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Date of Award
Doctor of Philosophy (PhD)
Pulsars are highly-magnetized rapidly-rotating neutron stars that emit energy throughout the electromagnetic spectrum. Despite decades of study, the emission mechanisms of pulsars are not well understood. New observations at the highest energy end of the spectrum can provide strong constraints on theoretical models of pulsar emission. The strong magnetic fields of pulsar magnetospheres accelerate charged particles to relativistic energies and these particles emit very high energy (VHE; E > 100 GeV) gamma rays. In addition to creating conditions to emit gamma rays, the magnetic fields are powerful enough to attenuate gamma rays through pair production. The attenuation of gamma rays limits the photon energies that may escape the magnetosphere, unless an additional physical process decreases the opacity of the magnetosphere to these photons. The interaction of axions or axionlike particles (ALPs) with magnetic fields is one such process. Some extensions of the Standard Model suggest the existence of axions, which are light pseudoscalar bosons with a two-photon coupling. As a result of this coupling photon-ALP oscillations can occur in the strong fields of a pulsar magnetosphere. For typical parameters of pulsar magnetospheres, VHE photons fall within the strong mixing-regime for oscillations when the axion mass is 10−3 eV < ma < 10 eV and the axion-photon coupling constant is 10−11 < gaγ < 10−6 . Axion-photon oscillations within the inner magnetosphere would decrease its opacity as axions would propagate unimpeded by pair attenuation. In this dissertation, the VHE photon emission and propagation from pulsars is studied in detail. New observations and analysis of the Crab pulsar from the VERITAS experiment are presented which extend the Crab spectrum to higher energies. The magnetospheres of pulsars are simulated using a retarded vacuum dipole solution for the magnetic field. VHE photon emission and propagation is studied using a Monte Carlo method. The emission regions are defined using the slot gap and outer gap models. The effects of pair production and axion-photon mixing are considered and light curves and spectra are produced to illustrate the influence of both processes on the observations of pulsars. For some geometries, VHE photons are heavily attenuated by pair production. Axion-photon mixing is shown to reduce the opacity of pulsar magnetospheres allowing a larger fraction of VHE photons to survive propagation. However, we find that the inclusion of QED effects on the effective photon mass limit the conversion probability over much of the region where strong pair attenuation is expected.
Chair and Committee
James H. Buckley
Francesc Ferrer, Viktor Gruev, Henric Krawzcynski, Michael Ogilvie
Archer, Avery Michael, "A Study of the Effects of Pair Production and Axionlike Particle Oscillations on Very High Energy Gamma Rays from the Crab Pulsar" (2016). Arts & Sciences Electronic Theses and Dissertations. 828.
Available for download on Friday, September 22, 2017