The flux of ultra high energy cosmic gamma rays observed by the HiRes Experiment

Ultra-high energy cosmic rays are particles of likely extragalactic origin which, upon entering the earth's atmosphere, produce extensive air showers of billions of charged particles. The energy of the primary cosmic rays---up to approximately 1020 eV---is deposited in the atmosphere and causes nitrogen molecules to fluoresce in the ultra violet which may be observed by ground-based instruments like the High Resolution Fly's Eye Experiment (HiRes). Certain parameters of the air shower are dependent on the particle type of the primary cosmic ray. In this work the properties of extensive air showers are subjected to a Bayesian composition analysis with the goal of setting an upper limit on the fraction of gamma ray primaries in the cosmic ray flux above 40 EeV. The upper limit is found to be 61% at the 95% confidence level. By using the existing gamma ray limits as prior knowledge, a combined upper limit of 21% is placed on the fractional gamma ray flux.