Physics and Astronomy

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This is the collection for the University of Waterloo's Department of Physics and Astronomy.

Research outputs are organized by type (eg. Master Thesis, Article, Conference Paper).

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Now showing 1 - 12 of 12

Cosmological beam plasma instabilities
(University of Waterloo, 2017-08-24) Shalaby, Mohamad; Broderick, Avery; Afshordi, Niayesh
Blazars are the main source of extragalactic very high energy gamma-rays. These gamma rays annihilate on the extragalactic background light, producing electron-positron pair beams with TeV energies. The pair beams are very dilute, with beam-IGM density ratio of $\alpha \sim 10^{-15}$, ultra-relativistic, $\gamma \sim 10^6$, and energetically subdominant ($\gamma \alpha \sim 10^{-9}$). Such pair beams suffer from prevailing cosmological scale, linear beam-plasma instabilities. The associated instability growth rates suggest that at least initially these overwhelmingly dominate inverse Compton cooling, currently the only alternative mechanism by which the pair beams lose energy. Therefore, the full non-linear evolution of the instabilities is key to determining the mechanism by which these pair-beams lose their energy. Kinetic numerical simulations are the only method by which we can currently study the full non-linear evolution of the blazar-induced beam-plasma instabilities. However, the extreme parameters of the pair beams make direct simulations via existing particle-in-cell (PIC) codes infeasible. To address this, we developed a new Spatially Higher-order Accurate, Relativistic PIC algorithm (SHARP). A one dimensional implementation of the SHARP algorithm (SHARP-1D) is given in detail. We show explicitly that SHARP-1D can overcome a number of the limitations of existing PIC algorithms. Using SHARP-1D, we demonstrate a number of points that are important to correctly simulate the full evolution of beam-plasma instabilities. We show that convergence for PIC algorithms requires increasing both spatial resolution and the number of particles per cell concurrently. For a beam-plasma system, we show that the spectral resolution is another important resolution criteria and under-resolved simulations can lead to erroneous physical conclusions. We quantify the required box sizes to faithfully resolve the spectral support of the instabilities. When the background plasmas contain structure, we show that a significant fraction of beam energy (similar to that in uniform plasma simulations; $\sim 20$\%) is, still, lost during the linear evolution of the electrostatic unstable modes. Compared to uniform plasma growth rates, we find lower growth rates, however, the non-uniform systems stay longer in the linear regime. Therefore, the IGM inhomogeneities are unlikely to affect the efficiency of beam-plasma instabilities to cool the blazar-induced pair beams.
Leveraging Polarization in the Era of Submillimeter VLBI
(University of Waterloo, 2024-06-14) Ni, Chunchong; Broderick, Avery
With the advancement of technology, global very long baseline interferometry (VLBI) observations at millimeter wavelengths become possible. The Event Horizon Telescope (EHT) is the first such experiment, which makes observing accretion disk and jet launching regions near supermassive black holes and active galactic nuclei (AGN) possible, including polarimetry observations. Centaurus A (Cen A) is a nearby radio-loud AGN, with large jet structures of angular size measured in degrees. It was observed by the EHT, whose first total intensity image shows a fork-shaped edge brightening jet structure. Chapter 2 applies Bayesian imaging method to the Cen A data. We first construct the total intensity image of Cen A, which we directly compare with the previous publication. Second, the Bayesian method produces the first polarization studies of Cen A jet. Both the total intensity imaging and the polarization mapping feature a full image posteriors with access to the image uncertainty. This proves to be essential in the case of Cen A, where the data is very challenging for various reasons. With polarization image posterior of Cen A, we are able to study different regions of the jet separately, eventually producing a robust estimate of a collection of important physics quantities, including magnetic field strength, the electron number density and the jet velocity. In Chapter 3, we explore the origin and influence of the interstellar scattering on observations of Sgr A*, and propose a novel method to mitigate this scattering via EHT and next-generation EHT (ngEHT) polarimetry in the future. In EHT and other radio astronomical observations of Sgr A*, scattering contaminates the image with external small-scale structures, essentially preventing further studies of the turbulence in the accretion disk. However, for credible interstellar magnetic field strengths, the scattering is proved to be insensitive to polarization. Therefore, it is possible to distinguish intrinsic and scattered structures via the image power spectra constructed in different polarization components. Via numerical experiments, we demonstrate a method for reconstructing intrinsic structural information from the scattered power spectrum. We demonstrate that this is feasible through a series of numerical experiments with general relativistic magnetohydrodynamic (GRMHD) simulation images. Specifically, we show that the ratio of the power spectra, obtained independently for different polarization components, is independent of the scattering screen. Therefore, these power spectra ratios provide a window directly into the MHD turbulence believed to drive accretion onto black holes.
Messier 87: A Laboratory for Exploring AGN Variability using the Event Horizon Telescope
(University of Waterloo, 2020-09-17) Jeter, Britton; McNamara, Brian; Broderick, Avery
At a distance of 50 million light years at the heart of the Virgo cluster, lies the giant elliptical galaxy Messier 87. This galaxy has at its center a brilliant relativistic jet, observable at all wavelengths, and at the jet's base, the black hole M87*. As of April 2017, this black hole was directly imaged by the Event Horizon Telescope (EHT), a network of eight radio telescopes scattered across the globe. These first images of a black hole will let us investigate not only the physics of black holes, but also the formation and launching mechanism of relativistic jets. The jet in M87 is known to exhibit variability on many different time-scales, from weeks to decades, and comparing the images produced by the EHT to models for jet variability will let us learn how relativistic jets are born. Synchrotron spot models have been used to model variability near black holes; the first part of this thesis extends these models by allowing spots to shear and deform in the jet velocity field. Depending on the position of the spot, shearing forces can significantly alter the structure of the spot, producing distinct signals in reconstructed images and light curves. The maximum intensity of the shearing spot can vary by as much as a factor of five depending on the spot azimuthal launch position, but the intensity decay time depends most significantly on the spot radial launch position. Spots launched by a black hole driven jet exhibit distinct arc structures in reconstructed images, and exhibit brighter and shorter-lived enhancements of the light curve. Spots launched by a wind-driven jet have exhibit much simpler structures in the image, and longer-lived light curve enhancements than spots launched by a black hole driven jet. The EHT measured the mass of M87* to be $6.5 \times 10^{9} ~M_{\odot}$, consistent with previous mass estimates from stellar kinematics, but inconsistent by up to $2\sigma$ with mass estimates made using gas dynamics models of the gas disk at parsec scales. Critical to gas-dynamical modeling is the assumed underlying dynamical state of the gas: that it lies on circular Keplerian orbits, potentially with some additional turbulent pressure support. This is inconsistent with models of the gas flow about low-accretion-rate SMBHs and at odds with observations of the Galactic Center. In the second part of this thesis, I present an extended model for non-Keplerian gas disks and explore their implications for SMBH mass measurements. I show that a larger central black hole with gas experiencing small amounts of sub-Keplerian motion and/or non-trivial disk thicknesses can produce velocity curves similar to models that just contain circular Keplerian motions and a lower black hole mass. These non-Keplerian models are distinguishable from low-mass Keplerian models primarily through measuremnets of the velocity dispersion, wherein non-Keplerian models produce higher and narrower peak dispersions. By combining the existing velocity measurements from \citet{M87gas:13} and the EHT mass estimate, we place constraints on the gas disk inclination and sub-Keplerian fraction. These constraints require the parsec-scale ionized gas disk be misaligned with the \mas radio jet by at least $2^{\circ}$, and more typically $15^{\circ}$. Modifications to the gas dynamics model either by introducing sub-Keplerian velocities or thick disks produces further misalignment with the radio jet. If the jet is driven by the black hole spin, this implies that the angular momentum of the black hole is decoupled with the angular momentum of the large scale gas feeding M87*. The velocity model presented in this thesis is capable of resolving the discrepancy between the ionized gas dynamics and stellar kinematics mass estimates, and is applicable to gas-dynamical mass estimates of SMBHs in general.
The Nature and Impact of Active Galactic Nuclei
(University of Waterloo, 2021-08-12) Tiede, Paul; Broderick, Avery
The gravitational interaction around the event horizon of black holes presents theoretical challenges. With the advent of the Event Horizon Telescope (EHT), we are now entering an era in physics where we can probe the structure of spacetime on horizon scales. The EHT presents the first opportunity to directly image the supermassive black holes at the center of the Milky Way and M 87. By imaging the central black hole, we can directly learn about the nature of spacetime and plasma physics on horizon scales. The black hole images produced by the EHT are dominated by a bright ring. The ellipticity of the ring could potentially signal deviations from general relativity. However, whether the EHT imaging techniques can robustly detect ellipticity has not been fully explored. Chapters 2–5 analyze the EHT’s ability to measure ellipticity in four parts. First, in Chapter 2, we develop a method to extract image features (e.g., ring ellipticity) called variational image domain analysis. Second, in Chapter 3, we apply variational image domain analysis to the M 87 image reconstruction pipeline and demonstrate that it is unable to measure ellipticity. The core reason for this failure is that traditional radio imaging techniques cannot quantify image uncertainty. To solve this issue, in Chapters 4 and 5 we use Themis, a Bayesian parameter estimation framework for the EHT, to robustly measure the ellipticity of M 87. To apply Themis to the problem of Bayesian imaging, we developed a new sampler interface in Chapter 4. In Chapter 5 we apply Themis to M 87 and construct the first Bayesian estimates of its ellipticity. Furthermore, we demonstrate that the measured ellipticity is consistent with the expected ellipticity from an accretion disk around a Kerr black hole. In Chapter 6 we describe a novel method to measure spacetime around Sgr A∗ using hot spots. While M 87 is static over an observation, Sgr A∗ is dynamic, changing on minute timescales. Furthermore, Sgr A∗ flares 1–3 times a day in sub-mm, infrared, and X-ray. The Gravity Collaboration recently demonstrated that hot spots near the innermost stable circular orbit explain Sgr A∗ flares. Using Themis, we construct an efficient semi- analytical model of hotspots and fit simulated Sgr A∗ data from the 2017 EHT observations. We demonstrate that the EHT could potentially make a sub-percent spin measurement of Sgr A∗ by tracking the evolution of these flares. Furthermore, by observing multiple flares, we can tomographically map spacetime around Sgr A∗ , providing a test of general relativity in the strong-field regime.
Photon Rings and Shadow Size for General Integrable Spacetimes
(University of Waterloo, 2023-08-08) Salehi, Kiana; Broderick, Avery
According to the no-hair theorem, the unique characteristics of astrophysical black holes are their masses and spins. However, recent observations from the Event Horizon Telescope (EHT) images of M87 and Sgr A* have allowed us to place constraints on possible deviations from this theory. To interpret these observations and compare them to other near horizon scale observations, we introduce a model-agnostic framework that explores deviations while maintaining generality. We start by considering a general spherically symmetric metric, which effectively applies for a polar observer in the slow rotation limit and then follow by relaxing these constraints to axi-symmetric and stationary spacetimes. We propose a nonperturbative, nonparametric spacetime-domain characterization of shadow size and related measurements that makes explicit the nature and power (or lack thereof) of shadow-size-based constraints, and facilitates comparisons among observations and targets. Furthermore, we demonstrate that relying solely on shadow size measurements does not impose a direct limitations on the value of the gtt component of the metric. However, in the case of spherically symmetric spacetime, it can impose a constraint on the radial derivative of gtt, while a more intricate constraint arises for the axi-symmetric spacetime. Moreover, the measurement of shadows and potential future observations of multiple photon rings do not provide any valuable information concerning the ergo-region and frame-dragging in axi-symmetric spacetime.
Placing Limits on Light Axions with EHT Polarimetry via Closure Traces
(University of Waterloo, 2023-08-31) Wang, Zhiren; Broderick, Avery
Black holes can amplify incoming bosonic waves via rotational superradiance, inducing bound states of ultralight bosons around them. This phenomenon has the potential to confine the parameter spaces of new bosons. Axions, and axion-like particles (ALPs) are candidates beyond-standard-model particles that can form such clouds around supermassive black holes (SMBHs) and impact the polarization signal in a similar fashion to Faraday rotation via axion-photon coupling. Prior research has used data from the Event Horizon Telescope (EHT) M87 2017 observations to limit the dimensionless axion-photon coupling to previously unexplored regions. With the novel calibration-insensitive quantities: clo sure traces and conjugate closure trace products, it is possible to constrain the existence of axion clouds while avoiding the most dominant sources of systematic uncertainties, e.g., station gains and polarization leakages. I utilize a simple geometric model for the polarization map of M87* to fit the model parameters with both simulated and real datasets to verify the applicability of this method and reach a comparable level of constraint in the accuracy with which an axion cloud may be excluded in M87. Such approach is feasible with application to future M87* and Sgr A* observations by EHT and next-generation EHT (ngEHT) and may provide stronger constraints on axions and ALPs.
Probing Accretion Turbulence in the Galactic Centre with EHT Polarimetry
(University of Waterloo, 2019-01-09) Ni, Chunchong; Broderick, Avery
We explore the origin and the influence of the interstellar scattering on the observation of Sgr A*, and the method to mitigate this scattering via Event Horizon Telescope (EHT) polarimetry. Interstellar scattering is due to the existence of inhomogeneous plasma screens between the Earth and Sgr A*. At radio wavelengths, this scattering adds and removes small structures in the images. In the EHT observation, the scattering contaminates the image by moving power from long baselines to short baselines in a fashion that may be described by a linear transformation characterized by the Scattering Shift Kernel. However, for credible interstellar magnetic field strengths, this transformation is insensitive to polarization. Therefore, it is possible to distinguish intrinsic and scattered structures via the image power spectra constructed in different polarization components. Via numerical experiments, we demonstrate a method for reconstructing intrinsic structure information. We do this for two cases: a toy model in which we show that this method accurately reproduces the characteristics of controlled image fluctuations, and general relativistic magnetohydrodynamic simulation images. Specifically, we show that the ratio of the power spectra obtained independently for different polarization components is independent of the scattering screen given the current observational limitations of the EHT. Therefore, these power spectra ratios provide a window directly into the magnetohydrodynamic turbulence believed to drive accretion onto black holes.
Probing the dark universe with gravitational lensing
(University of Waterloo, 2018-09-17) Karami, Mansour; Afshordi, Niayesh; Broderick, Avery
Since its early success as an experimental test of the theory of general relativity in 1919, gravitational lensing has come a long way and is firmly established as an indispensable element for many astrophysical applications. In this thesis, we explore novel applications of gravitational lensing that further our understanding of the dark sectors of the cosmos and other astrophysical objects, namely dark matter nanostructure, black holes and the Galactic disk. We pay particular attention to developing concrete and optimal statistical methodologies and numerical implemen- tations for these novel probes. We start by developing a statistical framework to measure the dark matter power spectrum in the deep nonlinear regime, using transient weak lensing, and simultaneously measure the time delays for strongly lensed quasars. We then outline how observations of microlensing in optical and radio can unravel the structure, dynamics, and content of the Galactic disk, and in particular, be used to detect stellar mass black holes. Lastly, using the shadow images of the super-massive black holes caused by extreme lensing effect, we can learn about the structure of space-time, accretion flows and astrophysical jets. We present a Bayesian framework for analyzing the data from the Event Horizon Telescope Collaboration.
Probing the large-scale clustering of fast radio bursts with CHIME/FRB
(University of Waterloo, 2021-08-23) Rafiei-Ravandi, Masoud; Smith, Kendrick; Broderick, Avery
Explaining the nature of extragalactic fast radio bursts (FRBs) has puzzled astrophysicists since 2007. In this thesis, we introduce the CHIME/FRB instrument, an FRB search engine that could solve this puzzle through FRB population studies. We explore CHIME/FRB science results, with an emphasis on the FRB-galaxy correlation. First, we formulate a framework for characterizing FRBs (e.g. by constraining their redshift and host dispersion measure distributions) through angular cross-correlations with large-scale structure. Using this machinery we model, forecast and simulate the FRB-galaxy correlation for two distinct FRB models. Then, we apply this technique to real data, using the first CHIME/FRB catalog along with five photometric redshift catalogs of galaxies. Computing the FRB-galaxy cross power spectrum, we find a statistically significant (p-value ~ 10⁻⁴, accounting for look-elsewhere factors) cross-correlation between CHIME FRBs and galaxies in the redshift range 0.3 ≲ z ≲ 􏰂0.5. The strength and angular scale of the cross-correlation are consistent with an order-one fraction of CHIME FRBs being in this redshift range, and in the same dark matter halos as the survey galaxies. Finally, we find statistical evidence for a subpopulation of FRBs with large host dispersion measure (host DM ~ 400 pc/cm³) at z ~ 0.4. We show that such large host DMs could be explained by a small population of FRBs near the centers (r ≲ 100 kpc) of large (M ~ 10¹⁴ M⊙) halos.
Selected topics in Computational Relativity
(University of Waterloo, 2022-08-18) Mukherjee, Soham; Schnetter, Erik; Broderick, Avery
This thesis addresses a collection of topics that are either directly related to, or have implications for, current challenges in computational relativity. In the first part, we explore a spacetime discretization method for computational relativity. This offers unique computational advantages, for distributing the computation over a large number of processes, as well as for studying spacetime regions close to black hole singularities. In the second part, we present a method to construct initial conditions for numerical evolution of charged, spinning black hole binaries. The evolution of these initial conditions provides a proxy for binary black hole waveforms in modified theories of gravity. In the third part of the thesis, we focus on building an empirical understanding of why Boolean Satisfiability (SAT) solvers are efficient for real-world problems, when, theoretically, the Boolean SAT problem is computationally intractable.
Semi-Analytic Radiatively Inefficient Accretion Flows in General Gravity
(University of Waterloo, 2018-01-05) Georgiev, Boris; Broderick, Avery; Lehner, Luis
The Event Horizon Telescope is capable of observing black holes on event-horizon scales and we can use it to explore deviations from General Relativity. We replicate the results of a previous study using a Newtonian prescription of gravity to set a suitable limit to justify viscosity and radiation choices. We also create a framework to explore semi-analytic models of Radiatively Inefficient Accretion Flows with a covariant prescription of gravity. We study a Kerr-like metric not described by General Relativity, and find only small deviations from Kerr solutions. We also find that the dynamics of the flow are sensitive to small General Relativistic effects.
Using Black Hole Environments as Laboratories for Testing Accretion and Gravity
(University of Waterloo, 2023-08-29) Georgiev, Boris; Broderick, Avery; Lehner, Luis
With the advent of the Event Horizon Telescope (EHT), we have the ability to observe the lensed emission from hot plasmas near event horizons. It contains entangled information about the turbulent magnetohydrodynamic accretion processes and the black hole spacetime. We present studies of both coherent and stochastic variable features measurable from EHT data, and by applying them to libraries of simulations, we describe how they inform on the underlying accretion flow. For a rapidly varying source like Sgr A*, we find that variability is greatest on the largest spatial scales and the longest timescale, and find a universal power-law variability prescription in the visibility domain. For a slowly varying source like M87*, we measure a correlation timescale and a rotation rate, which is inconsistent with the velocity of the fluid causing the emission. The strong predictions from the simulations of the variability measures we create are consistent with current EHT data. We also create a static analytical midplane accretion flow model and use variable simulations to motivate a prescription to incorporate turbulent effects. Our model matches SANE models well and struggles with the more variable MAD models. It can incorporate non-Kerr metrics, can connect the black hole scales to their large-scale environment, and can be incorporated in existing parameter estimation frameworks to fit EHT data.

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