Order & disorder: a study of the flaring properties and polarized emission of blazars
MacDonald, Nicholas Roy
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Blazars are the most luminous persistent and enigmatic objects in the sky. They constitute a sub-class of active galactic nuclei (AGN) whose relativistic plasma jets are closely aligned to our line of sight. By monitoring the polarized emission of these jets and subsequently modeling flares in the high-energy emission, we are able to gain insight into the parsec-scale physics of the jets close to the central engines. My dissertation develops and augments several theoretical models of high-energy blazar emission. The vast majority of gamma-ray flares detected in blazars are highly correlated with flares detected at longer wavelengths; however, a small subset of these gamma-ray flares appear to occur in isolation. These "orphan" gamma-ray flares challenge current models of blazar variability. I have developed a theoretical model of blazar emission to explain the origin of these orphan flares. This model invokes the presence of a sheath of plasma enshrouding the relativistic spine of the jet. The sheath supplies photons that are inverse-Compton scattered up to high energies by relativistic electrons contained within the jet, producing an orphan flare. This model is successfully applied to a number of such gamma-ray flares. In addition, I present stacked radio images that highlight the presence of jet sheaths in my sample of blazars. Circular polarization (CP) has been detected in a number of blazar jets. CP is very sensitive to the underlying plasma content of the jet. A. Marscher has developed the Turbulent Extreme Multi-Zone (TEMZ) model for blazar emission consisting of thousands of individual cells of plasma that propagate relativistically across a standing shock in the jet. The turbulent nature of the magnetic field within the TEMZ grid naturally creates a birefringent environment in which CP emission can be produced. In order to investigate whether the TEMZ model can indeed produce CP, I have developed a numerical algorithm to solve the full Stokes equations of polarized radiative transfer. I apply this algorithm to ray tracing through the TEMZ model. I am able to demonstrate that TEMZ can reproduce CP at the levels present in blazars.