Topics in solid-state astrophysics: Magnetized neutron star crusts and multicomponent crusts/white dwarfs

Engstrom, Tyler A The Pennsylvania State University 2015 해외박사(DDOD)

Two research endeavors are described in this dissertation; both undertake problems in solid-state astrophysics, which is a branch of solid-state physics concerning the extreme conditions found within white dwarfs and the solid crusts of neutron stars. As much of our knowledge about these compact objects comes from observation of astrophysical phenomena, Chapter 1 is devoted to the phenomena, and how they can be exploited as material property probes. Several of the most interesting phenomena involve the enormous magnetic fields (B ≥ 1012 gauss) harbored by many neutron stars, and the interaction between these fields and the charged particles within the solid crust. Accordingly, Chapter 2 reviews some theory of strongly-magnetized electrons, which both sets the stage for Chapter 3, and (hopefully) serves as a useful reference for future research. Let it now be made clear that this dissertation focuses exclusively on the "outer crusts," of neutron stars, where no free neutrons are present (rho < 4x1011 g/cc), and the similarly-composed interiors of white dwarfs, which have central densities ∼ 107 g/cc. For the most part we specialize to even lower densities. In Chapter 3, static and dynamic properties of low density (rho ≥ 106 g/cc) outer envelopes of neutron stars are calculated within the nonlinear magnetic Thomas-Fermi model, assuming degenerate electrons. A novel domain decomposition enables proper description of lattice symmetry and may be seen as a prototype for the general class of problems involving nonlinear charge screening of periodic, quasi-low-dimensionality structures, e.g. liquid crystals. We describe a scalable implementation of the method using Hypre. Over the density range considered, the effective shear modulus appears to be a factor of ≈ 20 larger than in the linearlyscreened Coulomb crystal model, which could have implications for observables related to astroseismology as well as low temperature phonon-mediated thermal conductivity. Other findings include incipient c' < 0 elastic instabilities for both bcc and fcc lattices, reminiscent of the situation in some light actinides, and suggestive of a symmetry-lowering transition to a tetragonal or orthorhombic lattice. Chapter 4 describes a systematic search for multicomponent crystal structures, carried out for five different ternary systems of nuclei in a polarizable background of electrons, representative of accreted neutron star crusts and some white dwarfs. Candidate structures are "bred" by a genetic algorithm, and optimized at constant pressure under the assumption of linear response (Thomas-Fermi) charge screening. Subsequent phase equilibria calculations reveal eight distinct crystal structures in the T = 0 bulk phase diagrams, five of which are complicated multinary structures not before predicted in the context of compact object astrophysics. Frequent instances of geometrically similar but compositionally distinct phases give insight into structural preferences of systems with pairwise Yukawa interactions, including and extending to the regime of low density colloidal suspensions made in a laboratory. As an application of these main results, we self-consistently couple the phase stability problem to the equations for a self-gravitating, hydrostatically stable white dwarf, with fixed overall composition. To our knowledge, this is the first attempt to incorporate complex multinary phases into the equilibrium phase layering diagram and mass-radius-composition dependence, both of which are reported for He-C-O and C-O-Ne white dwarfs. Finite thickness interfacial phases ("interphases") show up at the boundaries between single-component bcc crystalline regions, some of which have lower lattice symmetry than cubic. A second application---quasi-static settling of heavy nuclei in white dwarfs---builds on our equilibrium phase layering method. Tests of this nonequilibrium method reveal extra phases which play the role of transient host phases for the settling species.

An Analysis of Astrophysics and Fundamental Physics from the Lyman-alpha Forest

Day, Aaron University of California, San Diego 2013 해외박사(DDOD)

The Lyα forest, made up of the cosmologically redshifted hydrogen Lyα absorption lines in the spectra of distant quasars, is a sensitive probe of cosmology and astrophysics. The shape and distribution of the Lyα absorption lines is deter- mined by the column densities, peculiar velocities, redshifts, turbulent velocities, temperatures, and the distribution of systems of neutral hydrogen in the space between galaxies known as the intergalactic medium. The Lyα forest is sensitive to the total cosmological matter power spectrum at the smallest available scales. It provides a long lever arm for constraints on the shape of the matter power spectrum when used in conjunction with measurements at large scales, such as the cosmic microwave background. It also has the potential to be used to detect the small scale matter power suppression caused by the free streaming of fundamental particles including known active light neutrinos or potential warm dark matter candidates. Focusing on the line widths, optical depth, and the power spectrum of the flux of the Lyα forest, we find that previously published observations do not match the outputs of fully hydrodynamic cosmological computer simulations. We present a new measurement of the power spectrum of the flux of the Lyα forest, using 91 quasar spectra taken with the Keck HIRES spectrograph. This is the largest high resolution data set to date by a factor of about 3. The high resolution of our data sample gives fully resolved absorption lines. Compared to the low resolution data set from SDSS, the HIRES data allows for more accurate subtraction of metal line contamination, as well as a measurement of the flux power at the smallest scales. Finally, we run a suite of 44 new computer simulations, varying cosmological and astrophysical parameters in an attempt to find a model that matches our observational data. We find that no standard cosmological or astrophysical parameters provide an acceptable match. This calls into question previous constraints on cosmological parameters made from Lyα forest data sets.

Observations and Modeling of Merging Galaxy Clusters

Golovich, Nathan Ryan University of California, Davis ProQuest Dissertat 2017 해외박사(DDOD)

Context: Galaxy clusters grow hierarchically with continuous accretion bookended by major merging events that release immense gravitational potential energy (as much as ∼1065 erg). This energy creates an environment for rich astrophysics. Precise measurements of the dark matter halo, intracluster medium, and galaxy population have resulted in a number of important results including dark matter constraints and explanations of the generation of cosmic rays. However, since the timescale of major mergers (∼several Gyr) relegates observations of individual systems to mere snapshots, these results are difficult to understand under a consistent dynamical framework. While computationally expensive simulations are vital in this regard, the vastness of parameter space has necessitated simulations of idealized mergers that are unlikely to capture the full richness. Merger speeds, geometries, and timescales each have a profound consequential effect, but even these simple dynamical properties of the mergers are often poorly understood. A method to identify and constrain the best systems for probing the rich astrophysics of merging clusters is needed. Such a method could then be utilized to prioritize observational follow up and best inform proper exploration of dynamical phase space. Task: In order to identify and model a large number of systems, in this dissertation, we compile an ensemble of major mergers each containing radio relics. We then complete a pan-chromatic study of these 29 systems including wide field optical photometry, targeted optical spectroscopy of member galaxies, radio, and X-ray observations. We use the optical observations to model the galaxy substructure and estimate line of sight motion. In conjunction with the radio and X-ray data, these substructure models helped elucidate the most likely merger scenario for each system and further constrain the dynamical properties of each system. We demonstrate the power of this technique through detailed analyses of two individual merging clusters. Each are largely bimodal mergers occurring in the plane of the sky. We build on the dynamical analyses of Dawson (2013b) and Ng et al. (2015) in order to constrain the merger speeds, timescales, and geometry for these two systems, which are among a gold sample earmarked for further follow up. Findings: MACS J1149.5+2223 has a previously unidentified southern subcluster involved in a major merger with the well-studied northern subcluster. We confirm the system to be among the most massive clusters known, and we study the dynamics of the merger. MACS J1149.5+2223 appears to be a more evolved system than the Bullet Cluster observed near apocenter. ZwCl 0008.8+5215 is a less massive but a bimodal system with two radio relics and a cool-core "bullet" analogous to the namesake of the Bullet Cluster. These two systems occupy different regions of merger phase space with the pericentric relative velocities of ∼2800 km s--1 and ∼1800 km s--1 for MACS J1149.5+2223 and ZwCl 0008.8+5215, respectively. The time since pericenter for the observed states are ∼1.2 Gyr and ∼0.8 Gyr, respectivel. In the ensemble analysis, we confirm that radio relic selection is an efficient trigger for the identification of major mergers. In particular, 28 of the 29 systems exhibit galaxy substructure aligned with the radio relics and the disturbed intra-cluster medium. Radio relics are typically aligned within 20° of the axis connecting the two galaxy subclusters. Furthermore, when radio relics are aligned with substructure, the line of sight velocity difference between the two subclusters is small compared with the infall velocity. This strongly implies radio relic selection is an efficient selector of systems merging in the plane of the sky. While many of the systems are complex with several simultaneous merging subclusters, these systems generally only contain one radio relic. Systems with double radio relics uniformly suggest major mergers with two dominant substructures well aligned between the radio relics. Conclusions: Radio relics are efficient triggers for identifying major mergers occurring within the plane of the sky. This is ideal for observing offsets between galaxies and dark matter distributions as well as cluster shocks. Double radio relic systems, in particular, have the simplest geometries, which allow for accurate dynamical models and inferred astrophysics. Comparing and contrasting the dynamical models of MACS J1149.5+2223 and ZwCl 0008.8+5215 with similar studies in the literature (Dawson, 2013b; Ng et al., 2015; van Weeren et al., 2017), a wide range of dynamical phase space (∼ 1500 -- 3000 km --1 at pericenter and ∼ 500 -- 1500 Myr after pericenter) may be sampled with radio relic mergers. With sufficient samples of bimodal systems, velocity dependence of underlying astrophysics may be uncovered. (Abstract shortened by ProQuest.).

Development of Kinetic Inductance Detectors for Far-infrared Spectroscopy in Astrophysics

Barlis, Alyssa ProQuest Dissertations & Theses University of Penn 2019 해외박사(DDOD)

This thesis presents the development of kinetic inductance detectors targeted for applications in far-infrared spectroscopy in astrophysics. The formation and evolution of galaxies across cosmic time is one of the key areas of exploration in modern astrophysics. The star formation rate density peaks at a redshift of around z=2, when the universe was dominated by dusty star-forming galaxies whose optical and ultraviolet radiation are significantly obscured and thermally reprocessed by dust into infrared radiation. A swath of fine-structure lines in the far-infrared serve as tracers of star formation activity in these galaxies, and far-infrared continuum and line emission are unobscured by dust. However, detecting these lines in individual galaxies is difficult and time consuming with currently-available infrared instruments.The Terahertz Intensity Mapper (TIM) experiment is a balloon-borne telescope spectrometer that will observe these galaxies leading back to the era of peak star formation. TIM will use the intensity mapping technique to create a three-dimensional map, incorporating the spectral dimension as the line-of-sight coordinate. This measurement will survey the aggregate star-formation activity as a function of redshift of the total galaxy population without a flux limit. TIM will incorporate two grating spectrometer modules to observe the 240-420 micron wavelength band with spectral resolution R = 250, each with 1800 low-noise kinetic inductance detectors (KIDs) in its focal plane.I present the development and testing of prototype KID arrays targeted for use on TIM. KIDs are superconducting microresonators that serve as radiation detectors. They rely on the kinetic inductance effect, which causes a shift in resonant behavior when incident photons are absorbed by Cooper pairs in the superconductor material. I present characterization results from two 45-pixel KID arrays fabricated out of thin-film aluminum on silicon substrates. I demonstrate that their device performance meets the sensitivity and noise requirements for the TIM experiment.

Few-Body Problem in Astrophysics and Its Applications

Wang, Yihan State University of New York at Stony Brook ProQue 2022 해외박사(DDOD)

The few-body problem, that is how three or more particles gravitationally interact, has played a fundamental role in astrophysics. As the system cannot be solved analytically, numerical methods are typically used to study its evolution. However, due to the chaotic nature of the few-body problem and numerical errors, the long-term evolution of the few-body system is difficult to predict. I present a new few-body gravity integration toolkit. It contains several novel techniques which make it easier to accurately and precisely track the long-term evolution of few-body systems with extreme conditions, such as high eccentricities and extreme mass ratios that are quite common in astrophysics. With this tool, I have successfully estimated the cosmic merger rate of bound compact object binaries dynamically interacting with supermassive black hole binaries and explored the fate of dynamical encounters between black holes binaries in the vicinity of supermassive black hole binaries, which would help us to better understand the origin and merger process of black hole binaries. I also investigated the evolution of planetary systems in dense clusters which could be a promising way to explain the unexpected correlation between hot Jupiters and star clusters.

Semi-analytic modeling of stellar populations in astrophysical simulations

Crosby, Brian David Michigan State University 2016 해외박사(DDOD)

Understanding galaxy formation is an enduring questions in astrophysics. Galaxies are systems rich in interesting physical processes, and the vast range of times and environments in which galaxies form and evolve offer a wealth of challenges. One of the primary driving forces in galaxies is the interaction of stellar populations with their surroundings. The nature of this interaction drives the evolution of both components, and the resulting behavior has a profound impact on the observable universe. In this dissertation I discuss the results of modeling this interaction in a variety of contexts using semi-analytic methods in conjunction with high-performance numerical simulations to bridge the huge dynamic ranges spanned by these processes. With these techniques I explore the environments in which Population III stars form, studying the transition to chemically-enriched star formation, and quantifying the changing environment and assembly history of the dark matter halos which host Population III stars in a universe of increasingly chemical complexity. Chemical enrichment in high redshift proto-galaxies is investigated by coupling semi-analytic models of star formation and feedback to cosmological N-body simulations. The resulting elemental abundance ratios are compared to those observed in metal-poor stars and satellite systems of the Milky Way, with the comparison constraining the nature of Population III stars, galaxy formation at high redshift, and the transition from metal-free to chemically enriched star formation. Ensembles of semi-analytic models representing internal galactic processes are used to develop a new formalism for representing galaxies in cosmological simulations of galaxy clusters. This method is used to investigate galaxy formation in a cluster environment and the interaction between cluster galaxies and the intracluster medium. The interaction between stellar populations in disk galaxies and the diffuse circumgalactic medium is studied in simulations of idealized disk galaxies. The interplay of stellar feedback and the development of multiphase gas in the circumgalactic medium, and in turn the influence of this multiphase gas falling back onto the stellar disk is investigated.

The Astrophysics of Strongly Interacting Systems

Nerella, Tejaswi Venumadhav California Institute of Technology 2016 해외박사(DDOD)

This thesis presents investigations in four areas of theoretical astrophysics: the production of sterile neutrino dark matter in the early Universe, the evolution of small-scale baryon perturbations during the epoch of cosmological recombination, the effect of primordial magnetic fields on the redshifted 21-cm emission from the pre-reionization era, and the nonlinear stability of tidally deformed neutron stars. In the first part of the thesis, we study the asymmetry-driven resonant production of 7 keV-scale sterile neutrino dark matter in the primordial Universe at temperatures T >~ 100 MeV. We report final DM phase space densities that are robust to uncertainties in the nature of the quark-hadron transition. We give transfer functions for cosmological density fluctuations that are useful for N-body simulations. We also provide a public code for the production calculation. In the second part of the thesis, we study the instability of small-scale baryon pressure sound waves during cosmological recombination. We show that for relevant wavenumbers, inhomogenous recombination is driven by the transport of ionizing continuum and Lyman-alpha photons. We find a maximum growth factor less than ≈ 1.2 in 107 random realizations of initial conditions. The low growth factors are due to the relatively short duration of the recombination epoch. In the third part of the thesis, we propose a method of measuring weak magnetic fields, of order 10--19 G (or 10--21 G if scaled to the present day), with large coherence lengths in the inter galactic medium prior to and during the epoch of cosmic reionization. The method utilizes the Larmor precession of spin-polarized neutral hydrogen in the triplet state of the hyperfine transition. We perform detailed calculations of the microphysics behind this effect, and take into account all the processes that affect the hyperfine transition, including radiative decays, collisions, and optical pumping by Lyman-alpha photons. In the final part of the thesis, we study the non-linear effects of tidal deformations of neutron stars (NS) in a compact binary. We compute the largest three- and four-mode couplings among the tidal mode and high-order p- and g-modes of similar radial wavenumber. We demonstrate the near-exact cancellation of their effects, and resolve the question of the stability of the tidally deformed NS to leading order. This result is significant for the extraction of binary parameters from gravitational wave observations.

CMB Polarization Measurements with the POLARBEAR Experiment

Boettger, David University of California, San Diego 2014 해외박사(DDOD)

POLARBEAR is an experiment designed to measure the polarization of the cosmic microwave background (CMB). These measurements have scientific objectives spanning particle physics, cosmology, and astrophysics. The field of CMB polarization is rapidly evolving, with extraordinary progress being made on several fronts including the recently claimed detection of gravitational waves generated during cosmic inflation. This dissertation describes the POLARBEAR instrument and the measurements taken during its first season of observations. The results include the first evidence for a non-zero Cℓ BB power spectrum at sub-degree angular scales, consistent with theoretical expectations.

Cosmic gamma-ray propagation as a probe for intergalactic media and interactions

Huan, Hao The University of Chicago 2012 해외박사(DDOD)

Very-high-energy (VHE) gamma rays beyond 100 GeV, coming from galactic and extragalactic sources, reflect the most energetic non-thermal processes in the universe. The emission of these photons indicates the acceleration of charged particles to very high energies or the existence of exotic particles that annihilate or decay to photons. Observations of VHE gamma rays probing this highest energy window of electromagnetic waves thus can reveal the underlying acceleration processes or new astrophysical particles. The fluxes tend to be power-law spectra and this poses a difficulty for direct observation due to the low flux at the high-energy end and to the limited effective area of space-borne instruments. Ground-based VHE gamma-ray observatories therefore take advantage of the earth atmosphere as a calorimeter and observe the gamma rays indirectly via the electromagnetic cascade shower particles they produce. The shower particles are detected either directly or via the Cherenkov radiation they emit while propagating through the air. The current-generation telescopes adopting this ground-based methodology have confirmed several source categories and are starting to answer various physical and astronomical questions, e.g., the origin of cosmic rays, the nature of dark matter, the black hole accretion processes, etc. Together with multi-wavelength observations covering the full electromagnetic spectrum and astrophysical observatories of other particles (cosmic rays, neutrinos, etc.) VHE gamma-ray astronomy contributes as an indispensable part of the recently emerging field of multi-messenger particle astrophysics. When emitted by extragalactic sources, the VHE gamma rays undergo various interactions in the intergalactic medium as they propagate toward the earth. There is a guaranteed interaction, where the VHE gamma-ray photons are absorbed by the extragalactic background light (EBL), an isotropic background of optical-to-infrared photons coming from starlight or dust re-emission in the universe, producing electron-positron pairs. The pairs then upscatter ambient EBL and cosmic microwave background (CMB) photons to gamma rays, which are mostly high-energy (HE), i.e., between 100 MeV and 100 GeV. These secondary gamma rays could also trigger further pair production processes, resulting in an electromagnetic cascade in the cosmic voids. When there is no magnetic field present, all of the cascade gamma rays travel in virtually the same direction as the primary emissions from the source, adding to the observed gamma-ray flux. If the magnetic field in the voids is not negligible, however, the electron-positron pairs are deflected prior to inverse-Compton (IC) scattering on the background photons, impacting to the cascade photons an angular extension. The angular extension caused by the magnetic field both decreases the directly-observed source flux and creates a gamma-ray halo around the original source. An observation of the gamma-ray halo would therefore present a detection of the cosmic magnetic field, which so far has only upper limits imposed from Faraday rotation measurements of radio galaxies. On the other hand, by placing an upper limit on the HE gamma-ray flux of the source we can also derive a lower limit on the magnetic field. To address the processes involved in VHE gamma-ray propagation, I employ both semi-analytic models and full-scale Monte Carlo simulations derived from first principles. The two ways of approach give complementary perspectives on the physics involved and cross-check with each other to ensure a reliable result. By fitting the predicted cascade flux with observed data in both VHE and HE energy ranges by ground-based imaging atmospheric Cherenkov telescopes (IACTs) and the Fermi Large Area Telescope (LAT), I can place a robust lower limit on the extragalactic magnetic field (EGMF) strength at 10--16 to 10--15 Gauss, or at 10 --18 to 10--17 Gauss for a more conservative assumption on the source livetime. The lower limit rules out a large portion of the parameter space for the magnetic field cosmogenic models and my method with a rigorous statistical framework sets a baseline for the upcoming studies with accumulating data. Modeling the propagation of gamma rays in the intergalactic space also leads to a theoretical prediction of the isotropic diffuse gamma-ray background from unresolved gamma-ray sources. By employing the full-scale Monte Carlo simulation and building a complete database for gamma-ray calorimetry at different energies and different redshifts, the observed diffuse gamma-ray spectrum for any source category can be calculated as a convolution of the database and the bolometric gamma-ray emissivity history of the specific sources. I derive master equations for the convolution, predicting both the intensity and anisotropy of the diffuse background from the simulated gamma-ray transfer functions and theoretical models of bolometric emissivities. With the computational power of supercomputing clusters, the database can be built and put into application together with the growing data on the diffuse background to constrain the astrophysical populations of various VHE gamma-ray sources in the near future.

Observational Properties of Gigaelectronvolt-Teraelectronvolt Blazars and the Study of the Teraelectronvolt Blazar RBS 0413 with VERITAS

Senturk, Gunes Demet Columbia University 2013 해외박사(DDOD)

Blazars are active galactic nuclei with a relativistic jet directed towards the observer's line of sight. Characterization of the non-thermal continuum emission originating from the blazar jet is currently an essential question in high-energy astrophysics. A blazar spectral energy distribution (SED) has a typical double-peaked shape in the flux vs. energy representation. The low-energy component of the SED is well-studied and thought to be due to synchrotron emission from relativistic electrons. The high-energy component, on the other hand, is still not completely understood and the emission in this part of the blazar spectrum can extend to energies as high as tera electron volts in some objects. This portion of the electromagnetic spectrum is referred to as the very-high-energy (VHE or TeV, E > 0.1 TeV) regime. At the time of this writing, more than half a hundred blazars have been detected to emit TeV gamma rays, representing the high energy extreme of these objects and constituting a population of its own. Most of these TeV blazars have also been detected in the high-energy (HE or GeV, 0.1 GeV < E < 0.1 TeV) gamma-ray range. In this work, we report on our discovery of the TeV emission from the blazar RBS 0413 and perform a detailed data analysis on this source, including contemporaneous multi-wavelength observations to characterize the broad-band SED and test various emission models for the high-energy component. Further, we extend our focus on the high-energy component to all archival TeV-detected blazars and study their spectral properties in the framework of GeV and TeV gamma-ray observations. To do this, we assemble for the first time the GeV and TeV spectra of a complete sample of TeV-detected blazars available in the archive to date. In the Appendix we present an analysis method for improved observations of large zenith angle targets with VERITAS.