Physics

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Decoding Images of Debris Disks
(2010) Stark, Christopher; Kuchner, Marc J; Hamilton, Douglas C; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Current observations of debris disks reveal a wealth of radial and azimuthal structures likely created by planet-disk interactions. Future images of exozodiacal disks may reveal similar structures. In this work I summarize my observations and modeling of the structure of exozodiacal dust clouds. I present our observations of the 51 Ophiuchi circumstellar disk made with the Keck Interferometer operating in nulling mode at N-band. I modeled these data simultaneously with VLTI-MIDI visibility data and a Spitzer IRS spectrum and showed that the best-fit disk model is an optically thin disk with size-dependent radial structure. This model has two components, with an inner exozodiacal disk of blackbody grains extending to ~4 AU and an outer disk of small silicate grains extending out to ~1200 AU. This model is consistent with an inner "birth" disk of continually colliding parent bodies producing an extended envelope of ejected small grains and resembles the disks around Vega, AU Microscopii, and β Pictoris. I produced models of resonant ring structures created by planets in debris disks. I used a custom-tailored hybrid symplectic integrator to model 120 resonant ring structures created by terrestrial-mass planets on circular orbits interacting with collisionless steady-state dust clouds around a Sun-like star. I used these models to estimate the mass of the lowest-mass planet that can be detected through observations of a resonant ring, and showed that the resonant ring morphology is degenerate and depends on only two parameters: planet mass and a_p1/2/β, where a_p is the planet's semi-major axis and β is the ratio of radiation pressure force to gravitational force on a grain. I introduced a new computationally-efficient "collisional grooming" algorithm that enables us to model grain-grain collisions in structured debris disks and used this algorithm to show how collisions can alter the morphology of a resonant ring structure. My collisional models reveal that collisions act to remove azimuthal and radial asymmetries from the disk. I showed that the collision rate for background particles in a resonant ring structure is enhanced by a factor of a few compared to the rest of the disk, and dust grains in or near mean motion resonances have even higher collision rates. I also used this algorithm to model the 3-D structure of the Kuiper Belt's dust cloud at four different dust levels. I found that the Kuiper Belt dust would look like an azimuthally symmetric ring at 40-45 AU when viewed from afar at submillimeter wavelengths. At visible wavelengths, the Kuiper Belt dust cloud would reveal two resonant ring structures: one created by Saturn near 10 AU and one created by Neptune near 30 AU. A denser version of our Kuiper Belt dust cloud, with an optical depth 1000 times greater, would look qualitatively similar at submillimeter wavelengths, but would be void of Neptune's resonant ring structure at visible wavelengths. My simulations suggest that mean motion resonances with planets can play strong roles in the sculpting of debris disks even in the presence of collisions, though their roles are somewhat different than what has been anticipated.
Search for Quantum Gravity with IceCube and High Energy Atmospheric Neutrinos
(2010) Huelsnitz, Warren; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
IceCube is a cubic-kilometer neutrino telescope nearing completion in the South Pole Ice. Designed to detect astrophysical neutrinos from 100 GeV to about an EeV, it will contribute to the fields of high energy astrophysics, particle physics, and neutrino physics. This analysis looks at the flux of atmospheric neutrinos detected by IceCube while it operated in a partially-completed, 40-string configuration, from April 2008 to May 2009. From this data set, a sample of about 20,000 up-going atmospheric muon neutrino events with negligible background was extracted using Boosted Decision Trees. A discrete Fourier transform method was used to constrain a directional asymmetry in right ascension. Constraints on certain interaction coefficients from the Standard Model Extension were improved by three orders of magnitude, relative to prior experiments. The event sample was also used to unfold the atmospheric neutrino spectrum at its point of origin, and seasonal and systematic variations in the atmospheric muon neutrino flux were studied. A likelihood method was developed to constrain perturbations to the energy and zenith angle dependence of the atmospheric muon neutrino flux that could be due to Lorentz-violating oscillations or decoherence of neutrino flavor. Such deviations could be a signature of quantum gravity in the neutrino sector. The impact of systematic uncertainties in the neutrino flux and in the detector response on such a likelihood analysis were examined. Systematic uncertainties that need to be reduced in order to use a two-dimensional likelihood analysis to constrain phenomenological models for Lorentz or CPT violating neutrino oscillations were identified.
Numerical studies on new techniques for gravitational wave extraction and binary black hole simulations
(2009) Pazos, Enrique; Tiglio, Manuel; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This dissertation presents numerical studies of gravitational waves produced by black holes in two scenarios: perturbations of a single black hole, and the collision of a binary pair. Their detection plays a crucial roll in further testing General Relativity and opens a whole new field of observational astronomy. First, a technique called Cauchy--perturbative matching is revisited in one dimension through the use of new numerical methods, such as high order finite difference operators, constraint-preserving boundary conditions and, most important, a multi-domain decomposition (also referred to as multi-patch, or multi-block approach). These methods are then used to numerically solve the fully non-linear three-dimensional Einstein vacuum equations representing a non-rotating distorted black hole. In combination with a generalization of the Regge-Wheeler-Zerilli formalism, we quantify the effect of the background choice in the wave extraction techniques. It is found that a systematic error is introduced at finite distances. Furthermore, such error is found to be larger than those due to numerical discretization. Subsequently, the first simulations ever of binary black holes with a finite-difference multi-domain approach are presented. The case is one in which the black holes orbit for about twelve cycles before merging. The salient features of this multi-domain approach are: i) the complexity of the problem scales linearly with the size of the computational domain, ii) excellent scaling, in both weak and strong senses, for several thousand processors. As a next step, binary black hole simulations from inspiral to merger and ringdown are performed using a new technique, turduckening, and a standard finite difference, adaptive mesh-refinement code. The computed gravitational waveforms are compared to those obtained through evolution of the same exact initial configuration but with a pseudo-spectral collocation code. Both the gravitational waves extracted at finite locations and their extrapolated values to null infinity are compared. Finally, a numerical study of generic second order perturbations of Schwarzschild black holes is presented using a new gauge invariant high order perturbative formalism. A study of the self-coupling of first order modes and the resulting radiated energy, in particular its dependence on the type of initial perturbation, is detailed.
A Search for Muon Neutrinos from Gamma-Ray Bursts wih the IceCube 22-String Detector
(2009) Roth, A Philip; Hoffman, Kara; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Two searches are conducted for muon neutrinos from Gamma-Ray Bursts (GRBs) using the IceCube detector. Gamma-Ray Bursts are brief and transient emissions of keV/MeV radiation occuring with a rate of a few per day uniformly in the sky. Swift and other satellites of the Third Interplanetary Network (IPN3) detect these GRBs and send notices out via the GRB Coordinate Network (GCN). The fireball model describing the physics of GRBs predicts the emission of muon neutrinos from these bursts. IceCube is a cubic kilometer neutrino detector buried in the deep antarctic ice at the South Pole that can be used to find these prediceted but still unobserved neutrinos. It is sensitive to them by detecting Cherenkov light from secondary muons produced when the neutrinos interact in or near the instrumented volume. The construction of IceCube has been underway since the austral summer of 2004-2005 and will continue until 2011. The growing IceCube detector will soon be sensitivite to the high energy neutrino emission from GRBs that is predicted by the fireball model. A blind and triggered search of the 22-string IceCube data for this neutrino emission was conducted. The principal background to the observation of neutrinos in IceCube is muons generated in cosmic-ray air-showers in the atmosphere above the detector. Atmospheric neutrinos make up a separate irreducible background to the detection of extraterrestrial neutrinos. A binned stacked search of 41 bursts occuring in the northern hemisphere greatly reduces the muon background by looking for tracks moving up through the detector. The atmospheric neutrino background is greatly reduced by the temporal constraints of the search, making it effectively background free. 40 individual unbinned searches of bursts occuring in the southern hemisphere extend IceCube's sensitivity to the higher background regions above the horizon. No significant excesses over background expectations are found in either search. A 90% confidence upper limit on the neutrino fluence from northern hemisphere bursts is set at 6.52 x 10-3 erg cm-2 with 90% of the expected signal between 87.9 TeV and 10.4 PeV.
Non-linear Development of Streaming Instabilities in Magnetic Reconnection with a Strong Guide Field
(2009) Che, Haihong; Drake, James F.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Magnetic reconnection is recognized as a dominant mechanism for converting magnetic energy into the convective and thermal energy of particles, and the driver of explosive events in nature and laboratory. Magnetic reconnection is often modeled using resistive magnetohydrodynamics, in which collisions play the key role in facilitating the release of energy in the explosive events. However, in space plasma the collisional resistivity is far below the required resistivity to explain the observed energy release rate. Turbulence is common in plasmas and the anomalous resistivity induced by the turbulence has been proposed as a mechanism for breaking the frozen-in condition in magnetic reconnection. Turbulence-driven resistivity has remained a poorly understood, but widely invoked mechanism for nearly 50 years. The goal of this project is to understand what role anomalous resistivity plays in fast magnetic reconnection. Turbulence has been observed in the intense current layers that develop during magnetic reconnection in the Earth's magnetosphere. Electron streaming is believed to be the source of this turbulence. Using kinetic theory and 3D particle-in-cell simulations, we study the nonlinear development of streaming instabilities in 3D magnetic reconnection with a strong guide field. Early in time an intense current sheet develops around the x-line and drives the Buneman instability. Electron holes, which are bipolar spatial localized electric field structures, form and then self-destruct creating a region of strong turbulence around the x-line. At late time turbulence with a characteristic frequency in the lower hybrid range also develops, leading to a very complex mix of interactions. The difficulty we face in this project is how to address a long-standing problem in nonlinear kinetic theory: how to treat large amplitude perturbations and the associated strong wave-particle interactions. In my thesis, I address this long-standing problem using particle-in-cell simulations and linear kinetic theory.Some important physics have been revealed. 1: The lower hybrid instability (LHI) dominates the dynamics in low $beta$ plasma in combination with either the electron-electron two-stream instability (ETS) or the Buneman instability (BI), depending on the parallel phase speed of the LHI. 2: An instability with a high phase speed is required to tap the energy of the high velocity electrons. The BI with its low phase speed, can not do this. The ETS and the LHI both have high phase speed. 3: The condition for the formation of stable electron holes requires $|v_p -v_g|< sqrt{2e|phi|/m_e}$, where $|phi|$ is the amplitude of the electric potential, and $v_p$ and $v_g$ are the phase and group velocity of the relevant waves. Like ETS and BI, LHI all can form electron holes. 4: The overlapping resonance in phase space is the dominant mechanism for transporting the momentum and energy from high velocity electrons to low velocity electrons, which then couple to the ions.
The Search for Neutralino Dark Matter with the AMANDA Neutrino Telescope
(2008) Ehrlich, Ralf; Sullivan, Gregory; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
There is convincing indirect evidence based on cosmological data that approximately one quarter of the universe is made of dark matter. However, to this date there is no direct detection of the dark matter and its nature is unknown. Most theories suggest that this dark matter is made of Weakly Interacting Massive Particles (WIMPs), or more specifically: supersymmetric particles. The most promising candidate out of the supersymmetric particles is the lightest neutralino. These neutralinos can get trapped in the gravitational field of the Earth, where they accumulate and annihilate. The annihilation products decay and produce neutrinos (among other particles). These neutrinos (the focus is on muon-neutrinos here) can be detected with the AMANDA neutrino telescope located between one and two kilometers deep in the ice of the glacier near the South Pole. Neutrinos cannot be detected directly. However, there is a small possibility that they interact with nuclei of the ice and create charged leptons. These charged leptons continue to travel in the same direction as the neutrinos (accompanied by electromagnetic/hadronic cascades, and  electrons). As long as their speed is higher than the speed of light of the ice, they emit Cherenkov radiation which can be captured by photomultipliers installed inside the ice. The signals collected by the photomultipliers can be used to reconstruct the track of the lepton. AMANDA - the Antarctic Muon and Neutrino Detector Array - makes use of the unique properties of the neutrino: Since neutrinos interact only weakly, they can travel through the Earth without being stopped. Therefore all detected particles which have been identified as upward going (i.e. through the Earth coming) must have been produced by charged leptons originating from neutrinos after they reacted with the nuclei of the ice. All other particles which do not come from below are rejected. If the neutrino flux coming from the neutralino annihilation inside Earth is strong enough to be detected with AMANDA, it should show up as an excess over the expected neutrino flux, which comes from the atmospheric neutrinos produced in the northern hemisphere. This analysis which used data from 2001 and 2002 showed that there is no significant excess, yielding an upper limit on the neutrino flux that could have come from WIMP annihilation.
A Blind Search for Bursts of Very High Energy Gamma Rays with Milagro
(2008-08-03) Vasileiou, Vlasios; Goodman, Jordan A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Milagro is a water-Cherenkov detector that observes the extended air showers produced by cosmic gamma rays of energies E>100GeV. The effective area of Milagro peaks at energies E>10TeV, however it is still large even down to a few hundred GeV (~10m^2 at 100GeV). The wide field of view (~2sr) and high duty cycle (>90%) of Milagro make it ideal for continuously monitoring the overhead sky for transient Very High Energy (VHE) emissions. This study searched the Milagro data for such emissions. Even though the search was optimized primarily for detecting the emission from Gamma-Ray Bursts (GRBs), it was still sensitive to the emission from the last stages of the evaporation of Primordial Black Holes (PBHs) or to any other kind of phenomena that produce bursts of VHE gamma rays. Measurements of the GRB spectra by satellites up to few tens of GeV showed no signs of a cutoff. Even though multiple instruments sensitive to GeV/TeV gamma rays have performed observations of GRBs, there has not yet been a definitive detection of such an emission yet. One of the reasons for that is that gamma rays with energies E>100GeV are attenuated by interactions with the extragalactic background light or are absorbed internally at the site of the burst. There are many models that predict VHE gamma-ray emission from GRBs. A detection or a constraint of such an emission can provide useful information on the mechanism and environment of GRBs. This study performed a blind search of the Milagro data of the last five years for bursts of VHE gamma rays with durations ranging from 100 micro seconds to 316 seconds. No GRB localization was provided by an external instrument. Instead, the whole dataset was thoroughly searched in time, space, and duration. No significant events were detected. Upper limits were placed on the VHE emission from GRBs.
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.
Magnetic Fields Around Black Holes
(2008-01-22) Garofalo, David Andrea; Reynolds, Christopher S.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Active Galactic Nuclei are the most powerful long-lived objects in the universe. They are thought to harbor supermassive black holes that range from 1 million solar masses to 1000 times that value and possibly greater. Theory and observation are converging on a model for these objects that involves the conversion of gravitational potential energy of accreting gas to radiation as well as Poynting flux produced by the interaction of the rotating spacetime and the electromagnetic fields originating in the ionized accretion flow. The presence of black holes in astrophysics is taking center stage, with the output from AGN in various forms such as winds and jets influencing the formation and evolution of the host galaxy. This dissertation addresses some of the basic unanswered questions that plague our current understanding of how rotating black holes interact with their surrounding magnetized accretion disks to produce the enormous observed energy. Two magnetic configurations are examined. The first involves magnetic fields connecting the black hole with the inner accretion disk and the other involves large scale magnetic fields threading the disk and the hole. We study the effects of the former type by establishing the consequences that magnetic torques between the black hole and the inner accretion disk have on the energy dissipation profile. We attempt a plausible explanation to the observed ``Deep Minimum'' state in the Seyfert galaxy MCG-6-30-15. For the latter type of magnetic geometry, we study the effects of the strength of the magnetic field threading the black hole within the context of the cherished Blandford \& Znajek mechanism for black hole spin energy extraction. We begin by addressing the problem in the non-relativistic regime where we find that the black hole-threading magnetic field is stronger for greater disk thickness, larger magnetic Prandtl number, and for a larger accretion disk. We then study the problem in full relativity where we show that our Newtonian results are excellent approximations for slowly spinning black holes. We proceed to address the issue of the spin dependence of the Blandford \& Znajek power. The result we choose to highlight is our finding that given the validity of our assumption for the dynamical behavior of the so-called plunge region in black hole accretors, rotating black holes produce maximum Poynting flux via the Blandford \& Znajek process for a black hole spin parameter of about $a\approx0.8$. This is contrary to the conventional claim that the maximum electromagnetic flux is achieved for highest black hole spin.
Radiation reaction and self-force in curved spacetime in a field theory approach
(2007-11-28) Galley, Chad; Hu, Bei-Lok; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
This dissertation, in three parts, presents self-consistent descriptions for the motion of relativistic particles and compact objects in an arbitrary curved spacetime from a field theory approach and depicts the quantum and stochastic (part I), semiclassical (parts I and II), and completely classical regimes (part III). In the semiclassical limit of an open quantum system description, the particle acquires a stochastic component in its dynamics. The interrelated roles of noise, decoherence, fluctuations and dissipation are prominently manifested from a stochastic field theory viewpoint and highlighted with our derivations of Langevin equations for the particle in curved space, which are useful for studying influences imparted by a stochastic source. We also derive non-local and history-dependent equations for radiation reaction and self-force in a curved spacetime when the stochastic sources are negligible. When the scales of the mass and the field are very different, as for an astrophysical mass or compact object, the stochastic features of the system are strongly suppressed and the stochastic description yields a (semiclassical) effective field theory. The appropriate expansion parameter $\mu$ is the ratio formed by the size of the compact object and the background curvature scale. Within an effective field theory framework we derive the second order self-force and the leading order contributions to the equations of motion from spin-orbit and spin-spin interactions on a compact object. The finite size of the compact body affects its motion at $O(\mu^4)$ and the self-force at $O(\mu^5)$. These results are useful for constructing more accurate templates that the space-based interferometer LISA will need for parameter estimation. Within a purely classical setting we introduce a new framework that describes fully relativistic gravitating binary systems, possibly with comparable masses, and allows for the background geometry to dynamically respond with the motions and influences of the compact objects and gravitational waves. The approach self-consistently incorporates mutual action and backreaction on every component in the total system. We derive the equations of motion and identify the parameter regimes where this new approach is applicable with the aim of establishing a common framework applicable to the detection ranges of both LIGO and LISA interferometers.

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