A Search for the Galactic Origins of Ultrahigh-Energy Cosmic Rays


Glenn Edward Allen

Dissertation submitted to the faculty of the Graduate School
of The University of Maryland in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy

Advisory Committee:

Dr. Todd J. Haines, Cochairman/Advisor
Prof. Jordan A. Goodman, Cochairman
Dr. Cyrus M. Hoffman
Prof. Nicholas Hadley
Assoc. Prof. Douglas C. Hamilton
Assoc. Prof. David I. Bigio


After many decades of cosmic-ray research, the origins of cosmic-ray nuclei are still unknown. Because interstellar magnetic fields deflect charged particles, measurements of the incident directions of cosmic-ray nuclei do not reveal the sites of their production. Therefore, direct evidence of the sites of cosmic-ray acceleration requires detection of neutrally-charged particles, such as gamma rays, produced by the interactions of accelerated cosmic rays with other nuclei near the cosmic-ray accelerators.

It is generally accepted that supernova remnants predominantly accelerate Galactic cosmic rays at least up to energies ~ 100 TeV. No other single known class of objects can explain the properties of the cosmic rays observed at Earth.

Pulsars may accelerate some fraction of the galactic cosmic-ray electrons and nuclei. Pulsars are observed to accelerate particles to energies of at least ~ 1 TeV and are the strongest galactic gamma-ray sources at these energies and at energies ~ 100 MeV.

This thesis describes searches for evidence of gamma-ray emission from five supernova remnants and from the Geminga pulsar at energies ~ 100 TeV using the data set of the CYGNUS-I extensive air-shower array.

The analyses of the five supernova remnants, which are positionally coincident with gamma-ray sources observed with the EGRET detector at energies 100 MeV, reveal no evidence of gamma-ray emission at energies ~ 100 TeV. A comparison of the gamma-ray flux upper limits from the CYGNUS-I data to the fluxes measured with the EGRET instrument suggests that the integral cosmic-ray spectra of the five supernova remnants may soften by about 100 TeV or may be steeper than an E^-1.3 power-law distribution.

The analysis of the Geminga pulsar reveals no evidence of either pulsed or unpulsed gamma-ray emission from this object at energies ~ 100 TeV. This result is consistent with present models of particle acceleration by pulsars.