Physics Theses and Dissertations

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https://hdl.handle.net/1794/2048

This collection contains some of the theses and dissertations produced by students in the University of Oregon Physics Graduate Program. Paper copies of these and other dissertations and theses are available through the UO Libraries.

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Browsing Physics Theses and Dissertations by Subject "Astrophysics"

Now showing 1 - 5 of 5
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    Accurate and Precise Calibration of Advanced LIGO Detectors in the Era of Gravitational Wave Astronomy
    (University of Oregon, 2019-04-30) Karki, Sudarshan; Frey, Raymond
    The first direct detection of gravitational waves in 2015, and the multiple detections that followed ushered in the era of gravitational-wave astronomy. With these developments, the focus of the gravitational-wave community shifted from detection to precision measurement, requiring a factor of ten improvement in calibration accuracy to maximize the astrophysical information that can be extracted from these detected signals. This dissertation discusses the implementation and characterization of a radiation-pressure-based calibration system called the Photon calibrator that is used as the primary calibration reference for the Advanced LIGO detectors. It also discusses the techniques and procedures used to realize sub-percent accuracy calibration of absolute displacement fiducials introduced using the Photon calibrator system during Advanced LIGO’s first and second observing runs. Using the Photon calibrator systems, frequency dependent calibration of the interferometer responses was achieved at the level of 2-3% in magnitude and 3- 5 degrees in phase across the LIGO detection band. This level of calibration accuracy has already played a significant role in extracting astrophysical parameters from LIGO’s detections. With the LIGO and Virgo detectors operating at design sensitivity, updated rate estimates indicate that measurement of the Hubble constant with gravitational waves with 1% accuracy will be possible within the next decade. This will require absolute amplitude calibration of the detectors at the sub-1% level. This dissertation also discusses the improvements that have been implemented in the Photon calibrator systems that will reduce the uncertainty in absolute displacement to below 0.5%. The gravitational waves from the post-merger phase of binary neutron stars are expected to contain interesting features at frequencies up to few kHz, carrying rich information about neutron-star astrophysics. This dissertation discusses the calibration errors introduced by test mass deformations caused by calibration forces at frequencies above 1 kHz. The errors, estimated using Finite Element Analysis, is in reasonable agreement with measurement results in the 1 to 5 kHz band. These investigations have enabled the reduction of calibration uncertainty at these frequencies, which should enhance our ability to decipher the neutron star astrophysics encoded in the gravitational wave signals from the post-merger phase. This dissertation includes previously published co-authored material.
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    Environmental Noise in Gravitational Wave Detectors and the Search for Gravitational Wave Signals Associated with Gamma-Ray Burst During LIGO'S Third Observing Run
    (University of Oregon, 2022-10-26) Nguyen, Philippe; Frey, Raymond
    This dissertation describes methods used for characterizing the sources and effects of environmental noise in Advanced LIGO detectors from the end of the second LIGO-Virgo observing run through the end of the third observing run.We present the vibrational and magnetic noise budgets for the Hanford and Livingston observatories, results of noise studies focusing on scattered light sources in the Hanford detector and input beam jitter at both detectors, and a system of routine magnetic coupling measurements to track changes throughout an observing run. We describe the implementation of an automated pipeline for determining the effect of environmental noise signals on gravitational wave events. Noise plays a critical role in understanding the limitations and areas of potential improvement for analyses searching for gravitational waves.We discuss the targeted search for gravitational waves associated with gamma-ray bursts during the third observing run. No gravitational waves were detected coincident with the 86 GRBs analyzed. New upper limit exclusion distances are set for various signal waveform types, and we report exclusion probabilities for specific models. We discuss the relevance of transient noise on the performance of unmodeled search pipelines.
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    Host stellar population properties and the observational selection function of type Ia supernovae
    (University of Oregon, 2010-09) Johnson, Elsa M., 1971-
    Supernovae Ia are viable standard candles for measuring cosmological distances because of their enormous light output and similar intrinsic brightness. However, dispersion in intrinsic brightness casts doubt on the overall reliability of supernovae as cosmological distance indicators. Moreover, as shown in this thesis, the dependence of peak brightness on host galaxy properties significantly contributes to this dispersion. As a result, there is good reason to doubt that the nearby sample of supernovae Ia is identical to the distant samples, which occur in host galaxies that are billions of years younger. This study explores the validity of supernovae Ia as standard candles by examining regions of nearby galaxies that hosted supernovae and modeling their observational selection function. The approach is two-fold. First, photometry is performed on the stellar population environment of supernovae to characterize that region as a function of supernova type. Then, the observational selection function is simulated to determine the true supernovae production rate of the z < 0.1 redshift limit. We find that, on average, type Ia events occur in redder and older populations; underluminous supernovae Ia occur in regions that seem to be preferentially dusty, whereas normal Ia coming from the same galaxy type occur in a wide range of extinction environments. Furthermore, redder peak colors correspond to redder underlying population colors. This finding implies that dust extinction effects can cause systematic errors in the luminosity calibration of Ia events Finally, a single supernova rate does not adequately describe all supernovae Ia within z < 0.1. A rate of 0.25 SNu describes the population up to z < 0.03, and a much smaller rate, 0.1 SNu or less, describes supernovae past this distance. This finding indicates that observed supernova rates per galaxy remain biased by sample selection effects and that the intrinsic rate is likely uncertain by a factor of 2 to 3.
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    Linear stability analysis of nonaxisymmetric instabilities in self-gravitating polytropic disks
    (University of Oregon, 2011-03) Hadley, Kathryn Z., 1955-
    An important problem in astrophysics involves understanding the formation of planetary systems. When a star-forming cloud collapses under gravity its rotation causes it to flatten into a disk. Only a small percentage of the matter near the rotation axis falls inward to create the central object, yet our Sun contains over 99% of the matter of our Solar System. We examine how global hydrodynamic instabilities transport angular momentum through the disk causing material to accrete onto the central star. We analyze the stability of polytropic disks in the linear regime. A power law angular velocity of power q is imposed, and the equilibrium disk structure is found through solution of the time-independent hydrodynamic equations via the Hachisu self-consistent field method. The disk is perturbed, and the time-dependent linearized hydrodynamic equations are used to evolve it. If the system is unstable, the characteristic growth rate and frequency of the perturbation are calculated. We consider modes with azimuthal e im[varphi] dependence, where m is an integer and [varphi] is the azimuthal angle. We map trends across a wide parameter space by varying m , q and the ratios of the star-to-disk mass M * /M d and inner-to-outer disk radius r - /r + . We find that low m modes dominate for small r - /r + , increasing to higher r - /r + as M * /M d increases, independent of q . Three main realms of behavior are identified, for M * << M d , M * [approximate] M d and M * >> M d , and analyzed with respect to the I, J and P mode types as discussed in the literature. Analysis shows that for M * << M d , small r - /r + disks are dominated by low m I modes, which give way to high m J modes at high r - /r + . Low m J modes dominate M * [approximate] M d disks for small r - /r + , while higher m I modes dominate for high r - /r + . Behavior diverges with q for M * >> M d systems with high q models approximating M * [approximate] M d characteristics, while low q models exhibit m = 2 I modes dominating where r - /r + < 0.60.
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    A nonlinear investigation of corrugation instabilities in magnetic accretion shocks
    (University of Oregon, 2011-03) Ernst, Scott
    Accretion shock waves are present in many important astrophysical systems and have been a focus of research for decades. These investigations provide a large body of understanding as to the nature, characteristics, and evolutionary behaviors of accretion shock waves over a wide range of conditions. However, largely absent are investigations into the properties of accretion shock waves in the presence of strong magnetic fields. In such cases these strong magnetic fields can significantly alter the stability behaviors and evolution of the accretion shock wave through the production and propagation of magnetic waves as well as magnetically constrained advection. With strong magnetic fields likely found in a number of accretion shock systems, such as compact binary and protostellar systems, a better understanding of the behaviors of magnetic accretion shock waves is needed. A new magnetohydrodynamics simulation tool, IMOGEN, was developed to carry out an investigation of instabilities in strong, slow magnetic accretion shocks by modelling their long-term, nonlinear evolution. IMOGEN implements a relaxed, second-order, total variation diminishing, monotonic upwind scheme for conservation laws and incorporates a staggered-grid constrained transport scheme for magnetic advection. Through the simulated evolution of magnetic accretion shocks over a wide range of initial conditions, it has been shown, for sufficiently high magnetic field strengths, that magnetic accretion shocks are generally susceptible to corrugation instabilities, which arise in the presence of perturbations of the initial shock front. As these corrugation instabilities grow, they manifest as magnetic wave propagation in the upstream region of the accretion column, which propagate away from the accretion shock front, and as density columns, or fingers, that grow into the higher density downstream flow, defined and constrained by current loops created during the early evolution of the instability.