Date of Award
Doctor of Philosophy (PhD)
Blazars, among the most luminous objects in the sky, are extreme members of the class of active galactic nuclei featuring powerful outflows of relativistic particles in the form of jets pointed towards the observer. They display strong flux variability in all observed wavebands and on all observed timescales, with significant changes in flux observed in some sources over periods of as short as minutes. These remarkable observations have led to open questions regarding the nature of particle acceleration in the jets, location of the radiating particles along the jet, and the connection between jet emission and rate of accretion onto the black hole within the galactic nucleus. Recently, astronomers have devoted significant attention to the potential for these sources to show periodic variation in their fluxes, with numerous papers published annually claiming a significant finding in blazar fluxes across all energy bands. The potential for these periodicities to result from binary black hole orbits within active galaxies is frequently invoked.
This thesis addresses these questions of flux variability and periodic variation in a flux-limited sample of blazar light curves in the gamma-ray band from the Fermi Large Area Telescope. A commonly applied time series model is extended and used to characterize the flux variability of our sample. Marginal distributions of the fluxes are fit to a lognormal distribution. These results are analyzed and compared to the physical parameters of the sources. The time series model is then used as a null model, against which a novel method of significance testing for periodic variability is developed and applied. We find that the variability properties of the sources in our study agree with the predictions from standard leptonic scenarios of blazar emission, with some interesting implications for overlap between different source classes. Further, while we find that many claims of flux periodicities in the sources of our sample do not hold up to scrutiny, we see hints of flux oscillations in two sources, with two broad timescales. These are compared to the scenarios for blazar flux periodicities, and the potential explanations assessed. We conclude by discussing how this work could be expanded, as well as what other directions for research may be fruitful in understanding the particle acceleration and cooling in relativistic jets.
Chair and Committee
James Buckley, Henric Krawczynski, Ramanath Cowsik, Renato Feres,
Groebe, Benjamin, "Flux Variability in Gamma-Ray Blazars" (2020). Arts & Sciences Electronic Theses and Dissertations. 2319.