Physics & Astronomy - Virtual Senior Honor Thesis-Tara Sweeney, Dartmouth

Dartmouth Events

Physics & Astronomy - Virtual Senior Honor Thesis-Tara Sweeney, Dartmouth

Title: "An Analysis of Optical Polarization Angle Variations in Blazar Jets"

Thursday, May 28, 2020
Intended Audience(s): Public
Categories: Lectures & Seminars

Abstract: Blazars are supermassive black holes with relativistic jets that point near the Earth line of sight. These jets emit radiation across the electromagnetic spectrum, but are characterized by correlated, broadband variability and strong optical polarization. These characteristics are associated with the physical structure of the blazar’s jet. While we broadly understand the mechanism by which magnetic fields accelerate charged particles, the exact structure of the magnetic field within the jet and the degree to which it is coherent over large spatial scales remains unknown. Temporal changes in the blazar jet magnetic field structure cause the polarization angle (electric vector position angle or EVPA) to change over time, which we identify as an EVPA rotation. Recent theoretical models have been proposed to account for the relationship between EVPA rotations and the magnetic field, suggesting either an ordered or stochastic magnetic field structure. These models predict different responses of an EVPA rotation with respect to jet flares. In this thesis, I attempt to constrain these theoretical models using publicly available data from Steward Observatory. I analyze optical polarimetry and V-band photometry collected over ten years for a sample of 24 blazars, including 11 BL Lacs and 13 Flat Spectrum Radio Quasars (FSRQs). In order to define EVPA rotation candidates, I apply a novel suite of tools to identify robust EVPA rotation candidates, incorporating one phenomenological and two statistical tests, including LOWESS (Locally Weighted Scatterplot Smoothing) and ARIMA (Autoregressive Integrated Moving Average) modelling. I locate and rank the quality of EVPA rotation candidates in my sample and then evaluate the degree to which these EVPA rotation candidates are contemporaneous with blazar jet flares, as parameterized by gamma-ray flux or V-band magnitude. I find that while the most energetic blazar flares are consistent with a more structured, coherent jet, most blazars in my sample seem to have very little large scale magnetic field ordering, suggesting more stochastic processes are driving the EVPA rotation candidates I observed.

For more information, contact:
Tressena Manning

Events are free and open to the public unless otherwise noted.