Theses and Dissertations at Montana State University (MSU)
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Item The structure of energy-extracting black hole magnetospheres(Montana State University - Bozeman, College of Letters & Science, 2019) Thoelecke, Kevin; Chairperson, Graduate Committee: Yves U. IdzerdaSpinning black holes can store enormous amounts of rotational energy. Efficiently extracting that rotational energy can lead to significant energy outflows capable of powering very high energy astrophysical phenomena, such as gamma-ray bursts and active galactic nuclei. Black holes are unique in that they do not exist as physical objects in the same way a rock, planet, or star exists; instead, black holes exist only as spacetime curvature. As such processes for extracting a black hole's rotational energy are largely unique to black holes. This work explores one such process, the extraction of a black hole's rotational energy via an appropriately configured magnetosphere. Both analytic perturbation techniques and numerical codes are developed in order to solve for thousands of energy-extracting black hole magnetospheres. Those magnetospheres broadly sample the relevant solution space, allowing correlations to be drawn between different rates of black hole rotational energy and angular momentum extraction and global magnetosphere structure. The most fundamental behavior discovered is that magnetospheres that extract the most energy per unit angular momentum direct that energy away from the black hole's rotational axis, while magnetospheres that extract the least amount of energy per unit angular momentum direct that energy into jet-like structures aligned with the black hole's rotational axis. Exploration of the solutions obtained also suggests that magnetospheres most compatible with nearby accreting matter can very naturally launch jets, implying that black hole energy extraction and jet launching are likely to be concurrent and common features of astrophysical black hole magnetospheres.Item Testing alternative theories of gravity using low frequency gravitational waves(Montana State University - Bozeman, College of Letters & Science, 2019) O'Beirne, Logan Tyler; Chairperson, Graduate Committee: Neil J. Cornish; Bennett Link (co-chair); Logan O'Beirne, Stephen R. Taylor and Nicolas Yunes were co-authors of the article, 'Constraining alternative theories of gravity using pulsar timing arrays' in the journal 'Physical review letters' which is contained within this thesis.; Neil J. Cornish were co-authors of the article, 'Constraining the polarization content of gravitational waves with astrometry' in the journal 'Physical review D' which is contained within this thesis.; Neil J. Cornish, Sarah J. Vigeland and Stephen R. Taylor were co-authors of the article, 'Constraining alternative polarizations of continuous gravitational waves using pulsar timing arrays' submitted to the journal 'Physical review D' which is contained within this thesis.General Relativity aptly describes current gravitational observations. However, there is great theoretical interest in its validity in untested regimes. Alternative theories of gravity attempt to relax some of the assumptions made, leaving distinct signatures that are absent in Einstein's theory, namely the presence of alternative polarizations of gravitational waves that manifest from the emission of gravitational scalar and vector dipole radiation in black hole binaries. To study this lower order multipole of radiation, it is desirable to work in a regime where the quadrupolar tensor radiation of general relativity is as quiet as possible. This motivates working with supermassive black hole binaries in their slowly evolving inspiral phase, when they are well separated from merger, emitting low frequency gravitational waves. Using a frequentist framework, we study the detectability of a stochastic background of each polarization using pulsar timing arrays, which is currently the most technically developed and viable method for studying low frequency gravitational waves, correlating the observed time delays of pulsars. We also find that astrometry, which measures transverse displacements of the apparent position of stars, turns out to have a very similar correlation structure as the time delays measured by pulsar timing arrays. We lastly study how effective using a pulsar timing array is at studying a loud, foreground binary with these alternative polarizations, using a Bayesian framework. Low frequency gravitational wave astronomy proves advantageous for studying these exotic signatures.Item Rotational and thermal dynamics of neutron stars(Montana State University - Bozeman, College of Letters & Science, 2012) Price, Steven Curtis; Chairperson, Graduate Committee: Bennett LinkThis thesis explores the rotational and thermal dynamics of neutron stars. All neutron stars exhibit irregularities in their spin rates, which may be evidence of coupling between the solid crust and liquid components in the interior. We study short-time scale correlations in the stochastic variations in spin rate, timing noise, in 32 pulsars. Upon subtraction of low frequency wander, we find that in two stars a fluctuation in rotational phase at a given time is correlated with past fluctuations over a correlation time of ~̃ 10-40 d; over longer times, the fluctuations are uncorrelated. We interpret this result as the signature of a damped rotational mode in the star, excited by the noise process, and likely due to friction between the crust and interior liquid. In the second part of this thesis, we investigate the thermal and magnetic evolution of highly magnetized neutron stars, magnetars. We explore a thermo-resistive instability in the outer crusts of magnetars wherein a perturbation in temperature increases ohmic heating. We show that magnetars of characteristic age T age ~ 10 4 yr are unstable over timescales as short as days if strong current sheets are present in the outer crust. This instability could play an important role in the thermal and magnetic field evolution of highly magnetized neutron stars, and may be related to bursting activity in magnetars.Item Characterizing astrophysical sources of gravitational waves(Montana State University - Bozeman, College of Letters & Science, 2010) Key, Joey Shapiro; Chairperson, Graduate Committee: Neil J. CornishThe Laser Interferometer Space Antenna (LISA) and the Laser Interferometer Gravitational-wave Observatory (LIGO) are designed to detect gravitational waves from a wide range of astrophysical sources. The parameter estimation ability of these detectors can be determined by simulating the response to predicted gravitational wave sources with instrument noise and searching for the signals with sophisticated data analysis methods. A possible source of gravitational waves will be beams of radiation from discontinuities on cosmic length strings. Cosmic strings are predicted to form kinks and cusps that travel along the string at close to the speed of light. These disturbances are radiated away as highly beamed gravitational waves that produce a burst-like pulse as the cone of emission sweeps past an observer. The detection of a gravitational wave signal from a cosmic string cusp would illuminate the fields of string theory, cosmology, and relativity. Gravitational wave sources also include coalescing binary systems of compact objects. Colliding galaxies have central black holes that sink to the center of the merged galaxy and begin to orbit one another and emit gravitational waves. Previous LISA data analysis studies have assumed that binary black hole systems have a circular orbit or an extreme mass ratio. It is ultimately necessary to understand the general case of spinning black hole binary systems in eccentric orbits and how LISA observations can be used to measure the eccentricity of the orbits as well as the masses, spins, and luminosity distances of the black holes. Once LISA is operational, the comparison of observations of eccentric and circular black hole binary sources will constrain theories on galaxy mergers in the early universe.Item Studying cosmological sources of gravitational waves(Montana State University - Bozeman, College of Letters & Science, 2010) Corbin, Vincent Dominique Andre; Chairperson, Graduate Committee: Neil J. CornishThis dissertation presents two aspects of the study of cosmology through gravitational waves. The first aspect involves direct observation of past eras of the Universe's formation. The detection of the Cosmic Microwave Background Radiation was one of the most important cosmological discoveries of the last century. With the development of interferometric gravitational wave detectors, we may be in a position to detect its gravitational equivalent in this century. The Cosmic Gravitational Background is likely to be isotropic and stochastic, making it difficult to distinguish from instrument noise. The contribution from the gravitational background can be isolated by cross-correlating the signals from two or more independent detectors. Here we extend previous studies that considered the cross-correlation of two Michelson channels by calculating the optimal signal to noise ratio that can be achieved by combining the full set of interferometry variables that are available with a six link triangular interferometer. We apply our results to the detector design described in the Big Bang Observer mission concept study and find that it could detect a background with Omega gw > 2.2 x 10 -¹⁷. The second aspect consists in studying astrophysical sources that detain crucial information on the Universe's evolution. We focus our attention on black holes binary sytems. These systems contain information on the rate of merger between galaxies, which in turn is key to unlock the mystery of inflation. Pulsar timing is a promising technique for detecting low frequency sources of gravitational waves, such as massive and supermassive black hole binaries. Here we show that the timing data from an array of pulsars can be used to recover the physical parameters describing an individual black hole binary to good accuracy, even for moderately strong signals. A novel aspect of our analysis is that we include the distance to each pulsar as a search parameter, which allows us to utilize the full gravitational wave signal. This doubles the signal power, improves the sky location determination by an order of magnitude, and allows us to extract the mass and the distance to the black hole binary.Item A comprehensive Bayesian approach to gravitational wave astronomy(Montana State University - Bozeman, College of Letters & Science, 2009) Littenberg, Tyson Bailey; Chairperson, Graduate Committee: Neil J. CornishThe challenge of determining whether data from a gravitational wave detector contains signals which are cosmic in origin is the central problem in gravitational wave astronomy. The "detection problem" is particularly challenging for low amplitude signals embedded in "glitchy" instrument noise. It is imperative that we can robustly distinguish between the data being consistent with instrument noise alone, or noise and a weak gravitational wave signal. In response to this challenge we have set out to develop a robust, general purpose approach that can locate and characterize gravitational wave signals, and provided odds that the signal is of cosmic origin. Our approach employs the Markov Chain Monte Carlo family of algorithms to construct a fully Bayesian solution to the challenge - the Parallel Tempered Markov Chain Monte Carlo (PTMCMC) detection algorithm. The PTMCMC detection algorithm establishes which regions of parameter space contain the highest posterior weight, efficiently explores the posterior distribution function of the model parameters, and calculates the marginalized likelihood, or evidence, for the models under consideration. We illustrate our approach using simulated LISA and LIGO-Virgo data.