedented sensitivity will start operating in 2015. These experiments will look for gravitational waves with frequencies from 10 to about 1000 Hz, thus targeting astrophysical sources such as coalescing binaries of compact objects, core collapse supernovae, and spinning neutron stars, among others. The search strategy for gravitational waves emitted by compact-object binaries consists in filtering the output of the detectors with template waveforms that describe plausible signals, as predicted by general relativity, in order to increase the signal-to-noise ratio. In this work, we modeled these systems through the effective-one-body approach to the general-relativistic 2-body problem. This formalism rests on the idea that binary coalescence is universal across different mass ratios, from the test-particle limit to the equal-mass regime. It bridges the gap between post-Newtonian theory (valid in the slow-motion, weak-field limit) and black-hole perturbation theory (valid in the small mass-ratio limit, but not limited to slow motion). The project unfolded along two main avenues of inquiry, with the goal of developing faithful inspiral-merger-ringdown waveforms for generic spinning, stellar-mass black-hole binaries. On the one hand, we studied the motion and gravitational radiation of test masses orbiting Kerr black holes in perturbation theory, with the goal of extracting strong-field information that can be incorporated into effective-one-body models. On the other hand, we worked at the interface between analytical and numerical relativity by calibrating effective-one-body models against numerical solutions of Einstein's equations, and testing their accuracy when extrapolated to different regions of the parameter space. In the course of this project, we also studied conservative effects of the 2-body dynamics, namely the periastron advance, and devised algorithms for generating realistic initial conditions for spinning, precessing black-hole binaries. The waveform models developed in this project will be employed in data-analysis pipelines and gravitational-wave searches of advanced LIGO and Virgo. In the near future, natural extensions of this work will be the inclusion of tidal effects in the comparable-mass regime (relevant for neutron-star/black-hole binaries), and spin precession in the test-particle limit.Item LOOC UP: SEEKING OPTICAL COUNTERPARTS TO GRAVITATIONAL WAVE SIGNALS(2011) Kanner, Jonah Brian; Shawhan, Peter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Far-reaching investigations into astrophysics, precision measurements of cosmological parameters, and tests of fundamental physics are expected to be enabled through observations of gravitational wave (GW) signals. The gravitational waveform traces the bulk motion of matter in distant sources, and so observations of GWs would reveal the details of interesting astrophysical mechanisms that are inaccessible by any other means. Currently, the most sensitive GW instruments in the world are the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo observatory. These kilometer scale observatories make use of sensitive optics to record miniscule changes in the space-time metric induced by GW signals, and are located at three sites in the United States and Europe. Some models for sources of observable, transient GW signals predict that an electromagnetic (EM) counterpart will accompany the GW signal. These counterparts would be identifiable as transients which fade over the course of hours or days. Finding the EM counterpart to a GW signal would present significant benefits. The EM counterpart could confirm the astrophysical origin of the signal, and would also enhance the scientific investigations possible with the observation. This work describes the first prompt search for EM counterparts to GW event candidates. For the search, carried out between December 2009 and October 2010, event candidates from the LIGO/Virgo network were identified with latencies of only a few minutes. The sky position of each potential source was estimated, and was delivered to a collection of radio, optical, and x-ray telescopes with roughly thirty minutes of latency. The telescopes then observed the estimated source position, in an attempt to discover an EM counterpart. The low-latency GW data analysis, the methods for estimating source positions, and the observing strategy that guides telescopes based on GW data are all novel features of the search. The ability of the LIGO/Virgo network to correctly localize sources on the sky was studied using a Monte Carlo simulation. These developments lay the groundwork for similar searches in the future with the next-generation GW detectors Advanced LIGO and Advanced Virgo.Item Improving analytical templates and searching for gravitational waves from coalescing black hole binaries(2010) Ochsner, Evan Lee; Buonanno, Alessandra; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Laser Interferometer Gravitational-wave Observatory (LIGO) and Virgo are taking data at design sensitivity. They will be upgraded to Advanced LIGO and Virgo within the next 5 years and the detection of gravitational waves will be very likely. Binaries of two compact objects which inspiral and coalesce are one of the most promising sources for LIGO and Virgo. Most searches have focused solely on the inspiral portion of the waveform, and are consequently limited to low total mass. Recent breakthroughs in numerical relativity allow one to construct complete inspiral-merger-ringdown waveforms and search for the whole signal. This thesis will review some of the basic characteristics of gravitational waves from compact binaries and methods of searching for them. Analytical template waveforms for such systems will be presented including a comparison of different families of analytical waveforms, a study on the inclusion of spin effects in such waveforms, and a study of inspiral-merger-ringdown waveforms with amplitude corrections and the importance of these effects for parameter estimation. The thesis will culminate with a presentation of the first gravitational wave search to use inspiral-merger-ringdown templates, which was performed on data from the fifth science run of LIGO.Item APPLYING NUMERICAL RELATIVITY TO GRAVITATIONAL WAVE ASTRONOMY(2008-03-12) McWilliams, Sean Thomas; Shawhan, Peter; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)General relativity predicts the existence of gravitational waves produced by the motion of massive objects. The inspiral, merger, and ringdown of black hole binaries is expected to be one of the brightest sources in the gravitational wave sky. Interferometric detectors, such as the current ground-based Laser Interferometer Gravitational Wave Observatory (LIGO) and the future space-based Laser Interferometer Space Antenna (LISA), measure the influx of gravitational radiation from the whole sky. Each physical process that emits gravitational radiation will have a unique waveform, and prior knowledge of these waveforms is needed to distinguish a signal from the noise inherent in the interferometer. In the strong field regime of the merger, only numerical relativity, which solves the full set of Einstein's equations numerically, has been able to provide accurate waveforms. We present a comprehensive study of the nonspinning portion of parameter space for which we have generated accurate simulations of the late inspiral through merger and ringdown, and a comparison of those results with predictions from the adiabatic Taylor-expanded post-Newtonian (PN) and effective-one-body (EOB) PN approximations. We then focus on data analysis questions using the equal-mass nonspinning as well as the 2:1, 4:1, and 6:1 mass ratio nonspinning black hole binary (BHB) waveforms. We construct a full waveform by combining our results from numerical relativity with a highly accurate Taylor PN approximation, and use it to calculate signal-to-noise ratios (SNRs) for several detectors. We measure the mass ratio scaling of the waveform amplitude through the inspiral and merger, and compare our observations with predictions from PN. Lastly, we turn our focus to parameter estimation with LISA, and investigate the increased accuracy with which parameters can be measured by including both the merger and inspiral waveforms, compared to estimates without numerical waveforms which can only incorporate the inspiral. We use the equal mass, nonspinning waveform as a test case and assess the parameter uncertainty by means of the Fisher matrix formalism.Item Growing Intermediate-Mass Black Holes with Gravitational Waves(2006-06-05) Gultekin, Kayhan; Miller, M. Coleman; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present results of numerical simulations of sequences of binary-single scattering events of black holes in dense stellar environments. The simulations cover a wide range of mass ratios from equal mass objects to 1000:10:10 solar masses and compare purely Newtonian simulations with a relativistic endpoint, simulations in which Newtonian encounters are interspersed with gravitational wave emission from the binary, and simulations that include the effects of gravitational radiation reaction by using equations of motion that include the 2.5-order post-Newtonian force terms, which are the leading-order terms of energy loss from gravitational waves. In all cases, the sequence is terminated when the binary's merger time due to gravitational radiation is less than the arrival time of the next interloper. We also examine the role of gravitational waves during an encounter and show that close approach cross-sections for three 1-solar-mass objects are unchanged from the purely Newtonian dynamics except for close approaches smaller than 0.00001 times the initial semimajor axis of the binary. We also present cross-sections for mergers resulting from gravitational radiation during three-body encounters for a range of binary semimajor axes and mass ratios including those of interest for intermediate-mass black holes (IMBHs). We find that black hole binaries typically merge with a very high eccentricity --- extremely high when gravitational waves are included during the encounter such that when the gravitational waves are detectable by LISA, most of the binaries will have eccentricities e > 0.9 though all will have circularized by the time they are detectable by LIGO. We also investigate the implications for the formation and growth of IMBHs and find that the inclusion of gravitational waves during the encounter results in roughly half as many black holes ejected from the host cluster for each black hole accreted onto the growing IMBH. The simulations show that the Miller & Hamilton model of IMBH formation is a viable method if it is modified to start with a larger seed mass.