Doctor of Philosophy (PhD)


Physics and Astronomy

Document Type



The origin and mass composition of ultra-high energy cosmic rays (UHECRs) are among the mysteries of particle astrophysics. Since UHECRs are mostly or entirely charged particles, they will be deflected in cosmic magnetic fields on their way to Earth. UHECR deflections in the Galactic magnetic fields (GMF) should be considered to identify their origin and nature.

The calculated Galactic magnetic deflections of UHECRs from a source strongly depend on the GMF model. I use the recent GMF model of Jansson and Farrar (JF12), a 35-parameter model which includes coherent, striated and random components and is constrained by WMAP7 Galactic synchrotron emission map and more than forty thousand extragalactic rotation measures. I develop a new method of implementing a random field using a Kolmogorov spectrum and scale it with the JF12 random component strengths.

Simulated UHECRs are propagated through the GMF using the publicly available propagation code CRT. These particles are simulated from Centaurus A, which is the nearest AGN to Earth. I identify the expected arrival direction loci of UHECRs in the JF12 GMF model for different rigidities (energy divided by charge) between 2 and 100 EV (1 EV ≡ 1018 V) to determine whether Centaurus A could be a significant source of the UHECR excess reported by Pierre Auger Observatory. I calculate the excess of event arrival directions, with respect to isotropic expectations, for cases of pure protons, pure iron, and a mixture.

I also develop a method for identifying the mass composition of UHECRs under specific source and GMF hypotheses. Comparing Auger observations with simulations of different rigidities leads to the identification of the charges of the events. Consistent simulated rigidities are found for each Auger event based on their overlap probabilities. The charge of each event is the measured energy divided by the rigidity of the most consistent simulation.

Including a Kolmogorov random field component to the JF12 regular component does not significantly change the results of propagation at high rigidities whereas at low rigidities it smears the arrival distributions of events to such an extent that deflection studies can become difficult.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Matthews, James