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Date of Award

Summer 8-15-2016

Author's School

Graduate School of Arts and Sciences

Author's Department


Degree Name

Doctor of Philosophy (PhD)

Degree Type



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

elds 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 elds 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 elds 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 elds 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 < ga

< 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 eld. VHE photon

emission and propagation is studied using a Monte Carlo method. The emission regions

are dened using the slot gap and outer gap models. The eects of pair production and

axion-photon mixing are considered and light curves and spectra are produced to illustrate

the in

uence 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 nd that the inclusion of QED eects on the eective photon

mass limit the conversion probability over much of the region where strong pair attenuation

is expected.


English (en)

Chair and Committee

James H. Buckley

Committee Members

Francesc Ferrer, Viktor Gruev, Henric Krawzcynski, Michael Ogilvie


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